**4. Polymorphisms of DNA repair genes and amyotrophic lateral sclerosis**

Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND), is one of the major neurodegenerative diseases alongside AD and PD. It is a progressive disorder characterized by the degeneration of motor neurons of the motor cortex, brainstem and spinal cord. The incidence of the disease is similar worldwide and ranges from 1 to 3 cases per 100,000 individuals per year, with the exception of some high-risk areas around the Pacific Rim. Several studies report increased oxidative DNA damage and a compromised DNA repair activity, particularly BER activity, in spinal cords and other tissues of ALS patients (Bogdanov et al., 2000; Ferrante et al., 1997; Kikuchi et al., 2002; Kisby et al., 1997). Missense mutations in the gene encoding *APEX1* were found in DNA obtained from 8 of 11 ALS patients, including the common *APEX1* Asp148Glu polymorphism (Hayward et al., 1999), that was subsequently associated with increased ALS risk in a Scottish cohort of 117 ALS patients and 58 controls, and in an Irish group of 105 ALS individuals and 82 controls (Greenway et al., 2004). The analysis of 88 English ALS patients and 88 matched controls still revealed an increased frequency of the variant allele in the ALS cohort, even if not statistically significant (Tomkins et al. 2000). We have recently performed the largest casecontrol study aimed at clarifying the role of *APEX1* Asp148Glu in sporadic ALS pathogenesis. No difference in *APEX1* Asp148Glu allele and genotype frequencies was found between 134 ALS patients and 129 controls of Italian origin, nor was the polymorphism associated with disease age or site of onset, or duration of the disease, suggesting that it might not play a major role in ALS pathogenesis in the Italian population (Coppedè et al., 2010a). The ALSGene database (www.alsgene.org) is a public database containing all the ALS genetic association studies, genome-wide association studies and updated meta-analyses of the literature. A meta-analysis of the four studies described above revealed a significant increased frequency of the variant 148Glu allele in ALS cases with respect to controls, suggesting a protective role for the wild type 148Asp variant with an OR = 0.78 (95%CI=0.62-0.97) (www.alsgene.org). Our analysis of the *OGG1* Ser326Cys polymorphism in 136 ALS patients and 129 matched controls of Italian origin revealed a significant association of the variant allele with increased ALS risk (Coppedè et al., 2007b) (Table 3). At best of our knowledge this study is the first in the literature addressing this issue, still pending replication in other populations. More recently, we screened over 400 individuals, including 206 ALS patients and 203 matched controls of Italian origin for the presence of *XRCC1* Arg194Trp, Arg280His and Arg399Gln polymorphisms, observing a significant increased frequency of the 399Gln variant allele and a borderline significant decreased frequency of the 194Trp allele in ALS patients with respect to controls (Coppedè et al., 2010b). Interestingly, others have evaluated the same *XRCC1* polymorphisms and two

Variants and Polymorphisms of DNA Repair Genes and Neurodegenerative Diseases 575

Parkinson's disease is the second most common neurodegenerative disorder after AD, affecting 1–2% of the population over the age of 50 years, and is characterized by progressive and profound loss of neuromelanin containing dopaminergic neurons in the *substantia nigra* (SN) resulting in resting tremor, rigidity, bradykinesia, and postural instability. The majority of PD cases are sporadic idiopathic forms, resulting from three interactive events: an individual's inherited genetic susceptibility, subsequent exposure to environmental risk factors, and aging (Bekris et al., 2010). However, in a minority of the cases PD is inherited as a Mendelian trait. Parkin is an E3 ubiquitin ligase that acts on a variety of substrates, resulting in polyubiquitination and degradation by the proteasome or monoubiquitination and regulation of biological activity. Mutation of *parkin* is one of the most prevalent causes of autosomal recessive familial PD and a recent study has shown that parkin is essential for optimal repair of DNA damage. Particularly, DNA damage induces nuclear translocation of parkin leading to interactions with PCNA and possibly other nuclear proteins involved in DNA repair (Kao, 2009). Moreover, parkin protects mitochondrial genome integrity and supports mitochondrial DNA (mtDNA) repair (Rothfuss et al., 2009). DNA polymerase gamma (POLG1) participates in mtDNA replication and repair, thus playing a fundamental role in mtDNA maintenance. Missense mutations in *POLG1* co-segregate with a phenotype that includes progressive external ophthalmoplegia and parkinsonism (Hudson et al., 2007). Moreover, missense mutations in *POLG1* have been reported in case studies, in which parkinsonism was part of the clinical symptoms (Davidzon et al., 2006; Remes et al., 2008). *POLG1* mutations and polymorphisms have been also investigated in sporadic idiopathic PD, among them a polyglutamine (poly-Q) located in the *N-*terminal of POLG1, encoded by a CAG repeat in exon 2. The poly-Q tract normally consists of 10Q (frequency >80%), followed by 11Q (frequency > 6-12%), whereas non-10Q/11Q alleles are considered as less frequent alleles. Several authors investigated whether or not non-10Q alleles are more frequent in PD cases than in matched controls (Hudson et al., 2009; Luoma et al., 2007; Taanman & Shapira, 2005; Tiangyou et al., 2006). Eerola and coworkers recently screened 641 PD patients and 292 controls from USA and performed a pooled analysis of their data with those available in the literature (Hudson et al., 2009; Luoma et al., 2007; Taanman & Shapira, 2005; Tiangyou et al., 2006) for a total of 1163 sporadic PD patients and 1214 controls observing that variant alleles defined as non-10Q were significantly increased in PD patients than in controls (16.3%vs.13.4%, *p* = 0.005) (Eerola et al., 2010). A few months later Anvret and coworkers screened 243 PD patients and 279 matched controls from Sweden, observing that non10Q/11Q alleles were more frequent in PD cases than in controls with an OR of 2.0 (1.3-3.1, 95%CI) strengthening the evidence that non frequent *POLG1* alleles might be more frequent in sporadic PD patients than in controls, thus representing a PD risk factor (Anvret et al., 2010) (Table 4). We screened 139 sporadic PD patients and 211 healthy matched controls for the presence of the *OGG1* Ser326Cys polymorphism. The Cys326 allele frequency was similar between the groups (0.20 in PD patients and 0.19 in controls), and no difference in genotype frequencies was observed. Moreover, the *OGG1* Ser326Cys polymorphism was not associated with PD age at onset (Coppedè et al., 2010c). In human cells the oxidized purine nucleoside triphosphatase MTH1 efficiently hydrolyzes oxidized purines such as 8-oxo-guanine in the nucleotide pools, thus avoiding their incorporation into DNA or RNA. A Val83Met polymorphism of the *MTH1* gene was studied in 73

**5. Polymorphisms of DNA repair genes and Parkinson's disease** 

additional ones (rs939461 and rs915927) in 108 ALS patients and 39 controls from New-England, observing that rs939461 was associated with reduced ALS risk, and Arg399Gln with a borderline significant reduced risk (Fang et al., 2010) (Table 3). Overall, even if still inconclusive, the results of both studies suggest that additional investigation is required to clarify the role of *XRCC1* polymorphisms and haplotypes in ALS pathogenesis.

