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Few data exist on somatic mutation in the epithelial cell lineages that play a central role in human biology and disease. To delineate the "landscape" of somatic mutation in a human epithelial cell lineage, we determined the frequency and molecular nature of somatic mutations occurring in vivo in the X-linked HPRT gene of kidney tubular epithelial cells. Kidney epithelial mutants were frequent (range 0.5 to 4.2 x 10(-4)) and contained a high proportion of unreported HPRT base substitutions, -1-bp deletions and multiple mutations. This spectrum of somatic mutation differed from HPRT mutations identified in human peripheral blood T lymphocytes and from germ-line HPRT mutations identified in Lesch-Nyhan syndrome or hyperuricemia patients. Our results indicate that DNA damage and mutagenesis may have unusual or mechanistically interesting features in kidney tubular epithelium, and that somatic mutation may play a more important role in human kidney disease than has been previously appreciated.

Aberrant hypoxanthine phosphoribosyltransferase (HUGO-approved gene symbol HPRT1; MIM# 308000) mRNA splicing, promoted by splice site mutation or loss, is a common mechanism for loss of the purine salvage enzyme HPRT1 from human cells. We report here two in vivo somatic HPRT1 mutations in human kidney tubular epithelial cells that disrupt HPRT1 intron 1 splicing and lead to the inclusion of intron 1 sequence. We propose an explanation for the use of a common, cryptic intron 1 splice donor site by these two mutations, and by 14 additional human HPRT1 mutations that lead to aberrant splicing with the incorporation of intron 1 sequence into mRNA.

Aberrant hypoxanthine phosphoribosyltransferase (HUGO-approved gene symbol HPRT1; MIM# 308000) RNA splicing promoted by splice site mutation or loss is a common mechanism for loss of the purine salvage enzyme HPRT1 from human cells. We report here two in vivo somatic HPRT1 mutations in human kidney tubular epithelial cells that disrupt HPRT1 intron 1 splicing and lead to the inclusion of intron 1 sequence in mature mRNA. Analysis of these mutations and of 14 additional HPRT1 intron 1 inclusion mutations provides an explanation for use of a common, cryptic intron 1 splice donor site by all 16 mutations.