Pk. Cheung et al., Etiological point mutations in the hereditary multiple exostoses gene EXT1: A functional analysis of heparan sulfate polymerase activity, AM J HU GEN, 69(1), 2001, pp. 55-66
Citations number
64
Categorie Soggetti
Research/Laboratory Medicine & Medical Tecnology","Molecular Biology & Genetics
Hereditary multiple exostoses (HME), a dominantly inherited genetic disorde
r characterized by multiple cartilaginous tumors, is caused by mutations in
members of the EXT gene family, EXT1 or EXT2. The corresponding gene produ
cts, exostosin-1 (EXT1) and exostosin-2 (EXT2), are type II transmembrane g
lycoproteins which form a Golgi-localized heterooligomeric complex that cat
alyzes the polymerization of heparan sulfate (HS). Although the majority of
the etiological mutations in EXT are splice-site, frameshift, or nonsense
mutations that result in premature termination, 12 missense mutations have
also been identified. Furthermore, two of the reported etiological missense
mutations (G339D and R340C) have been previously shown to abrogate HS bios
ynthesis (McCormick et al. 1998). Here, a functional assay that detects HS
expression on the cell surface of an EXT1-deficient cell line was used to t
est the remaining missense mutant exostosin proteins for their ability to r
escue HS biosynthesis in vivo. Our results show that EXT1 mutants bearing s
ix of these missense mutations (D164H, R280G/S, and R340S/ H/L) are also de
fective in HS expression, but surprisingly, four (Q27K, N316S, A486V, and P
496L) are phenotypically indistinguishable from wild-type EXT1. Three of th
ese four "active" mutations affect amino acids that are not conserved among
vertebrates and invertebrates, whereas all of the HS-biosynthesis null mut
ations affect only conserved amino acids. Further, substitution or deletion
of each of these four residues does not abrogate HS biosynthesis. Taken to
gether, these results indicate that several of the reported etiological mut
ant EXT forms retain the ability to synthesize and express HS on the cell s
urface. The corresponding missense mutations may therefore represent rare g
enetic polymorphisms in the EXT1 gene or may interfere with as yet undefine
d functions of EXT1 that are involved in HME pathogenesis.