### **4.1 Less frequent BER gene variants and polymorphisms**

Alongside with common BER gene polymorphisms, less frequent gene variants or polymorphisms have been observed in the DNA of both ALS subjects and matched controls, but with very low allele frequencies and no significant difference between groups. Some examples are *APEX1* 1835C/A (Intron3), *APEX1* 2712A/T (3'UTR), *APEX1* 459C/T (Exon1), and *APEX1* rs1048945 (Q51H) (Hayward et al., 1999; Tomkins et al., 2000).


Table 3. DNA repair gene polymorphisms and risk of Amyotrophic Lateral sclerosisa OR are derived from the original paper and referred to (heterozygous+minor homozygous) vs major homozygous.

additional ones (rs939461 and rs915927) in 108 ALS patients and 39 controls from New-England, observing that rs939461 was associated with reduced ALS risk, and Arg399Gln with a borderline significant reduced risk (Fang et al., 2010) (Table 3). Overall, even if still inconclusive, the results of both studies suggest that additional investigation is required to

Alongside with common BER gene polymorphisms, less frequent gene variants or polymorphisms have been observed in the DNA of both ALS subjects and matched controls, but with very low allele frequencies and no significant difference between groups. Some examples are *APEX1* 1835C/A (Intron3), *APEX1* 2712A/T (3'UTR), *APEX1* 459C/T (Exon1),

> **Number of subjects ALS/Controls**

**Variant allele frequency ALS/Controls** 

Ser326Cys 136/129 0.26/0.18 1.62 (1.07-2.45)

Asp148Glu 117/58 0.62/0.49 1.66 (1.06-2.60)

Asp148Glu 88/88 0.51/0.45 1.28 (0.85-1.95)

Asp148Glu 105/82 0.60/0.51 1.46 (0.97-2.21)

Asp148Glu 134/129 0.44/0.45 0.99 (0.70-1.40)

Arg194Trp 206/195 0.05/0.08 0.58 (0.32-1.05)

Arg280His 205/203 0.09/0.08 1.25 (0.76-2.04)

Arg399Gln 197/194 0.39/0.28 1.39 (1.05-1.85)

Arg194Trp 108/39 0.06/0.03 2.4 (0.5-2.2)a

Arg280His 108/39 0.05/0.03 2.0 (0.4-2.0)a

Arg399Gln 108/39 0.35/0.47 0.4 (0.2-1.0)a

rs915927 108/39 0.45/0.33 2.4 (0.5-2.2)a

rs939461 108/39 0.06/0.15 0.4 (0.1-0.9)a

Table 3. DNA repair gene polymorphisms and risk of Amyotrophic Lateral sclerosisa OR are derived from the original paper and referred to (heterozygous+minor homozygous) vs

**Odds Ratio (95% CI)** 

clarify the role of *XRCC1* polymorphisms and haplotypes in ALS pathogenesis.

and *APEX1* rs1048945 (Q51H) (Hayward et al., 1999; Tomkins et al., 2000).

**4.1 Less frequent BER gene variants and polymorphisms** 

OGG1

APEX1

APEX1

APEX1

APEX1

XRCC1

XRCC1

XRCC1

**Reference Polymorphism** 

Coppedè et al., 2007b

Hayward et al. 1999

Tomkins et al. 2000

Greenway et al. 2004

Coppedè et al. 2010a

Coppedè et al. 2010b

Coppedè et al. 2010b

Coppedè et al. 2010b

major homozygous.

Fang et al. 2010 XRCC1

Fang et al. 2010 XRCC1

Fang et al. 2010 XRCC1

Fang et al. 2010 XRCC1

Fang et al. 2010 XRCC1

#### **5. Polymorphisms of DNA repair genes and Parkinson's disease**

Parkinson's disease is the second most common neurodegenerative disorder after AD, affecting 1–2% of the population over the age of 50 years, and is characterized by progressive and profound loss of neuromelanin containing dopaminergic neurons in the *substantia nigra* (SN) resulting in resting tremor, rigidity, bradykinesia, and postural instability. The majority of PD cases are sporadic idiopathic forms, resulting from three interactive events: an individual's inherited genetic susceptibility, subsequent exposure to environmental risk factors, and aging (Bekris et al., 2010). However, in a minority of the cases PD is inherited as a Mendelian trait. Parkin is an E3 ubiquitin ligase that acts on a variety of substrates, resulting in polyubiquitination and degradation by the proteasome or monoubiquitination and regulation of biological activity. Mutation of *parkin* is one of the most prevalent causes of autosomal recessive familial PD and a recent study has shown that parkin is essential for optimal repair of DNA damage. Particularly, DNA damage induces nuclear translocation of parkin leading to interactions with PCNA and possibly other nuclear proteins involved in DNA repair (Kao, 2009). Moreover, parkin protects mitochondrial genome integrity and supports mitochondrial DNA (mtDNA) repair (Rothfuss et al., 2009). DNA polymerase gamma (POLG1) participates in mtDNA replication and repair, thus playing a fundamental role in mtDNA maintenance. Missense mutations in *POLG1* co-segregate with a phenotype that includes progressive external ophthalmoplegia and parkinsonism (Hudson et al., 2007). Moreover, missense mutations in *POLG1* have been reported in case studies, in which parkinsonism was part of the clinical symptoms (Davidzon et al., 2006; Remes et al., 2008). *POLG1* mutations and polymorphisms have been also investigated in sporadic idiopathic PD, among them a polyglutamine (poly-Q) located in the *N-*terminal of POLG1, encoded by a CAG repeat in exon 2. The poly-Q tract normally consists of 10Q (frequency >80%), followed by 11Q (frequency > 6-12%), whereas non-10Q/11Q alleles are considered as less frequent alleles. Several authors investigated whether or not non-10Q alleles are more frequent in PD cases than in matched controls (Hudson et al., 2009; Luoma et al., 2007; Taanman & Shapira, 2005; Tiangyou et al., 2006). Eerola and coworkers recently screened 641 PD patients and 292 controls from USA and performed a pooled analysis of their data with those available in the literature (Hudson et al., 2009; Luoma et al., 2007; Taanman & Shapira, 2005; Tiangyou et al., 2006) for a total of 1163 sporadic PD patients and 1214 controls observing that variant alleles defined as non-10Q were significantly increased in PD patients than in controls (16.3%vs.13.4%, *p* = 0.005) (Eerola et al., 2010). A few months later Anvret and coworkers screened 243 PD patients and 279 matched controls from Sweden, observing that non10Q/11Q alleles were more frequent in PD cases than in controls with an OR of 2.0 (1.3-3.1, 95%CI) strengthening the evidence that non frequent *POLG1* alleles might be more frequent in sporadic PD patients than in controls, thus representing a PD risk factor (Anvret et al., 2010) (Table 4). We screened 139 sporadic PD patients and 211 healthy matched controls for the presence of the *OGG1* Ser326Cys polymorphism. The Cys326 allele frequency was similar between the groups (0.20 in PD patients and 0.19 in controls), and no difference in genotype frequencies was observed. Moreover, the *OGG1* Ser326Cys polymorphism was not associated with PD age at onset (Coppedè et al., 2010c). In human cells the oxidized purine nucleoside triphosphatase MTH1 efficiently hydrolyzes oxidized purines such as 8-oxo-guanine in the nucleotide pools, thus avoiding their incorporation into DNA or RNA. A Val83Met polymorphism of the *MTH1* gene was studied in 73

