Bone Abstracts

Osteogenesis imperfecta is a clinically heterogeneous heritable connective tissue disorder. Most OI cases are due to mutations in type I collagen genes, COL1A1 and COL1A2 encoding the pro-alpha1(I) and pro-alpha2(I) chains respectively. However, genotype–phenotype correlation has not been completely elucidated yet. In this study we evaluated a familial case including a mother and a daughter, classified as OI type I. The daughter had more severe clinical features compared to the mother. Both were carrying a 4005+1G>T mutation in COL1A1 gene, which leads to loss of a splicing site with retention of intron 49 and insertion of a stop codon in the mRNA. Accordingly, both patients had lower levels of COL1A1 transcripts compared to control subjects. Owing to the retention of intron 49, mRNA derived from the mutated allele should be longer than that derived from the wild-type allele. Differential expression analysis of the two alleles was performed on mRNA derived from dermal fibroblasts using RT-PCR. A low amount of transcripts derived from the mutated allele was present only in the daughter. Semi-quantitative determination of allele expression was evaluated by real-time PCR with primers and probe specific for the mutated and the wild-type mRNA. In the daughter, the levels of the mutated transcripts were 17 times higher than in the mother. In contrast, wild-type mRNA levels were similar in the two patients. Based on these results, it is tempting to speculate that the more severe clinical characteristics of the daughter might be due to the concomitant presence of a quantitative and a qualitative defect. Furthermore, these findings highlight the importance of a detailed molecular characterization of each genetic variant to explain the different phenotypic consequences of the same mutation.