What Causes Down Syndrome?

*Emine Ikbal Atli*

#### **Abstract**

Trisomy 21 (Down Syndrome) is the model human phenotype for all genome gain-dosage imbalance situations, including microduplications. Years after the sequencing of chromosome 21, the discovery of functional genomics and the creation of multiple cellular and mouse models provided an unprecedented opportunity to demonstrate the molecular consequences of genome dosage imbalance. It was stated years ago that Down syndrome, caused by meiotic separation of chromosome 21 in humans, is associated with advanced maternal age, but defining and understanding other risk factors is insufficient. Commonly referred to as Down syndrome (DS) in humans, trisomy 21 is the most cited genetic cause of mental retardation. In about 95% of cases, the extra chromosome occurs as a result of meiotic non- nondisjunction (NDJ) or abnormal separation of chromosomes. In most of these cases the error occurs during maternal oogenesis, especially in meiosis I.

**Keywords:** trisomy 21, chromosome 21, non- nondisjunction, down syndrome, genetics

#### **1. Introduction**

More than 50 years have passed since trisomy 21 was identified as the cause of Down syndrome. After that date, the first link between a clinical disorder and a chromosomal abnormality was established. In the intervening half century, the importance of numerical chromosome abnormalities for human disease pathology has been well established.

Studies with live births in the 1960s and 1970s showed that about 0,3% of newborns were trisomic or monosomic, while subsequent studies of spontaneous abortions found a much higher incidence of about 35%. Taken together, these studies revealed aneuploidy as the leading known cause of congenital birth defects and miscarriages, showing that most cases of aneuploidy disappear in utero [1–3].

In humans, trisomy 21, commonly referred to as Down syndrome (DS), is the most common genetic cause of mental retardation. In about 95% of cases, the excess chromosome occurs as a result of meiotic nondisjunction (NDJ) or incorrect dissociation of chromosomes [4, 5]. In most of the cases, the error occurs during maternal oogenesis, especially in meiosis I (MI) [6]. Advanced maternal age and defective recombination are two risk factors that have been reported to be associated with DS for cases where extra chromosome arises in the oocyte. The process of oogenesis is long and is a cycle that involves meiotic arrest, making it more vulnerable to improper assembly of chromosomes than spermatogenesis. Also, with increasing age, there is a rapid degradation of spindle thread formation in sister chromatid cohesion or anaphase separation of sister chromatids in oocytes, and this poses the risk of NDJ in both MI and MII [7–11].

Through recombinant DNA technology, a new technique has become available to study the origin and mechanisms of chromosomal abnormalities using DNA polymorphism analysis. Initially, such analyzes used chromosome 21-specific DNA probes to detect restriction fragment length polymorphisms. The development of the polymerase chain reaction (PCR) amplification technique has enabled the identification of new and highly informative classes of DNA polymorphisms (microsatellites or simple sequence repeat (SSR) polymorphisms) in the human genome. In particular, multi-allelic and easily typeable micro satellites have contributed to chromosomal nondisjunction studies in recent years [12–14].

Meiotic meiosis I or II examination of nondisjunction in trisomy 21 by DNA polymorphism analysis could not be performed due to the absence of centromeric markers. Alphaid DNA polymorphisms specific to the human chromosome 21 centromere were identified years ago, but these markers were unlikely to provide information on the process and were not useful for routine nondisjunction studies. However, alfoid DNA polymorphisms were localized in the genetic linkage map of chromosome 21(**Figure 1**), and an estimate of the genetic distance between the centromere and the closest pericentromeric markers on the long arm of chromosome 21 was made [4, 9, 16].

Two large collaborative studies used DNA polymorphism involving the long arm of human chromosome 21 to determine the parental origin of separation in trisomy 21. Such studies estimate that only 5% of trisomy 21 (of a total of 304 families studied) originates from the father and attributes the difference in cytogenetic studies to the increased accuracy of DNA polymorphism analysis as shown by inaccurate

#### **Figure 1.**

*Short tandem repeat (STR) markers used to infer the origin of the meiotic error and characterization of the recombination profile [15].*

#### *What Causes Down Syndrome? DOI: http://dx.doi.org/10.5772/intechopen.96685*

cytogenetic determinations in a subgroup of families. Other population-based studies show paternal meiotic errors in the 5–9% range [17, 18].