Variants and Polymorphisms of DNA Repair Genes and Neurodegenerative Diseases 577

Hereditary ataxias are a heterogeneous group of diseases with different patterns of inheritance. Some of them are caused by recessive mutations in genes involved in DNA repair pathways that likely predispose the affected individuals to neurodegeneration. Spinocerebellar ataxia with axonal neuropathy 1 (SCAN1) is caused by autosomal recessive mutations in the gene encoding tyrosyl-DNA phosphodiesterase 1 (TDP1), a protein required for the repair of DNA SSBs that arise independent of DNA replication from abortive topoisomerase 1 activity or oxidative stress. Ataxia-telangiectasia (AT), ataxiatelangiectasia-like disorder (ATLD), ataxia oculomotor apraxia type 1 (AOA1) and ataxia oculomotor apraxia type 2A (AOA2) are a subgroup of the autosomal recessive spinocerebellar ataxias characterized by cerebellar atrophy and oculomotor apraxia. The progressive neurodegeneration described in AT and ATLD is due to mutations in genes encoding for ATM and Mre11, respectively. ATM recognizes and signals DNA DSBs to the cell cycle checkpoints and the DNA repair machinery. The Mre11 DNA repair complex, composed of Rad50, Mre11 and Nbs1 proteins, is involved in DNA damage recognition, DNA repair, and initiating cell cycle checkpoints. ATM and the Mre11 complex combine to recognize and signal DNA DSBs. AOA1 is caused by mutations in the gene encoding aprataxin (APTX), a nuclear protein that interacts with several DNA repair proteins, including XRCC1, Polβ, DNA ligase III, PARP-1, and p53. It functions in the endprocessing of DNA SSBs removing 3′-phosphate, 5′-phosphate, and 3′-phosphoglycolate ends. AOA2 is caused by autosomal recessive mutations in the gene encoding senataxin (SETX). SETX is a member of the superfamily I DNA/RNA helicases, likely involved in oxidative DNA damage response. *SETX* mutations have been also linked to juvenile ALS. Overall, spinocerebellar ataxias deficient in DNA damage responses represent the most robust set of data linking mutations in DNA repair genes to neurodegeneration (Gueven et al., 2007;

Huntington's disease (HD) is a progressive neurodegenerative disorder resulting in cognitive impairment, choreiform movements and death which usually occurs 15–20 years after the onset of the symptoms. The disease is also characterized by psychiatric and behavioural disturbances. HD is an autosomal dominant disorder caused by a CAG repeat expansion within exon 1 of the gene encoding for huntingtin (*IT15*) on chromosome 4. In the normal population the number of CAG repeats is maintained below 35, while in individuals affected by HD it ranges from 35 to more than 100, resulting in an expanded polyglutamine segment in the protein. Age at onset of the disease is inversely correlated with the CAG repeat length; moreover the length of the expanded polyglutamine segment seems to be related to the rate of clinical progression of neurological symptoms and to the progression of motor impairment, but not to psychiatric symptoms. Somatic CAG repeat expansion in the gene encoding for huntingtin has been observed in several HD tissues, including the striatum which is the region most affected by the disease and the OGG1 protein has been involved in somatic CAG repeat expansion in HD, suggesting that it might contribute to disease age at onset (Kovtun et al. 2007). We recently observed a weak borderline association between the *OGG1* Ser326Cys polymorphism and HD age at onset in a small group of 91 HD subjects (Coppedè et al. 2010d). However, replication of the study in a

**6. Other neurodegenerative diseases** 

**6.1 Spinocerebellar ataxias** 

Martin, 2008).

**6.2 Huntington's disease** 

Japanese patients with sporadic PD and 151 age-matched controls but was not associated with sporadic PD risk (Satoh & Kuroda, 2000). Another *MTH1* polymorphism (Ile45Thr) was investigated in 106 PD patients and 135 unrelated controls from China. The variant allele frequency resulted borderline increased in PD males (Jiang et al., 2008). This finding is pending replication in other populations. *PARP1* promoter polymorphisms (-410C/T, - 1672G/A, and a (CA)n microsatellite) have been investigated in 146 Spanish PD cases and 161 matched controls. A protective effect against PD was found for heterozygosity at -410 (OR = 0.44) and (CA)n microsatellite (OR = 0.53) polymorphisms, and heterozygosity at -1672 polymorphism delayed by 4 years on the onset age of PD (Infante et al., 2007). Also these findings are original and waiting for replication in additional case-control groups (Table 4).


Table 4. DNA repair gene polymorphisms and risk of Parkinson's disease. a = pooledanalysis of (Eerola et al., 2010, Hudson et al., 2009; Luoma et al., 2007; Taanman & Shapira, 2005; Tiangyou et al., 2006). b = Allele frequency difference (PD/Controls) approached significance in the male subgroup (0.07/0.02, *P* = 0.05). c = Heterozygous genotype frequency. d = Associated with PD age at onset

#### **5.1 Other mutations and polymorphisms**

As previously observed, several *POLG1* mutations have been observed to co-segregate in families with parkinsonism. For a detailed description I suggest a recent review by Orsucci and coworkers (Orsucci et al., 2010).

#### **6. Other neurodegenerative diseases**

#### **6.1 Spinocerebellar ataxias**

576 DNA Repair

Japanese patients with sporadic PD and 151 age-matched controls but was not associated with sporadic PD risk (Satoh & Kuroda, 2000). Another *MTH1* polymorphism (Ile45Thr) was investigated in 106 PD patients and 135 unrelated controls from China. The variant allele frequency resulted borderline increased in PD males (Jiang et al., 2008). This finding is pending replication in other populations. *PARP1* promoter polymorphisms (-410C/T, - 1672G/A, and a (CA)n microsatellite) have been investigated in 146 Spanish PD cases and 161 matched controls. A protective effect against PD was found for heterozygosity at -410 (OR = 0.44) and (CA)n microsatellite (OR = 0.53) polymorphisms, and heterozygosity at -1672 polymorphism delayed by 4 years on the onset age of PD (Infante et al., 2007). Also these findings are original and waiting for replication in additional case-control

> **Number of subjects PD/Controls**

**Variant allele frequency PD/Controls** 

Poly-Q tract 641/292 0.17/0.12 OR = n.a.

Poly-Q tract 1163/1214a 0.16/0.13 OR = n.a.

