ARHI (Ras homologue member I) encodes a 26-kDa
GTPase with 50-60%
amino acid homology to Ras and Rap. ARHI and Ras share similar
GTP/
GDP binding domains, but exert opposite functions. ARHI is one of the first reported
tumor suppressors in the ras superfamily. ARHI is expressed consistently in normal breast and ovarian epithelial cells, but not in breast or
ovarian cancers. The loss of ARHI can be related to
tumor progression. Reexpression of ARHI induces apoptosis of breast and
ovarian cancer cells by a
caspase-independent,
calpain-dependent pathway. ARHI is consistently expressed in normal breast and ovarian epithelial cells but is dramatically downregulated in more then 70% of breast and
ovarian cancers. ARHI is maternally imprinted with methylation of the three CpG islands in the maternal allele of normal cells. ARHI is expressed only from the paternal allele whose three CpG islands are not methylated. Loss of ARHI expression can occur through a genetic event, with loss of heterozygosity observed in 40% of breast, ovarian, and
pancreatic cancers; but it can also occur through epigenetic mechanisms, including DNA methylation,
histone deacetylation,
histone methylation, and transcriptional regulation. Our data suggest that acetylation and methylation of
chromatin associated with the ARHI promoter leads to loss of both ARHI expression and the ability to suppress
tumor growth. Changes in
chromatin that silence ARHI may be driven by methylation-dependent and -independent pathways. Reactivation of both the silenced paternal and imprinted maternal alleles can be achieved by demethylation and inhibition of
histone deacetylation.