For example, the absence of detectable recombination or just a single telomeric change may be associated with MI NDJ errors, and this pattern is more common in the younger maternal age group than in the older maternal group. In contrast, it shows that MII errors are clearly associated with pericentromeric changes in older maternal age groups [19].

A molecular study found high differences in mean maternal ages between maternal origin cases and paternal origin cases. This demonstrated that the maternal age effect in Down syndrome is limited to maternal nondisjunction and does not provide evidence for a comfortable selection against trisomic fetuses in older women [20, 21].

#### **2. Sex-specific differences in meiosis**

As discussed in many studies, studies of clinically recognized pregnancies indicate that most human aneuploidy is of maternal origin. The question then arises: why is female meiosis so prone to error? In this section, we review oocyte development and summarize the latest evidence that errors in the oocyte that predispose to chromosome misgrouping are increased, and that gender-specific differences in meiotic cell cycle checkpoints allow oocytes with these errors to develop into mature eggs [3, 22].

In mammals, meiotic recombination occurs in the fetal ovary and the significance of the resulting physical connections for chromosome separation has been well observed. Studies in the 1990s identified transitions that could not be recombined and / or optimally positioned as significant contributors to human trisomy.

Changing recombination is essential here. It is related to mother-derived trisomies as well as those originating from the father. However, the female is clearly at greater risk, as most aneuploidy occurs during oogenesis. Therefore, either more recombination errors are made in the female or these errors are removed more efficiently in the male [23, 24].

The immunofluorescence methodology has made it possible to examine crosslinked proteins in pachytene spermatocytes and oocytes and thus test these alternatives. Interestingly, almost all chromosomes in males are joined by at least one crossover, but the same is not true for females [4, 14, 18].

Studies have shown that; The conclusion is that more than 10% of all human oocytes contain at least one "non-crossing" bivalent. Since half of all these divalent ones are expected to result in aneuploidy (**Figure 2**), the stage seems to have been adjusted for meiotic errors from the onset of oogenesis.

As suggested based on cytogenetic studies with no evidence of a difference in mean maternal age between maternal and paternal trisomy 21 cases. A factor associated with aging of the oocyte therefore appears to be responsible for the maternal age effect in Down syndrome.

Among maternal errors, approximately 75% are considered errors in meiosis I and 25% as errors in meiosis II. Maternal meiosis I and II errors are linked to increased maternal age [25–27]. Two studies of cytogenetic short-arm heteromorphisms and microsatellite DNA polymorphisms showed inconsistencies regarding the meiotic period of non-separation and suggested pericentromeric increased recombination associated with nondisjunction. The place where chiasma occurs is the middle of the chromosome arm and then recombination is necessary for proper

**Figure 2.**

*Homogeneous due to meiotic non-disjunction.*

chromosome separation as it holds the chromatids tightly and balances the attraction to opposite poles [28, 29].

Meiotic recombination was thought to stabilize matched homologs to ensure their proper separation. However, the process is stochastic and may not be handled properly even in euploid samples. Thus, achiasmate chromosomes are vulnerable to malsegregation and this condition gradually increases with age due to the rapid degradation of the protein mechanism within oocytes responsible for surveillance and separation of chromosomes. A chiasma located near the telomere of the chromosome probably attaches the homolog to the spindle weaklier due to loss of cohesion and directs the kinetochore precisely towards the opposite pole. On the other hand, chiasmata close to the center occurs during MI, which causes chromosome entanglement so the bivalent cannot be separated correctly. In this way the MII can pass into the anaphase plate and then result in the reduction section; as a result, a disomic gamete is produced [30, 31].

Epidemiological studies have identified some environmental, habitual and socio-economic factors that may pose a risk for Ch21 NDJ. These can be observed in both MI and MII errors depending on maternal age or independent of maternal age. When we consider these findings, it is clear that Ch21 NDJ risk is a multifactorial event that interacts with genetic and environmental factors.

About 5% of trisomy 21 cases are likely due to the mitotic (postzygotic) nondisjunction of chromosome 21 in the early embryo. This was demonstrated by the identification of pericentromeric DNA markers and the lack of recombination observed along the entire long arm of chromosome 21. Mitotic errors are not associated with advanced maternal age and do not show any preference depending on the parental origin of the replica chromosome 21. Mosaic with a normal cell line occurs in about 2–4% of newborns with Down syndrome. By DNA polymorphism analysis performed in 17 families with mosaic trisomy 21 probands, it showed that most cases were caused by a trisomic zygote with mitotic loss of one chromosome (**Table 1**) [32, 33].