Poly-Q tract 243/279 0.11/0.06 2.0 (1.3-3.1)

Ser326Cys 139/211 0.20/0.19 1.05 (0.72-1.53)

Val83Met 73/151 0.07/0.11 OR = n.a.

Ile45Thr 106/135 0.05/0.02 OR = n.a.

(-410) 146/161 0.35/0.53c 0.44 (0.26-0.75)

(-1672)d 146/161 0.29/0.27c 0.87 (0.50-1.52)

(CA)n 146/161 0.36/0.50c 0.53 (0.31-0.90)

Table 4. DNA repair gene polymorphisms and risk of Parkinson's disease. a = pooledanalysis of (Eerola et al., 2010, Hudson et al., 2009; Luoma et al., 2007; Taanman & Shapira, 2005; Tiangyou et al., 2006). b = Allele frequency difference (PD/Controls) approached significance in the male subgroup (0.07/0.02, *P* = 0.05). c = Heterozygous genotype

As previously observed, several *POLG1* mutations have been observed to co-segregate in families with parkinsonism. For a detailed description I suggest a recent review by Orsucci

**Odds Ratio (95% CI)** 

*P* = 0.004

*P* = 0.005

*P* = 0.219

*P* = 0.08b

groups (Table 4).

Eerola et al., 2010

Eerola et al., 2010

Anvret et al., 2010

Coppedè et al., 2010c

> Satoh & Kuroda, 2000

Jiang et al., 2008

Infante et al., 2007b

Infante et al., 2007b

Infante et al., 2007b

**Reference Polymorphism** 

POLG1

POLG1

POLG1

OGG1

MTH1

MTH1

PARP-1

PARP-1

PARP-1

frequency. d = Associated with PD age at onset

**5.1 Other mutations and polymorphisms** 

and coworkers (Orsucci et al., 2010).

Hereditary ataxias are a heterogeneous group of diseases with different patterns of inheritance. Some of them are caused by recessive mutations in genes involved in DNA repair pathways that likely predispose the affected individuals to neurodegeneration. Spinocerebellar ataxia with axonal neuropathy 1 (SCAN1) is caused by autosomal recessive mutations in the gene encoding tyrosyl-DNA phosphodiesterase 1 (TDP1), a protein required for the repair of DNA SSBs that arise independent of DNA replication from abortive topoisomerase 1 activity or oxidative stress. Ataxia-telangiectasia (AT), ataxiatelangiectasia-like disorder (ATLD), ataxia oculomotor apraxia type 1 (AOA1) and ataxia oculomotor apraxia type 2A (AOA2) are a subgroup of the autosomal recessive spinocerebellar ataxias characterized by cerebellar atrophy and oculomotor apraxia. The progressive neurodegeneration described in AT and ATLD is due to mutations in genes encoding for ATM and Mre11, respectively. ATM recognizes and signals DNA DSBs to the cell cycle checkpoints and the DNA repair machinery. The Mre11 DNA repair complex, composed of Rad50, Mre11 and Nbs1 proteins, is involved in DNA damage recognition, DNA repair, and initiating cell cycle checkpoints. ATM and the Mre11 complex combine to recognize and signal DNA DSBs. AOA1 is caused by mutations in the gene encoding aprataxin (APTX), a nuclear protein that interacts with several DNA repair proteins, including XRCC1, Polβ, DNA ligase III, PARP-1, and p53. It functions in the endprocessing of DNA SSBs removing 3′-phosphate, 5′-phosphate, and 3′-phosphoglycolate ends. AOA2 is caused by autosomal recessive mutations in the gene encoding senataxin (SETX). SETX is a member of the superfamily I DNA/RNA helicases, likely involved in oxidative DNA damage response. *SETX* mutations have been also linked to juvenile ALS. Overall, spinocerebellar ataxias deficient in DNA damage responses represent the most robust set of data linking mutations in DNA repair genes to neurodegeneration (Gueven et al., 2007; Martin, 2008).

#### **6.2 Huntington's disease**

Huntington's disease (HD) is a progressive neurodegenerative disorder resulting in cognitive impairment, choreiform movements and death which usually occurs 15–20 years after the onset of the symptoms. The disease is also characterized by psychiatric and behavioural disturbances. HD is an autosomal dominant disorder caused by a CAG repeat expansion within exon 1 of the gene encoding for huntingtin (*IT15*) on chromosome 4. In the normal population the number of CAG repeats is maintained below 35, while in individuals affected by HD it ranges from 35 to more than 100, resulting in an expanded polyglutamine segment in the protein. Age at onset of the disease is inversely correlated with the CAG repeat length; moreover the length of the expanded polyglutamine segment seems to be related to the rate of clinical progression of neurological symptoms and to the progression of motor impairment, but not to psychiatric symptoms. Somatic CAG repeat expansion in the gene encoding for huntingtin has been observed in several HD tissues, including the striatum which is the region most affected by the disease and the OGG1 protein has been involved in somatic CAG repeat expansion in HD, suggesting that it might contribute to disease age at onset (Kovtun et al. 2007). We recently observed a weak borderline association between the *OGG1* Ser326Cys polymorphism and HD age at onset in a small group of 91 HD subjects (Coppedè et al. 2010d). However, replication of the study in a

Variants and Polymorphisms of DNA Repair Genes and Neurodegenerative Diseases 579

interactions were missing, and only common polymorphisms have been included with little or no attention paid to rare gene variants. Concerning ALS, although results are still inconclusive, some studies performed in northern Europe suggest a possible association between the *APEX1* Asp148Glu polymorphism and disease risk, the *OGG1* Ser326Cys polymorphism was associated with increased ALS risk in Italy, and *XRCC1* variants gave conflicting results in different populations (Table 3). Overall, these studies (Section 4) suggest the need of further investigation aimed at addressing the contribution of haplotypes, gene-gene and gene-environment interactions. There is evidence for a contribution of *POLG1* mutations in PD, and parkin seems to be involved in mtDNA repair, thus strengthening the contribution of mtDNA mutations to disease pathogenesis (Section 5). Increasing evidence suggests that BER proteins might be involved in CAG repeat expansion in somatic cells of HD individuals (Section 6.2), however studies aimed at addressing the possible contribution of variant of BER genes to disease age at onset are still in their beginnings. Recent evidence also suggests a possible contribution of NER genes in MS (Section 6.3), and the impaired ability to repair oxidative DNA damage might cause neurodegeneration observed in progeroid syndromes caused by mutations of NER genes (Section 6.4). In summary, increasing evidence supports a role for DNA repair genes in

neurodegeneration, making this field a promising area for further investigation.

