KRAS MOSAIC MUTATION AND ITS TISSUE SPECIFICITY IN JUVENILE MYELOMONOCYTIC LEUKEMIA

Somatic or germline mutations in genes regulating Ras-MAPK pathway have been identified in majority of Juvenile myelomonocytic leukemia (JMML) cases. Mosaicism of KRAS and NRAS mutations in JMML were reported, with variation in the proportion of cells carrying these mutations across different organs. However, how this proportion is distributed across different organs had not been clarified whereas prevalence of somatic oncogenic KRAS mutations are known. Our study focused on one JMML patient with mosaic KRAS G12D mutation and uniparental disomy at the 12p locus, who developed aggressive transformation, invasion into multiple organs and subsequently died. DNA from FFPE biopsy samples of eighteen organs were analyzed using mutation-specific digital PCR (dPCR). The variant allele frequency (VAF) of KRAS G12D and hetero SNP at the 12p locus were analyzed in each samples to remove the impact of mutation by infiltrating tumor cells and to estimate the percentage of KRAS mutant cells. KRAS G12D mutations were identified in samples from sixteen of eighteen analyzed tissues. The frequency of KRAS G12D mutated alleles varied among different tissue type and the median VAF was 5.9% (2.0% -29.3%). The VAF were relatively high in gastrointestinal tract (stomach, colon, ileum), liver and spleen, whereas the VAF were low or undetected in testis, heart and spinal cord. Our findings suggest that KRAS mosaic mutations can exhibit tissue specificity like somatic oncogenic mutations. These tissue distribution might be associated with pathogenesis of JMML and other types of cancer with mosaicism.