Idh1 mutations contribute to the development of T-cell malignancies in genetically engineered mice.

Gain-of-function mutations in isocitrate dehydrogenase 1 (IDH1) are key drivers of hematopoietic malignancies. Although these mutations are most commonly associated with myeloid diseases, they also occur in malignancies of the T-cell lineage. To investigate their role in these diseases and provide tractable disease models for further investigation, we analyzed the T-cell compartment in a conditional knock-in (KI) mouse model of mutant Idh1. We observed the development of a spontaneous T-cell acute lymphoblastic leukemia (T-ALL) in these animals. The disease was transplantable and maintained expression of mutant IDH1. Whole-exome sequencing revealed the presence of a spontaneous activating mutation in Notch1, one of the most common mutations in human T-ALL, suggesting Idh1 mutations may have the capacity to cooperate with Notch1 to drive T-ALL. To further investigate the Idh1 mutation as an oncogenic driver in the T-cell lineage, we crossed Idh1-KI mice with conditional Trp53 null mice, a well-characterized model of T-cell malignancy, and found that T-cell lymphomagenesis was accelerated in mice bearing both mutations. Because both IDH1 and p53 are known to affect cellular metabolism, we compared the requirements for glucose and glutamine in cells derived from these tumors and found that cells bearing the Idh1 mutation have an increased dependence on both glucose and glutamine. These data suggest that mutant IDH1 contributes to malignancy in the T-cell lineage and may alter the metabolic profile of malignant T cells.

The IDH2 R172K mutation associated with angioimmunoblastic T-cell lymphoma produces 2HG in T cells and impacts lymphoid development.

Oncogenic isocitrate dehydrogenase (IDH)1 and IDH2 mutations at three hotspot arginine residues cause an enzymatic gain of function that leads to the production and accumulation of the metabolite 2-hydroxyglutarate (2HG), which contributes to the development of a number of malignancies. In the hematopoietic system, mutations in IDH1 at arginine (R) 132 and in IDH2 at R140 and R172 are commonly observed in acute myeloid leukemia, and elevated 2HG is observed in cells and serum. However, in angioimmunoblastic T-cell lymphoma (AITL), mutations are almost exclusively restricted to IDH2 R172, and levels of 2HG have not been comprehensively measured. In this study, we investigate the expression pattern of mutant IDH2 in the AITL tumor microenvironment and measure levels of 2HG in tissue and serum of AITL patients. We find that mutant IDH2 expression is restricted to the malignant T-cell component of AITL, and that 2HG is elevated in tumor tissue and serum of patients. We also investigate the differences between the three hotspot mutation sites in IDH1 and IDH2 using conditional knock-in mouse models. These studies show that in the lymphoid system, mutations in IDH2 at R172 produce high levels of 2HG compared with mutations at the other two sites and that lymphoid development is impaired in these animals. These data provide evidence that IDH2 R172 mutations may be the only variants present in AITL because of their capacity to produce significant amounts of the oncometabolite 2HG in the cell of origin of this disease.

Pan-cancer Analysis of Homologous Recombination Deficiency in Cell Lines.

Homologous Recombination Deficiency (HRD) drives genomic instability in multiple cancer types and renders tumors vulnerable to certain DNA damaging agents such as PARP inhibitors. Thus, HRD is emerging as an attractive biomarker in oncology. A variety of in silico methods are available for predicting HRD; however, few of these methods have been applied to cell lines in a comprehensive manner. Here we utilized two of these methods, "CHORD" and "HRDsum" scores, to predict HRD for 1,332 cancer cell lines and 84 non-cancerous cell lines. Cell lines with biallelic mutations in BRCA1 or BRCA2, which encode key components of the homologous recombination pathway, showed the strongest HRD predictions, validating the two methods in cell lines. A small subset of BRCA1/2-wildtype cell lines were also classified as HRD, several of which showed evidence of epigenetic BRCA1 silencing. Similar to HRD in patient samples, HRD in cell lines was associated with p53 loss, was mutually exclusive with microsatellite instability and occurred most frequently in breast and ovarian cancer types. In addition to validating previously identified associations with HRD, we leveraged cell line-specific datasets to gain new insights into HRD and its relation to various genetic dependency and drug sensitivity profiles. We found that in cell lines, HRD was associated with sensitivity to PARP inhibition in breast cancer, but not at a pan-cancer level. By generating large-scale, pan-cancer datasets on HRD predictions in cell lines, we aim to facilitate efforts to improve our understanding of HRD and its utility as a biomarker.