*What Causes Down Syndrome? DOI: http://dx.doi.org/10.5772/intechopen.96685*


MI: Meosis I, MII: Meosis II, Maternal and Paternal refer to parental origin of the chromosome that was duplicated by postzygotic nondisjunction

**Table 1.**

*Origin of nondisjunction in human trisomy 21 by DNA polymorphism analysis [31, 33–35].*

#### **3. Changes in recombination**

Failure to nondisjunction in maternal meiosis I is associated with reduced recombination between unallocated chromosomes 21, suggesting an important role for pairing / recombination errors or reduced recombination in the etiology of trisomy 21. Subsequent results showed an overall reduction in recombination, but with increased recombination in the distant region of 21q.


#### **Table 2.**

*Frequency distribution of observed recombinants and inferred exchanges for each meiotic outcome group stratified by maternal age group [10].*

Unpredictably, nondisjunction in meiosis II is due to the increased recombination occurring in meiosis I suggesting that all errors are due to meiosis I. The recombination rate remains constant with advancing maternal age. However, possible chiasmate configurations of chromosome 21 appear more susceptible to nondisjunction in older oocytes than younger oocytes (**Table 2**).

Analysis of the chiasma configuration showed that the failure of a proximal recombination (or the presence of a telomeric recombination) tends to be nondisjunction in meiosis I, while the presence of pericentromeric change appears to be nondisjunction in meiosis II [30, 31, 36, 37].

These findings are very effective in understanding the etiology of trisomy 21 and may explain why both maternal meiosis I and II errors are associated with increased maternal age. A two-hit nondisjunction model has been proposed where the first hit is the prenatal establishment of a sensitive tetrad and the second hit is the disruption of a meiotic process that increases the risk of nondisjunction of the susceptible configuration. The second hit can involve any element of the meiotic process and can be the basis for the maternal age effect. Recent studies have found signs indicating a reduction in the recombination rate in the total genome of eggs with chromosome 21 nondisjoined, meaning that the reduction in recombination is not limited to nondisjoined chromosomes but extends to other chromosomes as well [22].

The two-beat non-separation model needs to be validated with further study from other chromosomes and direct observation with oocytes.

#### **4. Paternal nondisjunction**

In the paternal nondisjunction of chromosome 21, there is mainly meiosis II error, as DNA polymorphisms show, in contrast to meiosis I errors and maternal nondisjunction.

Therefore, the mechanisms associated with paternal nondisjunction will likely differ from those associated with maternal nondisjunction.

In live births with Down syndrome; there is a well-known increasing ratio (about 1.15) between the sexes. This effect is limited to free trisomy 21 cases and does not include translocation-style trisomies, suggesting that increased sex ratio is associated with free trisomy 21 per se, not gender-based differential selection. As a result of molecular studies, it has been revealed that among the meiotic errors of the father, a rather high sex ratio (3.50) and male proband excess, in contrast to paternal mitotic errors and maternal errors, are specific to MII errors.

As with maternal meiosis, there is reduced recombination across the nondisjoined 21. chromosome involved in the 22 paternal nondisjunction cases, but there is no difference in recombination between the 27 paternal MII cases compared to controls [14, 18, 34].

#### **5. Recurrence risk of nondisjunction**

Two molecular studies with families with free trisomy 21 relapse showed that mosaicism in parents is an important etiological factor and that this possibility alone may explain recurrent trisomy 21 in most families. In only a small number of families, the possibility of genetic predisposition for chromosomal nondisjunction could not be excluded [32, 35].

It has been previously shown that live born children with free trisomy 21 for chromosomally normal parents whose maternal age is less than 30 years have a significantly increased risk of recurrence [35].

#### **6. Risk factors**

Many factors have been suggested as risk factors for nondisjunction in the past, but only in the last few studies identified the source of nondisjunction by DNA analysis. The increased frequency of the apolipoprotein E (APOE) allele Â4 was more observed in young mothers with MII errors in a population-scale study of Down syndrome in Denmark. This finding showed an increased risk of Alzheimer's disease in a subgroup of young Down syndrome mothers and suggested the APOE E4 allele as a risk factor for nondisjunction in young mothers [36–38].

