Prevalence, Correlation With Other Mutant Genes and Clinical Effect of IDH1/2 Mutations in Patients With Acute Myeloid Leukemia. The European IDH Research Alliance

Among the most notable cancer genome-wide sequencing discoveries in recent years was the finding of mutation hot-spots in the isocitrate dehydrogenase (IDH) genes in grade II/III astrocytomas and oligodendrogliomas and in secondary glioblastomas. This was rapidly followed by identification of recurrent IDH1/2 mutations in myeloid neoplasms (MN), including acute myeloid leukemia (AML), myelodysplastic syndromes (MDS) and myeloproliferative neoplasms (MPN).

Mutant IDH is now a therapeutic target of great interest in cancer research, especially in AML, given the limitations of current approved therapies and the encouraging early clinical data demonstrating proof of concept for investigational mutant IDH1/2 inhibitors.

There is evidence to suggest that IDH mutations may cooperate with other mutations to initiate and drive oncogenesis in myeloid malignancies. High levels of 2-hydroxyglutarate (2-HG, as a result of gene mutation) have been shown to inhibit αKG-dependent dioxygenases including histone and DNA demethylases, proteins that regulate cellular epigenetic status. Consistent with 2-HG promoting cancer via an effect on chromatin structure, tumors harboring IDH mutations display a CpG island methylator phenotype. More recent studies have shown that overexpression of mutant IDH enzymes can induce histone and DNA hypermethylation, as well as block cellular differentiation. Together, these data suggest that cancer-associated IDH mutations can induce a block in cellular differentiation through epigenetic modifications, which contributes to tumor initiation and progression, and thus support the clinical evaluation of agents targeted to mutant IDH

The origin of mutations in AML was explored by investigating the clonal evolution of genomes sequenced from patients with M1- or M3-AML and comparing them with hematopoietic stem/progenitor cells (HSPCs) from healthy volunteers. Six genes were found to have statistically higher mutation frequencies in M1 versus M3 genomes (NPM1, DNMT3A, IDH1, IDH2, TET2 and ASXL1), suggesting they are initiating rather than cooperating events. Furthermore, all of these genes have been shown to play a role in chromatin modification, suggesting that epigenetic alterations may function to initiate tumorigenesis.

Prospective evaluation of serial 2-HG levels during treatment of newly diagnosed AML treated with standard chemotherapy revealed that both 2-HG level and mutated IDH allele burden decreased with response to treatment but began to rise again as therapy failed.