Anvret, A., Westerlund, M., Sydow, O., Willows, T., Lind, C., Galter, D. & Belin, A.C. (2010)

associated with Parkinson's disease in Sweden. *Neurosci Lett*. 485(2): 117-20. Bekris, L.M., Mata, I.F. & Zabetian, C.P. (2010) The genetics of Parkinson disease. *J Geriatr* 

Bogdanov, M., Brown, R.H., Matson, W., Smart, R., Hayden, D., O'Donnell, H., Beal, F.M. &

Briggs, F.B., Goldstein, B.A., McCauley, J.L., Zuvich, R.L., De Jager, P.L., Rioux, J.D., Ivinson,

Brookmeyer, R., Johnson, E., Ziegler-Graham, K. & Arrighi, H.M. (2007) Forecasting the global burden of Alzheimer's disease. *Alzheimers Dement.* 3(3)*:* 186-191. Coppedè, F., Mancuso, M., Lo Gerfo, A., Manca, M.L., Petrozzi, L., Migliore, L., Siciliano, G.,

Coppedè, F. & Migliore, L. (2010) DNA repair in premature aging disorders and

Coppedè, F., Lo Gerfo, A., Carlesi, C., Piazza, S., Mancuso, M., Pasquali, L., Murri, L., Migliore,

Variations of the CAG trinucleotide repeat in DNA polymerase gamma (POLG1) is

Cudkowicz, M. (2000) Increased oxidative damage to DNA in ALS patients. *Free* 

A.J., Compston, A., Hafler, D.A., Hauser, S.L., Oksenberg, J.R., Sawcer, S.J., Pericak-Vance, M.A., Haines, J.L., Barcellos, L.F. & International Multiple Sclerosis Genetics Consortium (2010) Variation within DNA repair pathway genes and risk of

& Murri, L. (2007a) A Ser326Cys polymorphism in the DNA repair gene hOGG1 is not associated with sporadic Alzheimer's disease. *Neurosci Lett.* 414(3): 282-5. Coppedè, F., Mancuso, M., Lo Gerfo, A., Carlesi, C., Piazza, S., Rocchi, A., Petrozzi, L., Nesti,

C., Micheli, D., Bacci, A., Migliore, L., Murri, L. & Siciliano, G. (2007b) Association of the hOGG1 Ser326Cys polymorphism with sporadic amyotrophic lateral

L. & Siciliano, G. (2010a) Lack of association between the APEX1 Asp148Glu polymorphism and sporadic amyotrophic lateral sclerosis. *Neurobiol Aging.* 31(2): 353-5.

**8. References** 

*Psychiatry Neurol.* 23(4): 228-42.

multiple sclerosis. *Am J Epidemiol.* 172(2): 217-24.

sclerosis. *Neurosci Lett*. 420(2): 163-8.

neurodegeneration. *Curr Aging Sci* 3 (1): 3-19.

*Radic Biol Med.* 29(7): 652-8.

cohort of more than 400 HD individuals failed to confirm the association between *OGG1*  Ser326Cys and HD age at onset (Taherzadeh-Fard et al., 2010).

#### **6.3 Multiple sclerosis**

Multiple sclerosis (MS) has been classically regarded as an inflammatory demyelinating disease of the central nervous system. In recent years, it is also becoming increasingly apparent that there is a significant neurodegenerative component in the disease (Moore, 2010). MS is a complex autoimmune disease with a prominent genetic component. The primary genetic risk factor is the human leukocyte antigen *(HLA)-DRB1\*1501* allele; however, much of the remaining genetic contribution to MS remains to be elucidated. Briggs and collaborators screened 1,343 MS cases and 1,379 healthy controls of European ancestry for a total of 485 single nucleotide polymorphisms within 72 genes related to DNA repair pathways. Only a single nucleotide polymorphism (rs1264307) within the general transcription factor IIH polypeptide 4 gene (*GTF2H4*), a nucleotide excision repair gene, was significantly associated with MS risk (OR = 0.7) after correcting for multiple testing. However, using a nonparametric approach comprising the Random Forests and CART algorithms, authors observed evidence for a predictive relation for MS based on 9 variants in nucleotide excision repair (rs4134860, rs2974754, rs7783714, rs4134813, rs2957873 and rs4150454), homologous recombination (rs9562605), and nonhomologous end-joining genes (rs9293329 and rs1231201). Specifically, variants within nucleotide excision repair genes were most prominent among predictors of MS (Briggs et al., 2010). Variants of DNA repair genes, particularly *BRCA2* (rs1801406) and *XRCC5* (rs207906), might also increase the risk for the development of secondary acute promyelocytic leukemia in MS patients (Hasan et al., 2011).

#### **6.4 Diseases caused by mutations of NER genes**

Xeroderma pigmentosum (XP), Cockayne's syndrome (CS) and trichothiodystropy (TTD) represent a clinically heterogeneous group of progeroid syndromes characterized by defects in NER proteins. A subset of these patients exhibits neurological dysfunction and neurodegeneration, and many XP patients have high cancer predisposition, thus linking DNA repair defects to premature aging, cancer and neurodegeneration. Several studies performed in mice, as well as in cell cultures, suggest that neurodegeneration in XP and CS patients might arise as a consequence of impaired repair of oxidative DNA lesions caused by mutations of NER genes. Details are provided in our recent updated review (Coppedè & Migliore, 2010)

#### **7. Conclusions**

The present chapter describes the current knowledge concerning DNA repair genes and neurodegeneration. Studies in ataxias (section 6.1) have undoubtedly linked genes involved in DNA repair to neurodegeneration. These observations, alongside with evidence of increased DNA damage in affected brain regions, have driven researchers to search for variant and polymorphisms of DNA repair genes in major neurodegenerative diseases such as AD, ALS and PD. Studies in sporadic late onset AD patients (Section 3) suggest that common polymorphisms of BER genes, namely *OGG1* Ser326Cys, *APEX1* Asp148Glu, and *XRCC1* (Arg194Trp, Arg280His and Arg399Gln) are unlikely to represent major AD risk factors. However, further studies are required to replicate and clarify the associations observed between *PARP-1* haplotypes and disease risk. Moreover, the power of these studies was limited by the sample size of case-control groups (Table 2), gene-gene interactions were missing, and only common polymorphisms have been included with little or no attention paid to rare gene variants. Concerning ALS, although results are still inconclusive, some studies performed in northern Europe suggest a possible association between the *APEX1* Asp148Glu polymorphism and disease risk, the *OGG1* Ser326Cys polymorphism was associated with increased ALS risk in Italy, and *XRCC1* variants gave conflicting results in different populations (Table 3). Overall, these studies (Section 4) suggest the need of further investigation aimed at addressing the contribution of haplotypes, gene-gene and gene-environment interactions. There is evidence for a contribution of *POLG1* mutations in PD, and parkin seems to be involved in mtDNA repair, thus strengthening the contribution of mtDNA mutations to disease pathogenesis (Section 5). Increasing evidence suggests that BER proteins might be involved in CAG repeat expansion in somatic cells of HD individuals (Section 6.2), however studies aimed at addressing the possible contribution of variant of BER genes to disease age at onset are still in their beginnings. Recent evidence also suggests a possible contribution of NER genes in MS (Section 6.3), and the impaired ability to repair oxidative DNA damage might cause neurodegeneration observed in progeroid syndromes caused by mutations of NER genes (Section 6.4). In summary, increasing evidence supports a role for DNA repair genes in neurodegeneration, making this field a promising area for further investigation.

