NUCLEAR RETENTION OF COL1A1 MESSENGER-RNA IDENTIFIES NULL ALLELES CAUSING MILD OSTEOGENESIS IMPERFECTA

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder characterized by bone fragility. Most cases of severe OI result from mutations in the coding region of the COL1A1 or COL1A2 genes yielding an abnormal collagen alpha. chain. In contrast, many patients with mil d OI show evidence of a null allele due to a premature stop mutation i n the mutant RNA transcript. We have previously described a null allel e arising from a splice donor mutation where the transcript containing the included intron was sequestered in the nucleus. Here we demonstra te that transcripts from null alleles arising from premature stop muta tions are also present in the nucleus and absent in the cytoplasm. Usi ng reverse transcriptase-PCR and single-strand conformational polymorp hism of COL1A1 mRNA from patients with mild OI, we describe three pati ents with distinct null producing mutations identified from the mutant transcript within the nuclear compartment. A fourth patient with a Gl y-->Arg expressed point mutation exhibits the mutant transcript in bot h compartments. Defining the distribution of allelic variants of COL1A 1 mRNA in the nuclear and cytoplasmic compartments gives further insig ht into cell biology of OI and provides a strategy for investigating p otential causes of a null allele.