An association between an intron polymorphism in the presenilin-1 gene and maternal MII errors was identified in the same population-based study and the function of presenilin proteins in chromosome segregation was determined and thought to be related to subcellular localization.

Another population-based study revealed an association with young MII mothers and maternal smoking and oral contraceptive use.

Both studies have found an association in young MII mothers, and the proposed risk factors support the ovarian risky microcirculation hypothesis to explain the effect of maternal age on nondisjunction, and it should not be overlooked.

Oocytes from hypoxic follicles under heavy exposure showed abnormalities in the organization of chromosomes on the metaphase spindle at high frequencies.

When we look at the two hit nondisjunction model, the findings suggest that aging alone is sufficient to disrupt the meiosis process, but there is a higher requirement for a genetic or environmental factor for nondisjunction to occur in young women.

A different recent study showed abnormal folate metabolism in mothers with Down syndrome; It was reported that the C677T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene was higher in mothers with Down syndrome than in control mothers. However, the study included a small number of mothers and was not population-based, and so the source of nondisjunction could not be determined. Nevertheless, the study may support the at-risk microcirculation hypothesis as hyperhomocysteinemia is a known risk factor for vascular disease and the common MTHFR C677T mutation in the homozygous state is associated with mild hyperhomocysteinemia [39–42].

#### **7. Conclusions**

As a result, it shows that there is a high frequency of chromosome abnormalities throughout embryonic development as a result of accumulated errors during gametogenesis and early mitotic divisions. Advancing female age is associated with increased rates of aneuploidy in oocytes and embryos. Especially during female meiosis, excessive chromosome losses, anaphase delay of chromosomes and / or capturing of the spindle by microtubules (congression failure) are important mechanisms that cause aneuploidy during oogenesis and continue to have a significant effect during the first few mitotic divisions. Studies of abortions and molecular genetic analyzes of chromosomal abnormalities revealed that most aneuploidies occur during female meiosis, usually as a result of splitting in the first meiotic division. Aneuploidies and, to a lesser extent, male-meiotic errors due to both premature separation of sister chromatids during female meiotic divisions and mitotic chromosome malsegregation are quite common. The fact that aneuploidies caused by these disturbances are rarely seen later in pregnancy increases the likelihood that the origin of aneuploidy may somehow affect the impact on embryo viability.

While interest in the development and refinement of culture systems to support the development of functional gametes from stem cells for the treatment of infertility has been intense, so far those working in these areas have shown little interest in the meiotic process. Obviously, the successful production of normal gametes in vitro will require great attention to meiotic details and a full understanding of the differences between the sexes.

### **Acknowledgements**

Thanks to everyone to IntechOpen publishing team and Author Service Manager.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Acronyms and abbreviations**


### **Author details**

Emine Ikbal Atli Faculty of Medicine, Department of Medical Genetics, Edirne, Trakya University, Edirne, Turkey

\*Address all correspondence to: eikbalatli@trakya.edu.tr

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 6**

## Phenotypes Associated with Down Syndrome and Causative Genes

*Fatma Söylemez*

#### **Abstract**

Down syndrome (DS) is the most common chromosomal condition associated with mental retardation and is characterized by a variety of additional clinical findings. It occurs in approximately 1 of 800 births worldwide. DS is associated with number of phenotypes including heart defects, leukemia, Alzheimer's disease, hypertension etc. Individuals with DS are affected by these diseases to variable rates, so understanding the reason for this variation is an important challenge. Multiple genes located both on chromosome 21 and other regions of the genome such as the polymorphism of the amyloid precursor protein (APP) gene contribute to clinical variations. Information on these genetic variations allows early diagnosis and treatment of phenotypes associated with DS. In this chapter, an overview of disease management will be provided by reviewing the genes or miRNAs that cause DS-associated phenotypes.

