Novel Recurrent Mutations in the Ras-Like GTP-Binding Gene Rit1 in Myeloid Malignancies

Abstract 558

In addition to chromosomal and epigenetic abnormalities, somatic mutations constitute key pathogenic lesions in myeloid neoplasms. Individual somatic mutations or various combinations may be both valuable prognostic markers and targets for new rational therapies. Among them, RAS family genes are ubiquitous oncogenes associated with various cancers. Recurrent canonical mutations in the nucleotide binding domains in NRAS and KRAS result in constitutively activated proteins. In myeloid neoplasms, RAS mutations convey a poor prognosis and are often found in acute myeloid leukemia (AML), myelodysplastic syndromes (MDS) and, rarely, myeloproliferative neoplasms (MPN).

We applied whole exome sequencing to paired germline vs. leukemia samples in 65 cases of MDS, 36 MDS/MPN and 32 sAML. We focused our study on the RAS protein superfamily of small GTPases and identified mutations in 3% and 6% of KRAS and NRAS, respectively. Most significantly, we identified somatic recurrent mutations in the F82 residue of Ras-like without CAAX1 (RIT1) gene in 2 patients with chronic myelomonocytic leukemia (CMML) and secondary AML (sAML), respectively. We confirmed the somatic nature of both mutations in sorted CD3+ cells from each patient (pt).

RIT1 gene encodes a member of Ras-related GTPases, involved in the p38 MAPK and AKT signaling pathway that mediates cellular survival in response to stress. RIT1 gene amplification has been found in 26% of hepatocellular carcinoma. However, neither amplification nor mutations of this gene have been reported in myeloid malignancies. We thus focused this line of experimentation on this somatic mutation. To establish clinical associations we further studied a cohort of 322 patients with various myeloid malignancies by Sanger sequencing and detected somatic RIT1 mutations in an additional 6 (2%) cases. All mutations were located in exon 5, in the 81 and 82 residues, which encode the switch II domain of this protein, an effector region very close to the GTP-binding site G3, and which is highly conserved among species. Among the 8 mutant cases, 5 (63%) pts had CMML, resulting in a higher frequency of mutations in this subcohort of pts (5 out of 57 CMML, 9%). The other 3 mutations were found in one primary (p)AML (M5b subtype) (1 out of 58 pAML, 2%) and two high-grade MDS, one refractory anemia with excess blasts (RAEB)-2 and one sAML(RAEB-T in the FAB-classification) (2 out of 80, 2.5%).

RIT1 mutations were heterozygous in all cases except for one case with trisomy 1 and duplication of the mutant allele. In the cases of WES, we estimated an allelic frequency of ∼35%, consistent with the presence of a heterozygous mutation in ∼70% of sample cells. Because of the large size of the clone and serial samples showing RIT1 mutation since the time of initial diagnosis, it is likely that RIT1 may be of ancestral origin.

As RAS-family gene amplifications have been described in cancer, we also studied the presence of amplifications of the RIT1 locus (1q22) by SNP-A. We found 10 cases characterized by a gain involving the RIT1 region (1q21.1-q44): 4 (40%) cases had a diagnosis of CMML, 4 (40%) had myelofibrosis, whereas the remaining patients had MDS (one RAEB-1 and a RA). Quantitative RT-PCR showed RIT1 overexpression in mutants and in patients with 1q amplification (median normalized relative ratio 0,51 and 0,40, respectively) compared to patients with wild type RIT1 and no amplification in 1q (median normalized relative ratio 0,15; P=.039).

We theorized that activating RIT1 mutations may constitute a suitable therapeutic target. Because AKT inhibitors can block AKT phosphorylation and therefore reverse the antiapoptotic effect of mutant RIT1, we tested whether AKT inhibitor V (Triciribine) can selectively abrogate the growth of primary cells with RIT1 mutation. In in vitro suspension cultures, a 65% of reduction proliferation was observed with significant effects even at 0.1μM concentrations.

In sum, somatic recurrent RIT1 mutations are novel lesions involved in the molecular pathogenesis of myeloid cancers, presumably early in the development of the disease. Moreover, amplifications of RIT1 also lead to overexpression of this Ras-like GTP-ase. Specifically, these abnormalities appear to be more frequent in patients with CMML, but also can be found in other types of MDS.