DEFECTIVE SPLICING OF MESSENGER-RNA FROM ONE COL1A1 ALLELE OF TYPE-I COLLAGEN IN NONDEFORMING (TYPE-I) OSTEOGENESIS IMPERFECTA

Osteogenesis imperfecta (OI) type I is the mildest form of heritable bone fragility resulting from mutations within the COL1A1 gene. We studied fibroblasts established from a child with OI type I and demonstrated underproduction of alpha1 (I) collagen chains and alpha1 (I) mRNA. Indirect RNase protection suggested two species of alpha1 (I) mRNA, one of which was not collinear with fully spliced alpha1 (I) mRNA. The noncollinear population was confined to the nuclear compartment of the cell, and contained the entire sequence of intron 26 and a G --> A transition in the first position of the intron donor site. The G --> A transition was also identified in the genomic DNA. The retained intron contained an in-frame stop codon and introduced an out-of-frame insertion within the collagen mRNA producing stop codons downstream of the insertion. These changes probably account for the failure of the mutant RNA to appear in the cytoplasm. Unlike other splice site mutations within collagen mRNA that resulted in exon skipping and a truncated but in-frame RNA transcript, this mutation did not result in production of a defective collagen proalpha1 (I) chain. Instead, the mild nature of the disease in this case reflects failure to process the defective mRNA and thus the absence of a protein product from the mutant allele.