**Keywords:** Down syndrome, disease, phenotypes, genes, variation

#### **1. Introduction**

Down syndrome is one of the best-recognized and most common chromosome disorders caused by the presence of a third copy of chromosome 21 (Trisomy 21). It is the most common genetic cause of mental retardation. The incidence of Down syndrome is approximately 1/800 newborns [1, 2]. The risk for having a child with Down syndrome increases with maternal age. There are several features that occur in the entire DS population, including learning disability, craniofacial abnormality, and hypotonia [3]. In addition to learning difficulties, Down syndrome patients face a variety of health problems, including congenital heart disease, Alzheimer's diseases (AD), leukemia, cancers and gastrointestinal defects. The 200 to 300 genes on chromosome 21 have been identified as causatives to clinical features of the syndrome. Multiple genes such as polymorphisms of the Down syndrome cell adhesion molecule (DSCAM) and APP gene, both on chromosome 21 and other regions of the genome, are known to contribute to variation in clinical manifestations [4].

#### **2. Down syndrome genetics and typical features**

The most common reason for having a baby with DS is the presence of an extra copy of chromosome 21 that results in trisomy. Trisomy 21 (47,XX,+ 21 or 47,XY,+ 21) is caused by a failure of the chromosome 21 to separate during egg or sperm development (**Figure 1**). The other causes can be Robertsonian translocation and isochromosomal or ring chromosome [5]. Robertsonian translocation occurs in only 2–4%

#### **Figure 1.**

*47,XX,+21. Down syndrome karyotype demonstrating trisomy 21 (female) (Karyotype prepared by Fatma Soylemez).*

of cases and occurs when the long arm of the 21st chromosome is attached to another submetacentric chromosome. Mosaicism occurs as a result of an error in cell division or a false division after fertilization. This is why people with mosaic DS have two cell lines in their tissues, one containing a normal number of chromosomes and the other an extra chromosome 21 [5]. Mosaicism of trisomy 21 and partial trisomy 21 are other genetic diagnoses and are usually associated with fewer clinical features of DS. Trisomy 21 and partial trisomy 21 mosaicism are generally associated with less clinical features of DS [4].

DS has high genetic complexity and phenotype variability [6, 7]. DS individual has some physical characteristics like a small chin, slanted eye, poor muscle tone, a flat nasal bridge, a single crease of the palm, big toe, short fingers and large tongue [8]. DS patients may have an increased dosage or copy number that can lead to an increase in gene expression in Hsa 21 [8]. Specific genes such as Hsa21 or subsets of genes are able to control specific DS phenotypes [9]. In addition, phenotypic analyzes were performed on individuals with partial trisomy for Hsa21. It has been determined that a 3.8–6.5 Mb region called "Down syndrome critical regions" (DSCR) is responsible for most of the Down syndrome phenotypes at 21q21.22 [9]. With the sequencing of Hsa 21, more information was learned about DS-associated genotype–phenotype correlations and characterization of DSCR regions [3]. It has been suggested that the dual- specificity tyrosine phosphorylation-regulated kinase (DYRK1A), the regulator of calcineurin 1 (RCAN1) and Down syndrome cell adhesion molecule (DSCAM), play a critical role in brain development and the occurrence of heart defects in DS patients [10]. In particular, DSCAM plays a very important role in neuron differentiation, axon guidance and neural networks formation. Disruption of these processes contributes to the DS neurocognitive anomalies. All studies have shown that there is not a single critical gene region sufficient to cause DS phenotypes, and there must be a large number of critical regions or critical genes contributing to a DS-associated phenotype or phenotypes.

#### **3. Various pheotypes associated to Down syndrome**

The various clinical phenotypes associated with DS are Alzheimer's disease, heart defects, leukemia, hypertension and gastrointestinal problems (**Figure 2**). *Phenotypes Associated with Down Syndrome and Causative Genes DOI: http://dx.doi.org/10.5772/intechopen.96290*

#### **Figure 2.**

*Various phenotypes associated with Downs's syndrome with its responsible genes (GI: Gastrointestinal).*

The pathogenesis mechanism of these phenotypes associated with DS should be studied together with their causative agents to better understand the disease.

