While the natural history of LGGs almost invariably culminates in their transformation to WHO grade IV tumors, their tendency to exhibit prolonged periods of indolent growth sharply contrasts with the biological behavior of GBMs [1
]. This relative clinical stability likely reflects a less fundamentally altered physiological state, and provides an inviting “window of opportunity” for the implementation of appropriately targeted therapeutics. Elucidating the precise mechanisms driving LGG pathogenesis, therefore, is of vital importance to the advancement of their clinical management. The sheer frequency of IDH mutation in LGG demonstrates its biological relevance and highlights the unequivocal importance of epigenomics and metabolomics in gliomagenesis. Yet while its physiological effects are broad and profound, exactly how IDH mutation fundamentally drives a neoplastic phenotype remains unclear. Indeed, the inability of IDH mutation to promote glioma formation in mouse models thus far underscores this quandary, and further implies that additional molecular alterations are likely required for transformation.
We employed both whole exome and targeted next-generation sequencing approaches to identify ATRX
mutations in a significant percentage of LGGs. Intriguingly, the distribution of ATRX
mutations tracked with specific diagnostic and molecularly defined tumor subclasses. Specifically, their presence was entirely restricted to IDH-mutant, 1p/19q-intact LGGs, astrocytic and oligoastrocytic in their morphology, where they were found in ~70% of tumors and exhibited a tight correlation with TP53
mutation. These findings are strikingly consistent with those of a very recent study examining, among other parameters, ATRX
, and IDH
mutational status, along with 1p/19q codeletion in a large cohort of low- and high-grade gliomas [28
]. By contrast, a separate report, also very recent, found a somewhat lower rate of ATRX
abnormalities in astrocytic and oligoastrocytic LGGs (~42%) [29
]. This discrepancy likely reflects the latter study's employment of immunohistochemical staining in tissue microarrays, and not sequencing-based mutational analysis, as its primary screening technique for ATRX abnormalities. Indeed, frequent ATRX mutations (~79%) were revealed in a smaller cohort of tumors actually subjected to sequencing in this same study. Regardless, together with ours, both reports support a fundamental molecular stratification of LGGs based on IDH
mutational status and 1p/19q codeletion.
Additionally, our data indicate that abnormalities in ATRX
represent the defining molecular characteristic of the EPL subclass of astrocytic LGGs. We considered the possibility that functional loss of ATRX, a known chromatin regulator, might itself drive the EPL transcriptional signature. However, the recent finding that ATRX loss does not significantly alter H3.3 profiles across the coding genome argues against this [22
], and suggests a more fundamental association between EPL subclass and specific cells of origin for IDH-mutant LGG. Our previous work established biological links between EPL tumors and early-stage neuroglial precursor cells in the mammalian subventricular zone (SVZ) [21
]. The high frequency of ATRX
mutations in EPL tumors, therefore, implies either a unique propensity of this specific SVZ progenitor population to acquire ATRX
mutation or a particular sensitivity to its biological effects. Indeed, it has recently been suggested that regions harboring stem-like cells in the adult brain, including the SVZ, might be inherently more prone to oncogenic initiating events [31
The invariable co-occurrence of ATRX
mutations with IDH mutations, and their frequent association with TP53
mutation support a cooperative pathogenic mechanism by which dysfunction in all three proteins is required for oncogenesis in a large subset of LGG. Recent work has shown that IDH mutation dramatically reprograms the cellular epigenome, the physiological effects of which include impaired differentiation and the abnormal maintenance of stem and progenitor cell populations in physiological states permissive to self-renewal [15
]. Additionally, multiple studies have demonstrated that loss of ATRX protein or ATRX
mutation results in ALT and genomic instability [24
], and our own findings recapitulate these functional relationships in IDH-mutant LGGs. Combining genomic instability with inherent self-renewal potential could provide fertile ground for malignant transformation, particularly in the setting of TP53
loss, which would presumably allow affected cells to evade apoptosis and/or senescence [33
]. More extensive in vitro
and in vivo
modeling in disease-relevant experimental systems will be essential to test the validity of these and other related conjectures.
The recent identification of H3.3 histone protein mutations in pediatric GBM and diffuse intrinsic pontine glioma (DIPG) is particularly intriguing in light of our present findings [26
]. Specifically, their frequent association with ATRX
mutations suggests a functional equivalence with IDH mutation in adult glioma, with both fundamentally altering global epigenomic landscape and cellular differentiation state. Similarly, the mutual exclusivity of ATRX
mutation with 1p/19q deletion in adult IDH-mutant LGGs also implies analogous functionality, perhaps in the mediation of genomic instability. Thus, accruing evidence from direct molecular profiling in tumor tissue has bolstered the notion of a shared pathogenic mechanism, operative across a wide spectrum of glioma subtypes. While much work remains to be done, the detailed functional characterization of a common transformative pathway in LGG would have significant implications on future therapeutic development and disease management.