DISTRIBUTION OF MUTATIONS IN THE PEX GENE IN FAMILIES WITH X-LINKED HYPOPHOSPHATEMIC RICKETS (HYP)

Mutations in the PEX gene at Xp22.1 (phosphate-regulating gene with ho mologies to endopeptidases, on the X-chromosome), are responsible for X-linked hypophosphataemic rickets (HYP). Homology of PEX to the M13 f amily of Zn2+ metallopeptidases which include neprilysin (NEP) as prot otype, has raised important questions regarding PEX function at the mo lecular level. The aim of this study was to analyse 99 HYP families fo r PEX gene mutations, and to correlate predicted changes in the protei n structure with Zn2+ metallopeptidase gene function. Primers flanking 22 characterised exons were used to amplify DNA by PCR, and SSCP was then used to screen for mutations. Deletions, insertions, nonsense mut ations, stop codons and splice mutations occurred in 83% of families s creened for in all 22 exons, and 51% of a separate set of families scr eened in 17 PEX gene exons. Missense mutations in four regions of the gene were informative regarding function, with one mutation in the Zn2 +-binding site predicted to alter substrate-enzyme interaction and cat alysis. Computer analysis of the remaining mutations predicted changes in secondary structure, N-glycosylation, protein phosphorylation and catalytic site molecular structure. The wide range of mutations that a lign with regions required for protease activity in NEP suggests that PEX also functions as a protease, and may act by processing factor(s) involved in bone mineral metabolism.