#### **8. References**

578 DNA Repair

cohort of more than 400 HD individuals failed to confirm the association between *OGG1* 

Multiple sclerosis (MS) has been classically regarded as an inflammatory demyelinating disease of the central nervous system. In recent years, it is also becoming increasingly apparent that there is a significant neurodegenerative component in the disease (Moore, 2010). MS is a complex autoimmune disease with a prominent genetic component. The primary genetic risk factor is the human leukocyte antigen *(HLA)-DRB1\*1501* allele; however, much of the remaining genetic contribution to MS remains to be elucidated. Briggs and collaborators screened 1,343 MS cases and 1,379 healthy controls of European ancestry for a total of 485 single nucleotide polymorphisms within 72 genes related to DNA repair pathways. Only a single nucleotide polymorphism (rs1264307) within the general transcription factor IIH polypeptide 4 gene (*GTF2H4*), a nucleotide excision repair gene, was significantly associated with MS risk (OR = 0.7) after correcting for multiple testing. However, using a nonparametric approach comprising the Random Forests and CART algorithms, authors observed evidence for a predictive relation for MS based on 9 variants in nucleotide excision repair (rs4134860, rs2974754, rs7783714, rs4134813, rs2957873 and rs4150454), homologous recombination (rs9562605), and nonhomologous end-joining genes (rs9293329 and rs1231201). Specifically, variants within nucleotide excision repair genes were most prominent among predictors of MS (Briggs et al., 2010). Variants of DNA repair genes, particularly *BRCA2* (rs1801406) and *XRCC5* (rs207906), might also increase the risk for the development of secondary acute promyelocytic

Xeroderma pigmentosum (XP), Cockayne's syndrome (CS) and trichothiodystropy (TTD) represent a clinically heterogeneous group of progeroid syndromes characterized by defects in NER proteins. A subset of these patients exhibits neurological dysfunction and neurodegeneration, and many XP patients have high cancer predisposition, thus linking DNA repair defects to premature aging, cancer and neurodegeneration. Several studies performed in mice, as well as in cell cultures, suggest that neurodegeneration in XP and CS patients might arise as a consequence of impaired repair of oxidative DNA lesions caused by mutations of NER genes. Details are provided in our recent updated review (Coppedè & Migliore, 2010)

The present chapter describes the current knowledge concerning DNA repair genes and neurodegeneration. Studies in ataxias (section 6.1) have undoubtedly linked genes involved in DNA repair to neurodegeneration. These observations, alongside with evidence of increased DNA damage in affected brain regions, have driven researchers to search for variant and polymorphisms of DNA repair genes in major neurodegenerative diseases such as AD, ALS and PD. Studies in sporadic late onset AD patients (Section 3) suggest that common polymorphisms of BER genes, namely *OGG1* Ser326Cys, *APEX1* Asp148Glu, and *XRCC1* (Arg194Trp, Arg280His and Arg399Gln) are unlikely to represent major AD risk factors. However, further studies are required to replicate and clarify the associations observed between *PARP-1* haplotypes and disease risk. Moreover, the power of these studies was limited by the sample size of case-control groups (Table 2), gene-gene

Ser326Cys and HD age at onset (Taherzadeh-Fard et al., 2010).

**6.3 Multiple sclerosis** 

leukemia in MS patients (Hasan et al., 2011).

**7. Conclusions** 

**6.4 Diseases caused by mutations of NER genes** 


Variants and Polymorphisms of DNA Repair Genes and Neurodegenerative Diseases 581

Hirano, M., Yamamoto, A., Mori, T., Lan, L., Iwamoto, T.A., Aoki, M., Shimada, K., Furiya,

Hudson, G., Tiangyou, W., Stutt, A., Eccles, M., Robinson, L., Burn, D.J. & Chinnery, P.F.

Infante, J., Llorca, J., Mateo, I., Rodríguez-Rodríguez, E., Sánchez-Quintana, C., Sánchez-

Jiang, G., Xu, L., Wang, L., Song, S. & Zhu, C. (2007) Association study of human MTH1

Kao, S.Y. (2009) Regulation of DNA repair by parkin. *Biochem Biophys Res Commun* 382(2): 321-5. Kass, E.M. & Jasin, M. (2010) Collaboration and competition between DNA double-strand

Kikuchi, H., Furuta, A., Nishioka, K., Suzuki, S.O., Nakabeppu, Y. & Iwaki, T. (2002)

Kisby, G.E., Milne, J. & Sweatt, C. (1997) Evidence of reduced DNA repair in amyotrophic

Kovtun, I.V., Liu, Y., Bjoras, M., Klungland, A., Wilson, S.H. & McMurray, C.T. (2007) OGG1

Love, S., Barber, R. & Wilcock, G.K. (1999) Increased poly(ADP-ribosyl)ation of nuclear

Lovell, M.A., Xie, C. & Markesbery, W.R. (2000) Decreased base excision repair and increased helicase activity in Alzheimer's disease brain. *Brain Res* 855(1): 116-23 . Luoma, P.T., Eerola, J., Ahola, S., Hakonen, A.H., Hellström, O., Kivistö, K.T., Tienari, P.J. &

Mao, G., Pan, X., Zhu, B.B., Zhang, Y., Yuan, F., Huang, J., Lovell, M.A., Lee, M.P., Markesbery,

Kunkel, T.A. & Erie, D.A. (2005) DNA mismatch repair. *Annu Rev Biochem* 74: 681-710. Liu, H.P., Lin, W.Y., Wu, B.T., Liu, S.H., Wang, W.F., Tsai, C.H., Lee, C.C. & Tsai, F.J. (2010)

repair is impaired in aprataxin-related ataxia. *Ann Neurol* 61(2): 162-74. Hudson, G., Schaefer, A.M., Taylor, R.W., Tiangyou, W., Gibson, A., Venables, G., Griffiths,

ophthalmoplegia and Parkinsonism. *Arch Neurol.* 64(4): 553-7.

Parkinson's disease. *Mov Disord.* 24(7): 1092-4.

Parkinson's disease. *J Neurol Sci*. 256(1-2): 68-70.

break repair pathways. *FEBS Lett.* 584(17): 3703-8.

lateral sclerosis brain tissue. *Neuroreport.* 8(6): 1337-40.

proteins in Alzheimer's disease. *Brain*. 122(Pt 2): 247-53

idiopathic sporadic Parkinson disease. *Neurology*. 69(11): 1152-9.

in patients with Alzheimer's disease. *Nucleic Acids Res.* 35(8): 2759-66.

*Neuropathol.* 103(4): 408-14.

disease. *J Clin Lab Anal*. 24(3): 182-6.

(7143): 447-52.

