Somatic Mutation of TSSC 5 , a Novel Imprinted Gene from Human Chromosome llplS . 51

We previously reported the isolation of a 2.5 Mb tumor-suppressing subchromosomal transferable fragment (STF) from Ilpl5.5 and the iden tification of nine known genes and four novel genes within this STF. We now report the isolation of a fifth novel cDNA, tumor-suppressing STF cDNA 5, designated TSSC5, located within the STF. TSSC5 encodes a predicted protein of 424 amino acids. Sequence analysis suggests that TSSC5 is a membrane protein with 10 transmembrane segments, and it is located between two imprinted genes, p57KII>2and TSSC3. Northern blot hybridization revealed a 1.6-kb transcript in multiple adult tissues and in fetal liver and kidney, consistent with a potential role in embryonal tumors. We also found that TSSC5 is imprinted with preferential expres sion from the maternal chromosome. Reverse transcription-PCR analysis of TSSCS revealed frequent occurrence of aberrant RNA splicing, which deleted exons 4, 5. and 6 in Wilms' tumors. Mutational analysis of TSSCS by direct DNA sequencing of exons revealed a base substitution of G1120A in a Wilms' tumor, matched normal kidney, and the patient's mother, changed Arg at codon 309 to Gin. The G1120A substitution thus repre sents either a rare polymorphism or a tumor-predisposing mutation, because the mutant alÃ-elewas of maternal origin and preferentially ex pressed in the patient's tissue. A second base substitution, C892T, was found in a lung cancer, changing Ser at codon 233 to Phe. This substitu tion was absent from the matched normal tissue and thus represented a somatic mutation. We also found loss of heterozygosity in the lung cancer, suggesting that TSSCS may be a conventional tumor suppressor gene in the adult human lung and an imprinted tumor suppressor gene in the fetal kidney.