Genomic structure and expression profile of LPHH1, a 7TM gene variably expressed in breast cancer cell lines

Gene identification studies, centred on a region of overlapping loss of het erozygosity in breast tumours within band 1p31.1, lead to the characterisat ion of LPHH1, a novel human 7TM gene. The coding sequence of LPHH1 extends over a 60 kb region and comprises in excess of 28 exons. Alternative splici ng occurs minimally at five positions, four of which are within the coding sequence. The fifth region of alternative splicing occurs at the extreme 5' end of the transcript. A clear tissue specific bias in alternative exon se lection is observed to some degree at all five positions, including the ext reme 5' region, which raises the possibility of multiple and perhaps tissue specific promoters. One such putative promoter region, which appears to be utilised predominantly in breast cancer cells, has been identified. LPHH1 is highly evolutionarily conserved, with the simplest (19 exon) gene produc t being 95% identical between human and rat. Comparison of the alternativel y spliced exons between three species, where data are available, has so far revealed 100% identity in the encoded peptide sequences, suggesting conser vation of a functional aspect of this splicing. Gene expression has been ob served in all tissues and cell lines tested, with the exception of lymphobl astoid and multiple myeloma lines, where there appears to be only a very lo w level of transcription. LPHH1 also appears to be downregulated in human b one marrow. These data are consistent with a role for the gene products in adhesion-mediated signalling. Analysis of a panel of breast tumour cell lin es revealed that a number apparently overexpressed the gene whilst others s howed very low levels of transcription. In one case, the overexpression cor related with a low level increase in gene copy number in the tumour line. I n addition to differences in the overall levels of expression, LPHH1 mRNAs were alternatively spliced to varying degrees with shifts in the major gene product to truncated or altered forms in some lines. No somatic LPHH1 muta tions were detected through sequence analysis of four primary breast tumour s that showed loss of the adjacent 1p31.1 marker D1S207. (C) 2000 Elsevier Science B.V. All rights reserved.