Molekulargenetische Untersuchungen zum Spleißen ausgewählter High Mobility Group Protein-Gene.

Abstract

Chromosomal rearrangements of the HMGA2 locus belong to the most common aberrations in human benign tumors. HMGA2 rearrangements often result in chimeric genes expressing transcripts consisting of the first three exons of HMGA2 followed by ectopic sequences which have been assigned to chromosome 12 by CASH analysis. These ectopic sequences can be assumed either to result from aberrant splicing with the sequences derived from Intron 3 or 4 of HMGA2 or to represent true ectopic sequences derived from other genes on chromosome 12.To address that question, the entire introns 3 and 4 of HMGA2 have been cloned, sequenced and characterized. It was shown that 5 ectopic sequences formerly described as fused to HMGA2 exon 3 are localized within the third intron of that gene. 3 ectopic sequences formerly described as fused to HMGA2 exon 4 have been localized within the third intron of that gene. RT-PCR based expression studies revealed a co-expression of these transcripts in tumor samples as well as in normal tissues. As these additional HMGA2 transcripts can be detected in cells with a normal karyotype verified by FISH analysis it is quite obviously that these transcripts are alternative not aberrant splice-products of the HMGA2 gene.In silico analysis of the alternative terminal exons of HMGA2 and surrounding intronic sequences revealed a high homology to well established consensus sequences for the 5 splice donor site, 3 splice acceptor site, and the branch site. Potential polyadenylation signals were detected as well. Northern blot hybridizations of the lipoma cell line Li-14 revealed the expression of five additional HMGA2 transcripts consisting of exons 1 to 3 but not exons 4 to 5 besides the wild-type full-length HMGA2 transcript.Therefore, a model for the HMGA2-dependent tumorigenesis was postulated in which the expression of HMGA2 and its splice-variants is integrated in a complex network of mutual dependences.