Divergently transcribed overlapping genes expressed in liver and kidney and located in the 11p15.5 imprinted domain

Human chromosomal band 11p15.5 has been shown to contain genes involved in the development of several pediatric and adult tumors and in Beckwith-Wiedemann syndrome (BWS). Overlapping P1 artificial chromosome clones from this region have been used as templates for genomic sequencing in an effort to identify candidate genes for these disorders. PowerBLAST identified several matches with expressed sequence tags (ESTs) from fetal brain and liver cDNA libraries. Northern blot analysis indicated that two of the genes identified by these ESTs encode transcripts of 1-1.5 kb with predominant expression in fetal and adult liver and kidney. With RT-PCR and RACE, full-length transcripts were isolated for these two genes, with the largest open reading frames encoding putative proteins of 253 and 424 amino acids. Database comparison of the predicted amino acid sequence of the larger transcript indicated homology to integral membrane organic cation transporters; hence, we designate this gene ORCTL2 (organic cation transporter-like 2). An expressed sequence polymorphism provided evidence that the ORCTL2 gene exhibits "leaky" imprinting in both human fetal kidney and human fetal liver. The mouse orthologue (Orctl2) was identified, and a similar polymorphism was used to demonstrate maternal-specific expression of this gene in fetal liver from interspecific F1 mice. The predicted protein of the smaller gene showed no significant similarity in the database. Northern and RACE analyses suggest that this gene may have multiple transcription start sites. Determination of the genomic structure in humans indicated that the 5'-end of this transcript overlaps in divergent orientation with the first two exons of ORCTL2, suggesting a possible role for antisense regulation of one gene by the other. We, therefore, provisionally name this second transcript ORCTL2S (ORCTL2-antisense). The expression patterns of these genes and the imprinted expression of ORCTL2 are suggestive of a possible role in the development of Wilms tumor (WT) and hepatoblastoma. Although SSCP analysis of 62 WT samples and 10 BWS patients did not result in the identification of any mutations in ORCTL2 or ORCTL2S, it will be important to examine their expression pattern in tumors and BWS patients, since epigenetic alteration at these loci may play a role in the etiology of these diseases.