Allelic-expression imbalance of the insulin-like growth factor 2 gene in hepatocellular carcinomas and underlying disease

It has been well documented that the liver is an exceptional organ in which the monoallelic expression of insulin-like growth factor 2 (IGF2) due to genomic imprinting is relaxed during the postnatal period, resulting in biallelic expression thereafter. In the present study, changes in the status of genomic imprinting were examined in 15 hepatocellular carcinomas (HCCs) as well as in 29 liver biopsies of chronic hepatitis or liver cirrhosis without clinical evidence of HCC, following screening for heterozygotes with an ApaI polymorphism in IGF2 in 34 HCCs and 80 such non-HCC cases. Extreme allelic-expression imbalance, leading to restoration of monoallelic IGF2 expression, was observed in 15 (100%) of 15 informative HCCs for the polymorphism with this monoallelic IGF2 expression appearing to be non-random from the paternal allele. Interestingly, the same allelic-expression imbalance was also present in a significant fraction of noncancerous liver specimens of patients with underlying disease known to be associated with HCC development. In contrast, the status of genomic imprinting of H19, another gene closely mapped at 11p15 under opposite imprinting, was strictly maintained in seven (100%) of seven cases informative for an RsaI polymorphism of H19. Together with the previous reports on altered genomic imprinting of IGF2 and H19 in embryonal lesions such as Wilms tumors as well as in lung cancers, the results suggest that perturbations of imprinting status occur as locus and tumor-type specific events in the development of human cancers.