Isolation and mapping of a human septin gene to a region on chromosome 17q, commonly deleted in sporadic epithelial ovarian tumors.

Allele losses from chromosome 17 are common in sporadic ovarian tumors. Previously, we reported high rates of LOH (up to 70%) from 17q25 at the marker THH59 in a bank of malignant ovarian tumors. We have extended this study to 70 tumors with 17 markers from the long arm of chromosome 17. In most cases, the data are consistent with whole chromosome loss, but we have identified a minimal region of deletion that is centered around 4 microsatellites with zero recombination at map position 106.9 cM. A P1/BAC contig across the region (approximately 200 kb) was constructed and used to determine the precise position and order of the microsatellites. The contig was shown to hybridize to 17q25 by fluorescence in situ hybridization analysis. The DNA sequence of the entire contig was determined and analyzed by BLAST searches. A 4-kb cDNA was subsequently identified with homology to the yeast, Drosophila and mammalian septin family of genes. We have designated this gene Ovarian/Breast (Ov/Br) septin. Two splice variants were demonstrated within the 200-kb contig, which differ only at exon 1. Within the contig, approximately 45% of the septin alpha transcript was identified and 38% of the septin beta transcript. The septins are a family of genes involved in cytokinesis and cell cycle control. Their known functions are consistent with the hypothesis that the human 17q25 septin gene is a candidate for the ovarian tumor suppressor gene.

Genomic organization, complex splicing pattern and expression of a human septin gene on chromosome 17q25.3.

The Ov/Br septin gene, which is also a fusion partner of MLL in acute myeloid leukaemia, is a member of a family of novel GTP binding proteins that have been implicated in cytokinesis and exocytosis. In this study, we describe the genomic and transcriptional organization of this gene, detailing seventeen exons distributed over 240 kb of sequence. Extensive database analyses identified orthologous rodent cDNAs that corresponded to new, unidentified 5' splice variants of the Ov/Br septin gene, increasing the total number of such variants to six. We report that splicing events, occurring at non-canonical sites within the body of the 3' terminal exon, remove either 1801 bp or 1849 bp of non-coding sequence and facilitate access to a secondary open reading frame of 44 amino acids maintained near the end of the 3' UTR. These events constitute a novel coding arrangement and represent the first report of such a design being implemented by a eukaryotic gene. The various Ov/Br proteins either differ minimally at their amino and carboxy termini or are equivalent to truncated versions of larger isoforms. Northern analysis with an Ov/Br septin 3' UTR probe reveals three transcripts of 4.4, 4 and 3 kb, the latter being restricted to a sub-set of the tissues tested. Investigation of the identified Ov/Br septin isoforms by RT-PCR confirms a complex transcriptional pattern, with several isoforms showing tissue-specific distribution. To date, none of the other human septins have demonstrated such transcriptional complexity.