#### **3.1 Alzheimer disease**

It has been determined that the risk of early onset Alzheimer Disease (AD) is high in DS patients. After the age of 50, the risk of developing dementia increases up to 70% in patients with DS [11]. In the past decade, substantial progress has been made in the search for genetic risk factors for dementia in people with DS, and in understanding the neuropathological similarities and differences between AD with DS and without DS. For people with DS over the age of 40, dementia development has a similar progression to that of AD [12–14]. However, if dementia occurs in younger individuals (30–40 years of age), it manifests itself as personality and behavior changes such as increasing impulsivity and onset of apathy [10]. The most conspicuous parallel between AD and AD in DS are characteristic neuropathologies such as amyloid-β accumulation [15]. Results from post-mortem neurochemistry studies have showed a significant loss of choline acetyltransferase and noradrenaline in people with DS, which is similar to the changes seen in Alzheimer's disease [16]. Results obtained from studies, the cholinergic dysregulation in DS is controlled by the DYRK1A gene [17]. DYRK1A is a serine–threonine protein kinase. DYRK1A is involved in tau phosphorylation, and it's up-regulation may contribute to early onset formation of neurofibrillary tangles. In addition, the results obtained from microarray studies, pointed out that there is an up-regulation of the α2 subunit and down-regulation of the α3 and α5 subunits of GABAA receptor [18].

There are several genes known to cause early onset AD. The most important of these genes are APP (amyloid precursor protein), BACE2 (beta secretase 2), PICALM (Phosphatidylinositol binding clathrin assembly protein) and APOE (Apolipoprotein E) [19, 20]. APP is an integral membrane protein concentrated in the synapse of neurons. It is thought that the trisomy of this protein may contribute significantly to the increased frequency of dementia in individuals with DS. It has been shown that trisomic of APP along with Hsa 21 in non-DS individuals is associated with early onset AD. In a preliminary study, a tetranucleotide repeat, ATTT, in

intron 7 of the amyloid precursor protein, was associated with the onset of AD in DS [20]. It is also known that BACE2, encoding the enzyme beta secretase 2, plays a role in AD. Like APP, the BACE 2 gene is located on chromosome 21. The results of the studies are that the haplotypes in BACE2 are associated with AD [21]. A genome wide study, an important relationship was found between variants in BACE2 and age of onset of dementia in DS, with the rs2252576-T allele being associated with an earlier onset by 2–4 years [22]. However, there are other studies that reported no significant relationship between BACE2 and the age of onset of dementia [23]. There is still some uncertainty about the relationship between BACE2 variants and the development of dementia in DS.

In addition to the APP and BACE2 genes, other genes such as PICALM and APOE were found to be associated with early onset AD in DS [24]. PICALM, the other candidate risk gene for AD and DS were examined. PICALM is present in enlarged endosomes in early developing AD [25]. In a DS genome wide study, a relationship has been verified between the variation in the PICALM region of chromosome 11 and the age of onset of AD [26]. Three SNPS in this study, rs2888903, rs7941541 and rs10751134 has been associated with an earlier age of onset. The ε4 allele of the APOE gene, located on chromosome 19, is the most important genetic risk factor for late-onset Alzheimer's disease [27]. The APOE ε4 allele, known to be associated with increased amyloid burden and cholinergic dysfunction, is probably the most studied genetic risk factor. In individuals with DS, the presence of the APOE ε4 allele has been shown to increase the risk of Alzheimer's disease [28, 29]. Also, Aβ accumulation DS individuals carrying the APOE ε4 allele are increased [30].

#### **3.2 Heart defects**

The frequency of heart defects in newborns with DS is up to 50% [31]. The defect called atrioventricular cushion defect is the most common heart defect affecting 40% of DS patients. Ventricular septal defect (VSD) also affects 35% of patients [31]. In atrioventricular septal defect (AVSD), there is a common atrioventricular junction in contrast to normal heart. Other defects include muscular and membranous atrioventricular septum defects and an oval shape of the common atrioventricular junction. Pulmonary arterial hypertension occurs in 1.2 to 5.2% of people with DS [32]. Early repair of heart defects minimizes the risks of heart failure and irreversible pulmonary vascular disease [33]. Observation of specific anatomical patterns of heart defects that can be seen in DS showed that a locus on chromosome 21 plays a role in the development of cardiac malformations [34, 35]. Although up-regulation of genes mapped on chromosome 21 is thought to be related to heart defects, the molecular basis that regulating existence and anatomy of heart defects are still unclear [34]. It has been suggested that type VI collagen (COL6A1, COL6A2) is involved in the pathogenesis of AVSD in Down syndrome, in a similar way to other genes mapping on chromosome [36].

