HETEROALLELIC MISSENSE MUTATIONS OF THE GALACTOSAMINE-B-SULFATE SULFATASE (GALNS) GENE IN A MILD FORM OF MORQUIO-DISEASE (MPS IVA)

Morquio disease (RIPS IVA) is an autosomal recessive disorder caused b y a deficiency of N-acetylgalactosamine-6-sulfate sulfatase (GALNS) ac tivity. Patients commonly present in early infancy with growth failure , spondyloepiphyseal dysplasia, corneal opacification, and keratan sul faturia, but milder forms have been described. We report on a patient who grew normally until age 5 years. Her keratan sulfaturia was not de tected until adolescence, and she now has changes restricted largely t o the axial skeleton. She has experienced only mildly impaired vision. At age 22, thin-layer chromatography of purified glycosaminoglycans s howed some keratan sulfaturia. GALNS activity in fibroblast homogenate supernatants was 20 +/- 5% of controls (as compared to 5 +/- 3% of co ntrols in severe MPS IVA, P < .003). Kinetic analysis of residual fibr oblast GALNS activity in patient and parents revealed decreased K-m an d increased V-max in the mother and daughter, but not in the father, c ompatible with compound heterozygosity. GALNS exons were amplified fro m patient genomic DNA and screened by SSCP. Two missense mutations, a C to T transition at position 335 (predicting R94C) and a T to G trans version at position 344 (predicting F97V), were found on sequencing an abnormally migrating exon 3 amplicon. Digestion of the amplicon with FokI and AccI restriction enzymes (specific for the R94C and F97V muta tions, respectively) confirmed heterozygosity. In fibroblast transfect ion experiments, heterozygous R94C and F97V mutants independently expr essed as severe and mild GALNS deficiency, respectively. We interpret these findings to indicate that our patient bears heteroallelic GALNS missense mutations, leading to GALNS deficiency and mild RIPS IVA. Our findings expand the clinical and biochemical phenotype of RIPS IVA, b ut full delineation of the genotype-phenotype relationship requires fu rther study of native and transfected mutant cell lines. (C) 1996 Wile y-Liss, Inc.