Key Pointsâą The majority of mutations are found in genes that have low or no detectable biological expression.âą Mutated genes often show differential allelic expression in multiple myeloma patient samples.Recent work has delineated mutational profiles in multiple myeloma and reported a median of 52 mutations per patient, as well as a set of commonly mutated genes across multiple patients. In this study, we have used deep sequencing of RNA from a subset of these patients to evaluate the proportion of expressed mutations. We find that the majority of previously identified mutations occur within genes with very low or no detectable expression. On average, 27% (range, 11% to 47%) of mutated alleles are found to be expressed, and among mutated genes that are expressed, there often is allele-specific expression where either the mutant or wild-type allele is suppressed. Even in the absence of an overall change in gene expression, the presence of differential allelic expression within malignant cells highlights the important contribution of RNA-sequencing in identifying clinically significant mutational changes relevant to our understanding of myeloma biology and also for therapeutic applications. (Blood. 2014;124(20):3110-3117)
IntroductionMultiple myeloma (MM) is an incurable neoplastic disease involving the proliferation of monoclonal antibody producing plasma cells.
1MM is a heterogeneous disease but the hallmark genetic changes include several genomic rearrangements, such as translocations involving the IgH locus or hyperdiploidy. 2 The pathogenic molecular changes and the processes that drive genomic instability during the development and evolution of the disease are complex and incompletely understood. Elucidating the precise genetic changes that drive malignant transformation, affect the phenotypic behavior of the disease, and alter treatment response is an essential component of our drive toward individualized therapy.3 Recent studies have focused on attempts to identify individual driver mutations that might provide both prognostic information and unique therapeutic targets. Whole genome and whole exome sequencing of increasingly large numbers of patient samples have identified a number of commonly mutated genes in MM patients, such as CCND1, NRAS, KRAS, BRAF, TP53, and FAM46C. [4][5][6][7][8] However, none of these mutations are found in more than one quarter of patients, and most are found in less than 10% of samples sequenced.We recently reported a large cohort of MM exome sequences involving 84 samples from 67 patients. Of these patients, 15 contributed samples from multiple time points during disease evolution. 9 We again identified a diverse set of gene mutations with significant heterogeneity across our cohort, with a median of 52 (range, 21-488) mutations per sample. Computational approaches can be used to prioritize mutations that are expected to alter protein structure and function, or to lie in likely driver genes. It is more challenging to determine which mutations are likely to be clinically meani...