Apart from chromosome 21, other genes localized on different chromosomes have also been studied as the cause of heart defects in DS. Among these genes, the CRELD1 gene has been evaluated as increasing susceptibility to AVSD [31]. Mutations in the CRELD1 (Cysteine-rich EGF-like domain1) gene has been found to contribute to the development of AVSD in DS [37]. CRELD1 gene is located on chromosome 3p25 and contains 11 exons spanning approximately 12 kb [38]. This gene encodes a cell surface protein that functions as a cell adhesion molecule and is expressed during cardiac cushion development. There are studies suggesting that the CRELD1 gene probably plays a major role in the causation of the AVSD phenotype in DS individuals [39, 40]. Two heterozygous missense mutations (p.R329C

#### *Phenotypes Associated with Down Syndrome and Causative Genes DOI: http://dx.doi.org/10.5772/intechopen.96290*

and p.E414K) were identified with two subjects in DS and AVSD [31]. They also included 39 DS with complete AVSD and found the same mutations. No such mutation was detected in DS individuals without heart defects [37]. The R329C mutation reported in a person with sporadic partial AVSD and has also been detected in an individual with DS with AVSD. Although the mutation is the same in DS patients AVSD heart defect has created a more serious condition. Therefore, it has been suggested that the CRELD 1 mutation contributes to the pathogenesis of AVSD heart defects occurring in DS individuals.

#### **3.3 Hypertension**

Individuals with DS may have an increased risk of developing pulmonary hypertension (PH), in part due to congenital heart defects. Other factors such as upper airway obstruction, lung hypoplasia with DS, gastroesophageal reflux, abnormal pulmonary vascular function may play a role in increasing the risk of PH in DS. Findings from a study with DS in Mexico City (high altitude) showed that % 40 had congenital heart disease and 80% had PH [41, 42]. On the other hand, a reduced incidence of hypertension has been reported in individuals with DS [43, 44].

Some of the Hsa21-encoded miRs have been shown to be overexpressed in cells and tissues of DS patients. The direct cause of the overexpression of miRs in DS appears to be the extra copy of HSA21, whose miRs are at their normal chromosomal location [45]. It has been reported that trisomy of Hsa21 microRNA hsa-miR-155 causes this low incidence [45]. An allele of the type-1 angiotensin II receptor (AGTR1) gene is the specific target of HsamiR-155. In this study of twins (one twin was unaffected, and the other had a trisomy 21) to evaluate the expression of MiR-155 in trisomy 21, both twins are homozygous for the 1166A AGTR1 allele and therefore AGTR1 Reported to be the target of miR-155 [46]. This receptor has a vasopressor effect and regulates aldosterone secretion. It is an important factor controlling blood pressure and volume in the cardiovascular system. In this way, it is suggested that it contributes to the decrease of the risk of hypertension by reducing the expression of AGTR1. More studies are needed to validate these thoughts and to determine whether other genes could also protect DS people against hypertension.

#### **3.4 Leukemia**

Hematological abnormalities are common in patients with DS. Patients with DS have a wide risk of malignancy including leukemia. The first leukemia report in a DS patient was in 1930 [47]. It has been reported that leukemia may develop in DS individuals with subsequent systemic studies. Studies have shown that DS patients have an approximately 10–20 times higher risk of leukemia, with a 2% risk by age 5 and 2.7% at age 30 [48]. DS individuals account for about 2% of all childhood acute lymphoblastic leukemia (ALL) and about 10% of acute myeloid leukemia (AML).

Somatic mutations such as GATA 1 gene play a role in the development of acute megakaryoblastic leukemia (AMKL) in DS patients [49]. GATA 1 is a transcription factor localized on the X chromosome, which plays a role in erythroid and megakaryocytic differentiation. Mutations in GATA 1 cause a shorter GATA 1 protein to be expressed and consequently uncontrolled proliferation of immature megakaryocytes [49, 50]. Transient abnormal myelopoiesis, a form of myeloid preleukemia that occurs in about 10% of newborns with DS, is also caused by mutations in GATA1 [4]. A mutation in GATA1 in individuals with DS has been reported to cause transient myeloproliferative disorder (TMD) [51]. They thought it was likely that trisomy 21 and GATA1 causing hyperplasia of the fetal liver in some DS individuals to induce perinatal TMD.

