Deep intronic mutation in CRTAP results in unstable isoforms of the protein to induce type I collagen aggregation in a lethal type of osteogenesis imperfecta type VII.

Genetic mutations are involved in Mendelian disorders. Unbuffered intronic mutations in gene variants can generate aberrant splice sites in mutant transcripts, resulting in mutant isoforms of proteins with modulated expression, stability, and function in diseased cells. Here, we identify a deep intronic variant, c.794_1403A>G, in CRTAP by genome sequencing of a male fetus with osteogenesis imperfecta (OI) type VII. The mutation introduces cryptic splice sites in intron-3 of CRTAP , resulting in two mature mutant transcripts with cryptic exons. While transcript-1 translates to a truncated isoform (277 amino acids) with thirteen C-terminal non-wild-type amino acids, transcript-2 translates to a wild-type protein sequence, except that this isoform contains an in-frame fusion of non-wild-type twenty-five amino acids in a tetratricopeptide repeat sequence. Both mutant isoforms of CRTAP are unstable due to the presence of a unique 'GWxxI' degron, which finally leads to loss of proline hydroxylation and aggregation of type I collagen. Although type I collagen aggregates undergo autophagy, the overall proteotoxicity resulted in death of the proband cells by senescence. In summary, we present a genetic disease pathomechanism by linking a novel deep intronic mutation in CRTAP to unstable mutant isoforms of the protein in lethal OI type VII. • A novel deep intronic variant (c.794-1403A > G) of CRTAP leads to osteogenesis imperfecta (OI) type VII. • The mutation leads to alternative splicing of CRTAP , resulting in the formation of two mutant transcripts of CRTAP. • A novel degron in the mutant isoforms of CRTAP leads to their rapid degradation in cells of OI type VII. • Loss of CRTAP expression leads to aggregation of type I collagen and imbalance of proteostasis in proband cells. • Type I collagen aggregates are degraded by autophagy (aggrephagy) in the cells of proband affected by OI type VII. [ABSTRACT FROM AUTHOR]