A Novel Missense Mutation In An MDS Patient and Effects on TET2 mRNA Expression and Clinical Outcomes

The ten-eleven translocation 2 (TET2) gene has recently been recognized to be mutated at a relatively high frequency in all myeloid malignancies, representing a compelling unifying feature of these related but phenotypically distinct conditions. The TET2 paralog TET1 catalyzes the conversion of 5-methylcytosine to 5-hydroxymethylcytosine; TET2 shares a homologous domain thought to catalyze this conversion, and is hypothesized to act as a tumor suppressor gene (TSG) by regulating DNA methylation and epigenetic control of gene expression at critical loci important for myelopoiesis and leukemogenesis. The prognostic impact of TET2 mutations is largely unknown. The hypomethylating agents 5-azacytidine and decitabine, approved agents with activity in myelodysplastic syndrome (MDS), are incorporated into DNA and/or RNA, inhibiting DNA methyltransferase and preventing promoter methylation and TSG silencing. Given the role of TET2 in epigenetic regulation, the aim of this study was to determine whether TET2 mutations were associated with prognosis in MDS patients (pts) treated with hypomethylating therapies.

Bone marrow aspirates from 12 MDS pts who received hypomethylating agents were obtained, and PCR and bidrectional sequencing of TET2 coding exons was performed. RT-PCR was performed in triplicate and analyzed by the comparative Ct method.

The median age of the 12 pts was 68 (49-80), the median international prognostic scoring system (IPSS) score was 1.75 (0-3) and 7/12 (58%) had normal cytogenetics. Two of 12 (17%) pts had exonic TET2 mutations. Pt 7 exhibited a previously reported heterozygous V1718L missense mutation, while pt 3 had 2 previously unreported heterozygous mutations: a nonsense mutation in exon 3, Q1068X, and a missense mutation in exon 6, R1214V. TET2 mRNA expression from all pt samples that expressed GAPDH was quantified. Pt 3 had decreased TET2 mRNA expression compared to pt 7, MDS patients with wild type TET2 and non-diseased control samples (p=.01). Pts with wild-type TET2 had an overall response rate (defined as complete responses plus partial responses) of 20%, while neither of the patients with TET2 mutations responded to hypomethylating agents. The post-treatment median survival was significantly different between patients harboring TET2 mutations and those with wild type TET2 (log rank test p<.003). Both pts with mutated TET2 ultimately died, 4 months and 6 months after treatment, while 7/10 (70%) pts with wild type TET2 died, with a median time to death or last follow-up of 26 months (95% CI 6.1–47.6)

We report the existence of two novel mutations in TET2, one of which is a heterozygous nonsense mutation that is predicted to result in a truncated protein lacking the conserved domains. The reduced expression of TET2 observed in this pt may be due to nonsense-mediated decay, a post-transcriptional process that recognizes and degrades mRNA that contains a premature translation termination codon. The adverse association seen in this series between TET2 mutations and clinical outcomes in MDS pts who received hypomethylating drugs merits replication in a larger sample.

No relevant conflicts of interest to declare.