Distribution of mutations in the PEX gene in families with X-linked hypophosphataemic rickets (HYP)

Abstract

Mutations in the PEX gene at Xp22.1 (phosphate-regulating gene with homologies to endopeptidases, on the X-chromosome), are responsible for X-linked hypophosphataemic rickets (HYP). Homology of PEX to the M13 family of Zn2+ metallopeptidases which include neprilysin (NEP) as prototype, has raised important questions regarding PEX function at the molecular level. The aim of this study was to analyse 99 HYP families for PEX gene mutations, and to correlate predicted changes in the protein 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 mutations, stop codons and splice mutations occurred in 83% of families screened for in all 22 exons, and 51% of a separate set of families screened 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 catalysis. 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 align 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.

Mutational analysis of PHEX, FGF23, DMP1, SLC34A3 and CLCN5 in patients with hypophosphatemic rickets

“…The overall median detection rate of gene mutations reported in previous studies comprising 15 or more HR probands was 66% (range 43-100%), 20,[27][28][29][30][31][32][37][38][39][40][41] in familial probands 66% (44-100%), and in sporadic probands the detection rate was 50% (29-100%). 20,[27][28][29][30]32,37,38 In our study, the MLPA analysis added three PHEX mutations not identified by PCR, dHPLC or sequencing, thus increasing our overall detection rate from 78 to 88%, in familial probands from 67 to 83% and in sporadic probands from 83 to 92%. Our high detection rate of gene mutations, especially among the sporadic patients, may also be due to the robust inclusion/ exclusion criteria of this study.…”

MEPE has the properties of an osteoblastic phosphatonin and minhibin

“…The primary defect in HYP is a mutated Zn-metalloendopeptidase (PHEX) and MEPE is dramatically up-regulated [2,3,29,33,74]. There is overwhelming evidence implicating the osteoblast as the cell intrinsically defective [13,18,[20][21][22]30,37,50,58,59,67,69,80,101,102] and the HYP osteoblast directly secretes factors (phosphatonins) that impact adversely on renal phosphate uptake and mineralization in vivo and in vitro.…”

“…MEPE is exclusively expressed in osteoblasts, osteocytes and odontoblasts and markedly up-regulated in murine X-linked hypophosphatemic rickets (Hyp) osteoblasts and OHO tumors [2,3,15,26,29,45,62,71]. The primary defect in HYP is due to mutations and functional inactivation of the gene product PHEX resulting in defective calcification of bone, renal phosphate wasting (hypophosphatemia) and abnormal vitamin D metabolism [33,72,74]. The mechanisms responsible for these abnormalities are not known.…”

Non-random distribution of mutations in the PHEX gene, and under-detected missense mutations at non-conserved residues

“…[1][2][3][4][5][6] Cloning of the human [7][8][9] and the mouse 7,10-12 PHEX cDNA has now been reported by six independent teams.…”