HUMAN GNAL, C18ORF2, AND MPPE1 GENES Genomic organization of the human GNAL gene and characterization of two novel genes, C18orf2 and MPPE1, on chromosome 18p11.2, a susceptibility region for schizophrenia and bipolar disorder

The genomic organization and mRNA expression of the human GNAL gene on chromosome 18p11.2, a region that has been associated with bipolar disorder and schizophrenia, was determined. The GNAL gene was shown to span over 80 kb and consist of twelve exons, and its structure was very similar to adenyl cyclase stimulating G protein GSα. The start site of transcription was revealed by 5'-RACE. Two polyadenylation signals were found, and 3'-RACE assay was used to verify the functional site. The GNAL gene was expressed as approximately 6 kb transcripts in various regions of the human brain, and no alternative splicing was detected. One informative CA-dinucleotide repeat of 11 alleles and 74% heterozygosity was found in intron 5, and two single nucleotide polymorphisms in introns 3 and 10 were detected by SSCP. During characterization of the GNAL gene, two previously unknown genes were found. A novel intronless gene C18orf2 coding for a functionally unknown protein was localized to intron 5 of the GNAL gene. By semiquatitative RT-PCR, C18orf2 mRNA was found to be moderately expressed in all tissues studied here. Another novel gene, metallophosphoesterase MPPE1, was found to reside adjacent to the 3'-end of the GNAL gene in a tail-to-tail orientation. The deduced amino acid sequence revealed a highly conserved metallophosphoesterase motif gDxH..(16-60)..GDxxdr..(1334)..GNH[DE], which is typical for various phosphate hydrolyzing enzymes, especially serine/ threonine protein phosphatases. The MPPE1 gene contained fourteen exons and spanned about 27 kb. MPPE1 was expressed as a single mRNA of 2.2 kb in various regions of the human brain but not in any other tissues. Four different alternatively spliced forms of MPPE1 were detected by RT-PCR, and each transcript was shown to partially overlap with the 3'-untranslated region of the GNAL gene.