Y., Kariya, S., Asai, H., Yasui, A., Nishiwaki, T., Imoto, K., Kobayashi, N., Kiriyama, T., Nagata, T., Konishi, N., Itoyama, Y. & Ueno S. (2007) DNA single-strand break

P., Burn, D.J., Turnbull, D.M. & Chinnery, P.F. (2007) Mutation of the linker region of the polymerase gamma-1 (POLG1) gene associated with progressive external

(2009) No association between common POLG1 variants and sporadic idiopathic

Juan, P., Fernández-Viadero, C., Peña, N., Berciano, J. & Combarros, O. (2007) Interaction between poly(ADP-ribose) polymerase 1 and interleukin 1A genes is associated with Alzheimer's disease risk. *Dement Geriatr Cogn Disord*. 23(4): 215-8. Infante, J., Sánchez-Juan, P., Mateo, I., Rodríguez-Rodríguez, E., Sánchez-Quintana, C.,

Llorca, J., Fontalba, A., Terrazas, J., Oterino, A., Berciano, J. & Combarros O. (2007) Poly (ADP-ribose) polymerase-1 (PARP-1) genetic variants are protective against

Ile45Thr polymorphism with sporadic Parkinson's disease. *Eur Neurol.* 59(1-2): 15-7.

Impairment of mitochondrial DNA repair enzymes against accumulation of 8-oxoguanine in the spinal motor neurons of amyotrophic lateral sclerosis. *Acta* 

initiates age-dependent CAG trinucleotide expansion in somatic cells. *Nature* 447

Evaluation of the poly(ADP-ribose) polymerase-1 gene variants in Alzheimer's

Suomalainen, A. (2007) Mitochondrial DNA polymerase gamma variants in

W.R., Li, G.M. & Gu, L. (2007) Identification and characterization of OGG1 mutations


Coppedè, F., Migheli, F., Lo Gerfo, A., Fabbrizi, M.R., Carlesi, C., Mancuso, M., Corti, S.,

Coppedè, F., Ceravolo, R., Migheli, F., Fanucchi, F., Frosini, D., Siciliano, G., Bonuccelli, U. &

Coppedè, F., Migheli, F., Ceravolo, R., Bregant, E., Rocchi, A., Petrozzi, L., Unti, E., Lonigro,

Coppedè, F. (2011) Variants and polymorphisms of DNA base excision repair genes and

Davidzon, G., Greene, P., Mancuso, M., Klos, K.J., Ahlskog, J.E., Hirano, M. & DiMauro, S. (2006) Early-onset familial parkinsonism due to POLG mutations. *Ann Neurol.* 59(5): 859-62 Dogru-Abbasoglu, S., Inceoglu, M., Parildar-Karpuzoglu, H., Hanagasi, H.A., Karadag, B.,

Dorszewska, J., Kempisty, B., Jaroszewska-Kolecka, J., Rózycka, A., Florczak, J., Lianeri, M.,

Fang, F., Umbach, D.M., Xu, Z., Ye, W., Sandler, D.P., Taylor, J.A. & Kamel, F. (2010) No

Ferrante, R.J., Browne, S.E., Shinobu, L.A., Bowling, A.C., Baik, M.J., MacGarvey, U., Kowall,

Greenway, M.J., Alexander, M.D., Ennis, S., Traynor, B.J., Corr, B., Frost, E., Green, A. &

Gueven, N., Chen, P., Nakamura, J., Becherel, O.J., Kijas, A.W., Grattan-Smith, P. & Lavin,

Hasan, S.K., Buttari, F., Ottone, T., Voso, M.T., Hohaus, S., Marasco, E., Mantovani, V.,

Hayward, C., Colville, S., Swingler, R.J. & Brock, D.J. (1999) Molecular genetic analysis of

sporadic late-onset Alzheimer's disease. *Neurosci Lett*. 404(3):258-61. Doğru-Abbasoğlu, S., Aykaç-Toker, G., Hanagasi, H.A., Gürvit, H., Emre, M. & Uysal, M.

sporadic late-onset Alzheimer's disease. *Neurol Sci*. 28(1):31-4.

sclerosis. *Amyotroph Lateral Scler.* 11(1-2): 122-4.

Huntington's disease. Toxicology 278(2):199-203.

Alzheimer's disease. *J Neurol Sci.* 300(1-2): 200-1.

Parkinson's disease. *Neurosci Lett.* 477(1): 1-5.

Fleck, O & Nielsen, O. (2004) DNA repair. *J Cell Sci* 117(Pt4): 515-7.

of DNA repair genes. *Neurology*. 76(12): 1059-65.

responses. *Neuroscience*. 145(4): 1418-25.

*Neurobiol Aging*. Epub Aug 16.

*Neurology.* 63(10): 1936-8.

sporadic Parkinson's disease. *Neurosci Lett.* 473(3): 248-51.

Mezzina, N., del Bo, R., Comi, G.P., Siciliano, G. & Migliore, L. (2010b) Association study between XRCC1 gene polymorphisms and sporadic amyotrophic lateral

Migliore, L. (2010c) The hOGG1 Ser326Cys polymorphism is not associated with

R., Siciliano, G. & Migliore, L. (2010d) The hOGG1 Ser326Cys polymorphism and

Gurvit, H., Emre, M., Aykac-Toker, G. & Uysal, M. (2006) Polymorphisms in the DNA repair genes XPD (ERCC2) and XPF (ERCC4) are not associated with

(2007) The Arg194Trp polymorphism in DNA repair gene XRCC1 and the risk for

Jagodziński, P.P. & Kozubski, W. (2009) Expression and polymorphisms of gene 8 oxoguanine glycosylase 1 and the level of oxidative DNA damage in peripheral blood lymphocytes of patients with Alzheimer's disease. *DNA Cell Biol*. 28(11): 579-88. Eerola, J., Luoma, P.T., Peuralinna, T., Scholz, S., Paisan-Ruiz, C., Suomalainen, A., Singleton

,A.B. & Tienari, P.J. (2010) POLG1 polyglutamine tract variants associated with

association between DNA repair gene XRCC1 and amyotrophic lateral sclerosis.

N.W., Brown, R.H. Jr. & Beal, M.F. Evidence of increased oxidative damage in both sporadic and familial amyotrophic lateral sclerosis. *J Neurochem*. 69(5): 2064-74.

Hardiman, O. (2004) A novel candidate region for ALS on chromosome 14q11.2.

M.F. (2007) A subgroup of spinocerebellar ataxias defective in DNA damage

Garagnani, P., Sanz, M.A., Cicconi, L., Bernardi ,G., Centonze, D. & Lo-Coco, F. (2011) Risk of acute promyelocytic leukemia in multiple sclerosis: Coding variants

the APEX nuclease gene in amyotrophic lateral sclerosis. *Neurology.* 52(9): 1899-901.


**Part 5** 

**Telomeres and DNA Repair** 


**Part 5** 

**Telomeres and DNA Repair** 

582 DNA Repair

Martin, L.J. (2008) DNA damage and repair: relevance to mechanisms of neurodegeneration.

