Professor AIIMS, New Delhi New Delhi, Delhi, India
Introduction: Several nonsynonymous somatic mutations are known to occur in the genome of Multiple Myeloma (MM) that may lead to loss or gain of gene functions in the growing tumor. The somatic codonic changes translate to corresponding alternate amino acid substitutions, some of which may facilitate the oncogenic circuitry and thus impact disease progression or clinical outcomes.
Methods: Codonic changes were identified in whole exomes of malignant plasma cells (PCs) obtained from 109 PCPD patients including 71 MM patients. Whole exome sequencing data (generated using Nextera Exome library prep kit) were analyzed with Illumina Dragen pipeline. Circulating arginine levels in blood plasma were measured with Abcam’s L-Arginine assay kit.
Results: A preferential net loss of arginine specific codons (CGG (73%) > CGC (66%) > CGA (47%) > CGT (35%) >AGG (32%) > AGA (22%)) was observed in MM exome. The circulating arginine levels measured in blood plasma of such patients were also significantly reduced (p=0.03). Arginine (R) is not only a key player in cell metabolism but also marks the conserved cleavage sites (such as RXXR↓R) of proprotein convertase enzyme Furin. Mutations involving arginine over furin cleavage sites were mapped and highlighted the functional involvement of different furin sensitive proteins across PCPD. For example, furin sensitive RNF43 and TG were mutated at their furin cleavage sites in MGUS, while TNC, ADAM22 IGSF10 were mutated in SMM and others like MMP15, SLC9A3 were found to be mutated in MM. Kaplan Meier curve analyses showed a significant correlation between no loss of arginine with inferior PFS (p=0.012). Plausibly, if there is no loss of arginine, its continued abundance would tend to favor tumor progression and thus inferior outcomes. Further, mutations in arginine at furin cleavage sites in neoplastic plasma cells were found to be significantly associated with shorter PFS in MM (p=0.011) and could have prognostic value.
Conclusions: The main novel finding of this study is that there is a selective net loss of arginine codon usage in MM that may have an impact on PFS. A targeted deprivation of Arginine in the tumor microenvironment is a potential therapeutic modality under trials in certain cancers and could be deeply investigated in MM. Arginine substitutions affect furin cleavage of important proteins involved in myelomatogenesis and contribute to PFS. Identification of furin sensitive substrates in early precursor stages of MGUS/SMM may help unveil potential targets for early therapy/ early prognosis and warrant further studies in this direction.
