**6.2. IDH1/2 mutations**

IDHs catalyze the oxidative decarboxylation of isocitrate to α-ketoglutarate with production of NADH/NADPH and they are involved in the Krebs cycle.

Recurrent somatic point mutations affect the arginine (Arg) residue at codon 132 in the IDH1 gene on chromosome 2q33.3. Less frequently, they occur at the homolog Arg (R) residue at codon 172 in the IDH2 gene on chromosome 15q26.1. The IDH1/2 mutation rate is in the range of 70– 80% in OII and OIII [50–53] and less in OAII and OAIII, with a higher frequency in 1p/ 19q co-deleted tumors [37]. IDH1 mutations prevail in astrocytic tumors whereas IDH2 mutations are more common in oligodendroglial tumors [51, 54].

In low grade gliomas, they are prognostic favorable factors [53, 55, 56].

The c.395G>A (p.R132H) mutation can be easily detected by a anti-IDH1R132H mutation-specific antibody by immunohistochemical techniques [53, 57]. Tumor cells show IDH1R132H immuno‐ positivity in their cytoplasms, whereas reactive astrocytes and normal glia cells are negative. This is particularly evident in the picture of cortical perineuronal satellitosis where positive (tumor) and negative (normal) satellites can be found, at the beginning of invasion.

## *6.2.1. ATRX mutations*

The ATRX gene is located on chromosome Xq21.1, contains 35 exons, and encodes a 2,492 amino acid protein. ATRX belongs to the H3.3-ATRX-DAXX chromatin remodeling path‐ way, involved in chromatin stabilization [58]. ATRX and its binding factor death-associated protein 6 (DAXX) incorporates the histone protein H3.3 into the nucleosome at telomeres and pericentric heterochromatin [59, 60]. Alterations of this function lead to loss of structural integrity at telomeres leading to tumorigenesis. In fact, ATRX or DAXX protein loss is associated to the alternative lengthening of telomeres (ALT), a telomerase-independent mechanism of telomere lengthening [61–66].

Germline ATRX mutations give rise to a syndrome characterized by severe mental retarda‐ tion [67] and to α-thalassemia.

Somatic ATRX mutations occur in gliomas of different types and histologic grades [38, 61–65, 68–70]. They are more frequent in grade II (67%) and in grade III (73%) astrocytic tumors and in secondary GBMs (57%), as well as in mixed gliomas (25% in grade II and 27–53.8% in grade III tumors) [64, 65, 69, 70]. In contrast, they are rare in primary GBMs (4%), pediatric GBMs (20%) and in pure oligodendroglial tumors (<10%) [63, 64, 68]. Very importantly, ATRX mutations do not affect pilocytic astrocytomas [64].

ATRX mutations occur in 70% of IDH mutant and intact 1p/19q low grade gliomas [62, 64, 65]. Restricted to IDH mutant tumors, they are significantly associated to TP53 mutations and nuclear p53 overexpression and to astrocytic differentiation; they are mutually exclusive with 1p/19q co-deletion [71, 72]. ATRX and IDH1/2 mutations occur in association and they may represent early genetic alterations in the development of gliomas affecting progenitors before their differentiation along the two lineages.

In pediatric gliomas, all ATRX mutations cluster nearthe C-terminal helicase domains; in adult tumors, they are evenly distributed across the gene, mainly as frameshift mutations leading to truncated proteins [63, 64, 69].

The relatively large size of the ATRX gene makes the mutation analysis difficult to be applied in the routine diagnostic procedures. The immunohistochemical evaluation of the ATRX protein expression could represent an alternative method to assess the ATRX status. Although studies reported concordant results between the mutation analysis and IHC [38, 72], tumor heterogeneity in the ATRX expression and concurrent normal non-tumor cells with constitu‐ tive ATRX expression may explain possible discrepancy. As a matter of fact, ATRX muta‐ tions/ATRX protein loss characterizes astrocytic gliomas, whereas retained ATRX immunoreactivity characterizes oligodendroglial gliomas. Referred to OA, the former is typical of the astrocytic component while the latter of the oligodendroglial one [71].

## *6.2.2. ATRX and prognosis*

**6.2. IDH1/2 mutations**

346 Neurooncology - Newer Developments

*6.2.1. ATRX mutations*

mechanism of telomere lengthening [61–66].

mutations do not affect pilocytic astrocytomas [64].

before their differentiation along the two lineages.

tion [67] and to α-thalassemia.

IDHs catalyze the oxidative decarboxylation of isocitrate to α-ketoglutarate with production

Recurrent somatic point mutations affect the arginine (Arg) residue at codon 132 in the IDH1 gene on chromosome 2q33.3. Less frequently, they occur at the homolog Arg (R) residue at codon 172 in the IDH2 gene on chromosome 15q26.1. The IDH1/2 mutation rate is in the range of 70– 80% in OII and OIII [50–53] and less in OAII and OAIII, with a higher frequency in 1p/ 19q co-deleted tumors [37]. IDH1 mutations prevail in astrocytic tumors whereas IDH2

The c.395G>A (p.R132H) mutation can be easily detected by a anti-IDH1R132H mutation-specific antibody by immunohistochemical techniques [53, 57]. Tumor cells show IDH1R132H immuno‐ positivity in their cytoplasms, whereas reactive astrocytes and normal glia cells are negative. This is particularly evident in the picture of cortical perineuronal satellitosis where positive

The ATRX gene is located on chromosome Xq21.1, contains 35 exons, and encodes a 2,492 amino acid protein. ATRX belongs to the H3.3-ATRX-DAXX chromatin remodeling path‐ way, involved in chromatin stabilization [58]. ATRX and its binding factor death-associated protein 6 (DAXX) incorporates the histone protein H3.3 into the nucleosome at telomeres and pericentric heterochromatin [59, 60]. Alterations of this function lead to loss of structural integrity at telomeres leading to tumorigenesis. In fact, ATRX or DAXX protein loss is associated to the alternative lengthening of telomeres (ALT), a telomerase-independent

Germline ATRX mutations give rise to a syndrome characterized by severe mental retarda‐

Somatic ATRX mutations occur in gliomas of different types and histologic grades [38, 61–65, 68–70]. They are more frequent in grade II (67%) and in grade III (73%) astrocytic tumors and in secondary GBMs (57%), as well as in mixed gliomas (25% in grade II and 27–53.8% in grade III tumors) [64, 65, 69, 70]. In contrast, they are rare in primary GBMs (4%), pediatric GBMs (20%) and in pure oligodendroglial tumors (<10%) [63, 64, 68]. Very importantly, ATRX

ATRX mutations occur in 70% of IDH mutant and intact 1p/19q low grade gliomas [62, 64, 65]. Restricted to IDH mutant tumors, they are significantly associated to TP53 mutations and nuclear p53 overexpression and to astrocytic differentiation; they are mutually exclusive with 1p/19q co-deletion [71, 72]. ATRX and IDH1/2 mutations occur in association and they may represent early genetic alterations in the development of gliomas affecting progenitors

(tumor) and negative (normal) satellites can be found, at the beginning of invasion.

of NADH/NADPH and they are involved in the Krebs cycle.

mutations are more common in oligodendroglial tumors [51, 54].

In low grade gliomas, they are prognostic favorable factors [53, 55, 56].

Patients harboring ATRX mutations would show a better outcome [65]. ATRX has important prognostic implications in anaplastic gliomas [65]. ATRX loss is a prognostic factor in IDH mutant and non 1p/19q co-deleted low grade gliomas [62, 65]. In GBMs, ATRX loss affects younger patients [72].