Moore, G.R. (2010) Current concepts in the neuropathology and pathogenesis of multiple

Orsucci, D., Caldarazzo Ienco, E., Mancuso, M. & Siciliano, G. (2011) POLG1-Related and other "Mitochondrial Parkinsonisms": an Overview. *J Mol Neurosci*. Epub Jan 8. Parildar-Karpuzoğlu, H., Doğru-Abbasoğlu, S., Hanagasi, H.A., Karadağ, B., Gürvit, H.,

Qian, Y., Chen, W., Wu, J., Tao, T., Bi, L., Xu, W., Qi, H., Wang, Y. & Guo L. (2010) Association

disease and age of onset in elderly Han Chinese. *J Neurol Sci.* 295(1-2): 62-5. Remes, A.M., Hinttala, R., Kärppä, M., Soini, H., Takalo, R., Uusimaa, J. & Majamaa, K.

Rothfuss, O., Fischer, H., Hasegawa, T., Maisel, M., Leitner, P., Miesel, F., Sharma, M.,

Satoh, J. & Kuroda, Y. (2000) A valine to methionine polymorphism at codon 83 in the 8-oxo-

Subba Rao, K. (2007) Mechanisms of disease: DNA repair defects and neurological disease.

Taanman, J.W. & Schapira, A.H. (2005) Analysis of the trinucleotide CAG repeat from the

Taherzadeh-Fard, E., Saft, C., Wieczorek, S., Epplen, J.T. & Arning, L. (2010) Age at onset in

Takahashi, T., Tada, M., Igarashi, S., Koyama, A., Date, H., Yokoseki, A., Shiga, A., Yoshida,

Tiangyou, W., Hudson, G., Ghezzi, D., Ferrari, G., Zeviani, M., Burn, D.J. & Chinnery, P.F. (2006) POLG1 in idiopathic Parkinson disease. Neurology. 67(9):1698-700. Tomkins, J., Dempster, S., Banner, S.J., Cookson, M.R. & Shaw, P.J. (2000) Screening of AP

Weissman, L., de Souza-Pinto, N.C., Stevnsner, T. & Bohr, V.A. (2007) DNA repair, mitochondria, and neurodegeneration. *Neuroscience* 145 (12): 1318-29. Weissman, L., Jo, D.G., Sørensen, M.M., de Souza-Pinto, N.C., Markesbery, W.R., Mattson,

and 3'-phosphoglycolate ends. *Nucleic Acids Res* 35(11): 3797-809.

Emre, M. & Uysal, M. (2008) Single nucleotide polymorphisms in base-excision repair genes hOGG1, APE1 and XRCC1 do not alter risk of Alzheimer's disease.

of polymorphism of DNA repair gene XRCC1 with sporadic late-onset Alzheimer's

(2008) Parkinsonism associated with the homozygous W748S mutation in the

Bornemann, A., Berg, D., Gasser, T. & Patenge N. (2009) Parkin protects mitochondrial genome integrity and supports mitochondrial DNA repair. *Hum Mol* 

dGTPase gene MTH1 is not associated with sporadic Parkinson's disease. *Eur J* 

DNA polymerase gamma gene (POLG) in patients with Parkinson's disease.

Huntington's disease: replication study on the associations of ADORA2A, HAP1

Y., Tsuji, S., Nishizawa, M. & Onodera O. (2007) Aprataxin, causative gene product for EAOH/AOA1, repairs DNA single-strand breaks with damaged 3'-phosphate

endonuclease as a candidate gene for amyotrophic lateral sclerosis (ALS).

M.P. & Bohr V.A. (2007b) Defective DNA base excision repair in brain from individuals with Alzheimer's disease and amnestic mild cognitive impairment.

*J Neuropathol Exp Neurol.* 67(5): 377-87.

*Neurosci Lett*. 442(3): 287-91.

*Genet.* 18(20): 3832-50.

*Nat Clin Pract Neurol* 3(3): 162-72.

and OGG1. *Neurogenetics.* 11(4): 435-9.

*Neurosci Lett.* 376(1): 56-9.

*Neuroreport.* 11(8): 1695-7.

*Nucleic Acids Res* 35(16): 5545-55.

*Neurol.* 7(6): 673-7.

sclerosis. *Can J Neurol Sci.* 37 (Suppl 2) :S5-15.

POLG1 gene. *Parkinsonism Relat Disord.* 14(8): 652-4.

**29** 

*Japan* 

**Characterization of 5'-Flanking Regions of** 

**Various Human Telomere Maintenance** 

*1Department of Gene Regulation, Faculty of Pharmaceutical Sciences 2Department of Biochemistry, Faculty of Pharmaceutical Sciences* 

*4Research Center for RNA Science, RIST, Tokyo University of Science* 

Telomeres are the unique nucleoprotein complex structures located at the end of linear eukaryotic chromosomes (Blackburn, 2000; de Lange, 2006). They are composed of TTAGGG repeats that are typically 10 kb at birth and gradually shorten with cell divisions (de Lange, 2006). Telomerase is composed of the protein subunit TERT and the RNA subunit TERC (TR). It elongates the telomere by adding telomeric repeats (Greider & Blackburn, 1987). The 50 to 300 nucleotides from the terminal end of the telomeres are single stranded 3'-protluded Goverhang structures which make the t-loop configuration (de Lange, 2006; Griffith et al., 1999). Mammalian telomeres are included in heterochoromatin and attached to the nuclear matrix (Oberdoerffer & Sinclair, 2007; Gonzalez-Suarez & Gonzalo, 2008). Telomere shortening causes instability of the ends of chromosomes to lead to replicative senescence (O'Sullivan & Karlseder, 2010; Lundblad & Szostak, 1989). Therefore, the ends of telomeres should be protected from damaging or cellular activities. The t-loop structures are regulated by shelterin protein factors, TRF1, TRF2, Rap1, TIN2, TPP1, POT1 (Gilson & Geli, 2007; O'Sullivan & Karlseder, 2010), and Rec Q DNA helicases, WRN and BLM (Chu & Hickson, 2009). TRF1 and TRF2, which bind to duplex telomeric DNA and retain shelterin on the telomere repeats, were shown to interact with various functional proteins (Giannone et al., 2010). Molecular structural analysis of Rap1 revealed that its mechanism of action involves interaction with TRF2 and Taz1 proteins (Chen et al., 2011). A recent study showed that depletion of TPP1 and its partner TIN2 causes a loss of telomerase recruitment to telomeres (Abreu et al., 2010). POT1 is an important regulator of telomerase length, in stimulating the RecQ helicases WRN and BLM (Opresko et al., 2005). Tankyrase-1 (TANK1), which is classified as a poly(ADP-ribose) polymerase family protein, is also known to regulate telomere homeostasis by modifying TRF1 (Smith et al., 1998; Schreiber et al., 2006). Dyskerin, which is encoded by the *DKC1* gene, is a key auxiliary protein that is contained in a Cajal body with TERT (Cohen et al., 2007). Defects in the shelterin components and telomerase are thought to down-regulate telomere structure

**1. Introduction**

**Factor-Encoding Genes** 

Fumiaki Uchiumi1,4, Takahiro Oyama1, Kensuke Ozaki1 and Sei-ichi Tanuma2,3,4

*3Genome and Drug Research Center* 
