Quasipalindrome Instability in the C1 Inhibitor Gene • 20

Abstract

Mutations disrupting the function or production of C1 inhibitor cause hereditary angioneurotic edema. Frameshift mutations in separate kindred identified an imperfect inverted repeat sequence that was postulated to play a mechanistic role in the mutations. This inverted repeat was cloned into the chloramphenicol acetyl transferase gene in pBR325 and its mutation rate was studied in four bacterial strains. These strains were selected to assay for effects of recombination and superhelical tension on mutation frequency. Mutations that revert bacteria to chloramphenicol resistance (Cmr) were scored. The recombination proficient cell line RR1 did not produce revertants. The other recombination deficient cell lines had reversion frequencies of approximately 10-9. These rare reversion events in bacteria were most often a frameshift that involved the imperfect inverted repeat with a deletion or a tandem duplication, an event very similar to the human mutations. Increased DNA superhelical tension, that would be expected to enhance cruciform extrusion, did not accentuate mutagenesis (p=0.081). This finding suggests that the imperfect inverted repeat may form a stem loop structure in the single stranded DNA created by the duplex DNA melting prior to replication, and models explaining the slippage can be drawn using the lagging strand of the replication fork. Such intrinsically unstable DNA may have facilitated the apparent enhanced regional evolutionary rate that has been observed in the serine protease inhibitors, the protein superfamily to which C1 inhibitor belongs.