MUTATION PRODUCING ALTERNATIVE SPLICING OF EXON-26 IN THE COL1A2 GENECAUSES TYPE-IV OSTEOGENESIS IMPERFECTA WITH INTRAFAMILIAL CLINICAL VARIABILITY

We have characterized a familial form of osteogenesis imperfecta (OI), Following the identification by ultrasound of short limbs and multipl e fractures in a fetus at 25 weeks of gestation, the family was referr ed with a provisional diagnosis of severe OI. We detected subtle clini cal and radiological signs of OI in the father and in the paternal gra ndmother of the proposita, who had never received a diagnosis of OI. L inkage analysis indicated COL1A2 as the disease locus, Heteroduplex an alysis of reverse transcription-polymerase chain reaction (RT-PCR) amp lification products of pro alpha 2(I) mRNA from an affected member and subsequent sequencing of the candidate region demonstrated the presen ce of normal transcripts and a minority of transcripts lacking exon 26 (54 bp) of COL1A2, Sequencing of PCR-amplified genomic DNA identified an A --> G transition in the moderately conserved +3 position of the IVS 26 donor splice site. The mutant pre-mRNA molecules were alternati vely spliced, yielding both full-length and deleted transcripts that r epresented less than 30% of the total pro alpha 2(I) mRNA, The biochem ical data on type I collagen synthesized by dermal fibroblasts showed intracellular retention of the mutant protein; failure to detect the s hortened alpha 2(I) chains either in the medium or in the cell layer m ay be the consequence of their instability at physiological temperatur e, These observations justified the mild resulting phenotype. (C) 1997 Wiley-Liss, Inc.