Hepatocellular Carcinoma in Mongolia Delineates Unique Molecular Traits and a Mutational Signature Associated with Environmental Agents

Purpose: Mongolia has the world's highest incidence of hepatocellular carcinoma (HCC), with ∼100 cases/100,000 inhabitants, although the reasons for this have not been thoroughly delineated. Experimental Design: We performed a molecular characterization of Mongolian (n = 192) compared with Western (n = 187) HCCs by RNA sequencing and whole-exome sequencing to unveil distinct genomic and transcriptomic features associated with environmental factors in this population. Results: Mongolian patients were younger, with higher female prevalence, and with predominantly HBV-HDV coinfection etiology. Mongolian HCCs presented significantly higher rates of protein- coding mutations (121 vs. 70 mutations per tumor in Western), and in specific driver HCC genes (i.e., APOB and TSC2). Four mutational signatures characterized Mongolian samples, one of which was novel (SBS Mongolia) and present in 25% of Mongolian HCC cases. This signature showed a distinct substitution profile with a high proportion of T>G substitutions and was significantly associated with a signature of exposure to the environmental agent dimethyl sulfate (71%), a 2A carcinogenic associated with coal combustion. Transcriptomic-based analysis delineated three molecular clusters, two not present in Western HCC; one with a highly inflamed profile and the other significantly associated with younger female patients. Conclusions: Mongolian HCC has unique molecular traits with a high mutational burden and a novel mutational signature associated with genotoxic environmental factors present in this country.