Recurrent KIT D816V Mutation in Atypical Chronic Myeloid Leukemia

INTRODUCTION: Atypical Chronic Myeloid Leukemia (aCML) is a heterogeneous disorder belonging to the group of myelodysplastic/myeloproliferative syndromes, characterized by a poor prognosis with a median survival time of 37 months. In 2013, by applying Next Generation Sequencing (NGS) technologies on 8 aCML cases, we demonstrated the presence of a recurrent somatic mutations in the SETBP1 gene (Piazza et al, Nat Gen 2013). SETBP1 mutations were identified in approximately 30% of aCML cases.

AIM: To further characterize the molecular pathogenesis of aCML and to possibly identify other recurrent lesions responsible for SETBP1 unmutated cases, we extended our initial NGS effort: we applied whole-exome and transcriptome sequencing to a total of 16 matched samples taken at onset of the disease.

MATERIAL and METHODS: Whole-exome and transcriptome sequencing data were generated using an Illumina Genome Analyzer IIx following standard library-preparation protocols. Alignment to the reference GRCh37/hg19 genome was performed using BWA. Alignment data were processed using Samtools. Single nucleotide and small indel detection was performed using in-house software. Copy number analyses from whole-exome data were generated using CEQer (Piazza et al, PLoS One 2013) and gene fusions transcriptome data were screened using FusionAnalyser (Piazza et al, Nucleic Acids Res. 2012).

RESULTS: The application of NGS techniques to the cohort of aCML cases led to the identification of a somatic, non-synonymous single-nucleotide mutation (chr4:g.55599321A>T) in the KIT gene in 1/16 (6%) cases. At protein level this mutation translated into the D816V variant that has been already described in several clonal disorders, such as systemic mastocytosis, gastrointestinal stromal tumors and acute myeloid leukemia.

To assess whether the mutation identified by NGS was recurrent, we extended our analysis by targeted resequencing on a larger cohort of 68 aCML cases. This analysis revealed the presence of KIT mutations in 3 additional patients, thus confirming the recurrence of KIT variants in aCML. All the KIT mutations identified correspond to the D816V that is responsible for the constitutive activation of the tyrosine kinase. This finding suggests that the activation of the KIT tyrosine kinase signaling may play an important role in this subset of aCML patients. It is known from the literature that KIT D816V is highly sensitive to the tyrosine kinase inhibitor dasatinib (Schittenhelm MM, Cancer Res 2006). To test whether dasatinib is able to affect the growth of the leukemic clone in KIT mutated aCML cases, we performed ex vivo tritiated thymidine proliferation assays on bone marrow (BM) cells from one of the KIT D816V positive aCML patients in presence of either dasatinib, imatinib or vehicle alone: the proliferation assay showed that dasatinib was able to inhibit the proliferation of the leukemic clone with an IC50 of 1nM, while, as expected, neither imatinib nor vehicle alone were able to significantly impair cell growth. In line with these data, western blot with an anti- Phospho-KIT antibody on KIT+ lysates after treatment with increasing concentration of dasatinib showed that the drug was highly effective in inhibiting KIT autophosphorylation. To further confirm the inhibitory activity of dasatinib, we performed a colony assay on peripheral blood cells from a KIT D816V positive aCML patient grown in presence of increasing concentrations of the drug: treatment with 100nM dasatinib was able to completely inhibit cell growth, leading to a virtually complete absence of colonies in the D816V-positive plates.

CONCLUSION: These data indicate that KIT D816V is a pro-oncogenic lesion recurrently present in aCML, albeit with low frequency (5/84, 6%) and that aCML cells bearing this mutation are highly sensitive to dasatinib, at least ex vivo. Given the very poor prognosis of this disorder, these findings suggest a new, highly efficient targeted treatment for a subset of aCML patients.