Another mutation that has been suggested to play a role in ALL cases occurring in DS is in the Janus Kinase 2 (JAK 2) gene and is present in approximately 30% of ALL cases in DS [52]. Mutations in the JAK–STAT pathway are at high risk for the development of ALL in individuals with DS [53]. JAK2 is a non-receptor tyrosine kinase and a member of the Janus kinase family. It has been implicated in signaling by members of some receptor families (e.g. interferon receptors and interleukin receptors) [54]. Mutations in JAK2 have been associated with polycythemia vera, essential thrombocythemia, myelofibrosis, and other myeloproliferative disorders. Also, it has been reported that the JAK1, JAK2 and JAK3 genes are mutated in AMKL patients with DS [55–57].

#### **3.5 Gastrointestinal defects**

Individuals with DS consist about 12% of Hirschprung disease (HD) cases. HD is an intestinal obstruction caused by the absence of normal myenteric ganglion cells in part of the colon [58]. In this gastrointestinal (GI) defect, peristaltic waves do not pass through the aganglionic segment and cause obstruction as there is no normal defecation. Other GI defects that can be seen in individuals with DS are duodenal stenosis (DST) and imperforate anus (IA). They are seen 260 and 33 times more respectively in DS [59]. In newborns with duodenal blockage or DST, bilious vomiting occurs in the early neonatal period. If left untreated, there is a risk of death due to severe dehydration and electrolyte imbalance. IA is a birth defect that causes rectal malformation and is associated with the increase of some other specific anomalies such as tracheoesophageal fistula and esophageal atresia.

It has been suggested that changes in genes unrelated to Hsa21 play a role in these diseases. DSCAM has long been viewed as a candidate gene explaining the increased prevalence of this GI defect in HD patients with DS. DSCAM is Down syndrome cell adhesion molecule and plays a crucial role in the development of DS. It is a trans-membrane protein and a member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. It is expressed in the developing nervous system with the highest level of expression occurring in the fetal brain. When over-expressed in the developing fetal central nervous system, it leads to Down syndrome. DSCAM gene is expressed in neural crest that gives rise to enteric nervous system. The overlapping critical region is defined for both DST and IA [58]. Alterations in the DSCAM gene have been shown to play a role in HD development. In connection with HD, two SNPs, rs2837770 and rs8134673, spanning a 19 kb exon-free region of the DSCAM gene was identified [60].

#### **4. Conclusions**

DS, the most common chromosomal abnormality among newborns, is associated with a number of congenital malformations, primarily mental retardation caused by the trisomy of chromosome 21. In addition to its own characteristics, DS can be accompanied by different phenotypes. Different theories such as "gene dosage" have been considered to understand the interactions between phenotype and genotype. The DS phenotype is mainly due to the dosage imbalance of genes located on human chromosome 21 (Hsa 21). The most common cause of DS is presence extra copy chromosome 21. A critical region in 21q22 is thought to be responsible for various DS phenotypes such as craniofacial abnormalities, congenital heart defects, clinodactyly and mental retardation. The health problems and life period of DS people are quite complex and are associated with many different medical, psychological and social problems from infancy to adulthood. In this chapter, it is

*Phenotypes Associated with Down Syndrome and Causative Genes DOI: http://dx.doi.org/10.5772/intechopen.96290*

to reveal the common genes involved in DS related phenotypes such as APP, BACE2, PICALM, APOE, GATA 1, JAK 2.

The association of DS with various clinical phenotypes requires continuous following of these patients with a multidisciplinary approach. For example, there are numerous epidemiological and molecular studies linking the pathological changes observed in the brains of individuals with Down syndrome and the neurodegeneration seen in Alzheimer's disease. Knowing the genes and pathology associated with such changes is very important for a good clinical follow-up of DS patients. Due to the insufficient knowledge of the molecular pathogenesis of DS, an effective therapeutic intervention is unlikely to be found yet. The situation is further complicated by the complex phenotypes accompanying DS. It may be a good option to use pharmacological approaches to key target molecules that are crucial for dysregulated metabolic pathways or phenotypic characteristics. In conclusion, elucidating the phenotypic consequences of gene dose imbalance in DS and knowing the genes that cause accompanying phenotypes may provide new opportunities for therapeutic interventions.

#### **Author details**

Fatma Söylemez Department of Food Processing, Vocational School of Technical Sciences, Mersin University, Mersin, Turkey

\*Address all correspondence to: soylemez\_fatma@yahoo.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## Section 4
