UT M.D. Cancer Center
A novel imprinted gene ARHI was defined by differential display PCR. ARHI is expressed consistently in normal ovarian and breast epithelial cells, but not in ovarian or breast cancers. This gene encodes a small G protein of 26 kD that is homologous to Ras and Rap (1). Transgenic mice bearing the human ARHI transgene are small in size. Furthermore, overexpression of the ARHI transgene is associated with a defect in mammary gland development, sterility, and atrophy of multiple organs and degeneration of the hippocampus and cerebellar cortex. Loss of ARHI expression results from multiple mechanisms. The gene is expressed monoallelically and is maternally imprinted. Loss of heterozygosity of the gene was detected in 41% of ovarian and breast cancers on chromosome lp31. In subsequent studies, the gene was localized with greater precision using radiation hybrid mapping. One intragenic TA repeat marker and 6 flanking lp31 microsatellite markers were also used for 1p31 deletion mapping by PCR-LOH analysis in 49 ovarian and breast cancers. Among the polymorphic markers studied, the highest frequency of LOH occurred in ARHI and in its linked marker D1S2829 (41% and 42% respectively). Two other discrete regions of LOH were found centered on D1S207 and D1S488, consistent with the possibility that other suppressor genes might be linked to ARHI. Consequently, the ARHI gene appeared to be maternally imprinted and loss of its expression in a fraction of breast and ovarian cancers could result from deletion of the nonimprinted allele. Among 9 informative cases in which both LOH and imprinting could be evaluated, the retained allele was methylated and presumably silenced in 7. Expression of ARHI can also be transcriptionally regulated. ARHI promoter was sequenced and analyzed. We have generated a series of deletion constructs in the regulation region of ARHI and found one of the constructs can completely shut down the promoter activity. Specific transcription factors involved in ARHI expression will be characterized. Thus, ARHI is an imprinted putative tumor suppressor gene whose expression can be lost by multiple mechanisms.