Identification of 31 novel mutations in the N-acetylgalactosamine-6-sulfatase gene reveals excessive allelic heterogeneity among patients with Morquio A syndrome

Mutation analysis of the N-acetylgalactosamine-6-sulfate sulfatase gene was performed in a group of 35 patients with mucopolysaccharidosis type IVA from 33 families, mainly of European origin. By nonradioactive SSCP screening, 35 different gene mutations were identified, 31 of them novel. Together they account for 88.6% of the disease alleles of the patients investigated. The vast majority of the gene alterations proved to be point mutations, 23 missense, 2 nonsense, and 3 affecting splicing. Six small deletions (1-27 bp) and one insertion were also characterized. In a Polish family, two mildly affected siblings were compound heterozygotes for R94G and R259Q. Their mother was homozygous for the latter point mutation, leading to enzyme deficiency and a borderline disease phenotype.

Molecular heterogeneity in mucopolysaccharidosis IVA in Australia and Northern Ireland: nine novel mutations including T312S, a common allele that confers a mild phenotype

Mucopolysaccharidosis IVA (MPS IVA) is an autosomal recessive lysosomal storage disorder caused by a genetic defect in N-acetylgalactosamine-6-sulfate sulfatase (GALNS). Previous studies of patients from a British-Irish population showed that the I113F mutation is the most common single mutation among MPS IVA patients and produces a severe clinical phenotype. We studied mutations in the GALNS gene from 23 additional MPS IVA patients (15 from Australia, 8 from Northern Ireland), with various clinical phenotypes (severe, 16 cases; intermediate, 4 cases; mild, 3 cases). We found two common mutations that together accounted for 32% of the 44 unrelated alleles in these patients. One is the T312S mutation, a novel mutation found exclusively in milder patients. The other is the previously described I113F that produces a severe phenotype. The I113F and T312S mutations accounted for 8 (18%) and 6 (14%) of 44 unrelated alleles, respectively. The relatively high residual GALNS activity seen when the T312S mutant cDNA is overexpressed in mutant cells provides an explanation for the mild phenotype in patients with this mutation. The distribution and relative frequencies of the I113F and T312S mutations in Australia corresponded to those observed in Northern Ireland and are unique to these two populations, suggesting that both mutations were probably introduced to Australia by Irish migrants during the 19th century. Haplotype analysis using 6 RFLPs provides additional data that the I113F mutation originated from a common ancestor. The other 9 novel mutations identified in these 23 patients were each limited to a single family. These data provide further evidence for extensive allelic heterogeneity in MPS IVA in British-Irish patients and provide evidence for their transmission to Australia by British-Irish migrants.

Fourteen novel mucopolysaccharidosis IVA producing mutations in GALNS gene

Mucopolysaccharidosis IVA (MPS IVA) is an autosomal recessive disorder caused by a deficiency of the lysosomal N-acetylgalactosamine-6-sulfate sulfatase. Here, we report our analysis of data on 21 patients of diverse ethnic and geographic origins studied by SSCP and sequencing analysis. Sixteen mutations were detected, including 14 new mutations (11 missense, one premature termination, one splice site alteration, and one cryptic site alteration). The donor splice site mutation (IVS4 + 1G-->A) predicts that normal splicing will be abolished and that translation would lead to an immediate premature termination (W141X). Another novel nucleotide change outside the coding sequence is an intronic alteration (IVS9-42C-->T:ggtcggtgcggttggtgc) creating a potential cryptic donor site. The nucleotide sequence surrounding this alteration is highly suggestive of a consensus donor splice site. All 12 missense and nonsense mutations were shown by transient expression to abolish or greatly reduce GALNS activity, thereby providing an explanation as to why they produce MPS IVA. All mutations were readily confirmed by restriction enzyme or by allelic specific oligonucleotide analysis (ASO). These findings, coupled with previously reported mutations, bring the total of different mutations to 41 among independent families with MPS IVA, illustrating the extensive allelic heterogeneity among mutations producing MPS IVA.

Biochemical and structural analysis of missense mutations in N-acetylgalactosamine-6-sulfate sulfatase causing mucopolysaccharidosis IVA phenotypes

Mucopolysaccharidosis IVA (MPS IVA; OMIM#253000), a lysosomal storage disorder caused by a deficiency of N -acetylgalactosamine-6-sulfate sulfatase (GALNS), has variable clinical phenotypes. To date we have identified 65 missense mutations in the GALNS gene from MPS IVA patients, but the correlation between genotype and phenotype has remained unclear. We studied 17 missense mutations using biochemical approaches and 32 missense mutations, using structural analyses. Fifteen missense mutations and two newly engineered active site mutations (C79S, C79T) were characterized by transient expression analysis. Mutant proteins, except for C79S and C79T, were destabilized and detected as insoluble precursor forms while the C79S and C79T mutants were of a soluble mature size. Mutants found in the severe phenotype had no activity. Mutants found in the mild phenotype had a considerable residual activity (1.3-13.3% of wild-type GALNS activity). Sulfatases, including GALNS, are members of a highly conserved gene family sharing an extensive sequence homology. Thus, a tertiary structural model of human GALNS was constructed from the X-ray crystal structure of N -acetylgalacto-samine-4-sulfatase and arylsulfatase A, using homology modeling, and 32 missense mutations were investigated. Consequently, we propose that there are at least three different reasons for the severe phenotype: (i) destruction of the hydrophobic core or modification of the packing; (ii) removal of a salt bridge to destabilize the entire conformation; (iii) modification of the active site. In contrast, mild mutations were mostly located on the surface of the GALNS protein. These studies shed further light on the genotype-phenotype correlation of MPS IVA and structure-function relationship in the sulfatase family.

The mouse N-acetylgalactosamine-6-sulfate sulfatase (Galns) gene: cDNA isolation, genomic characterization, chromosomal assignment and analysis of the 5'-flanking region

Deficiency of lysosomal enzyme N-acetylgalactosamine-6-sulfate sulfatase (GALNS) leads to mucopolysaccharidosis IV A (MPS IV A), for which there is no definitive treatment so far. Although a number of mutations of the GALNS gene of MPS IV A patients have been described, pathogenesis of the disorder still remains elusive. In order to facilitate in vivo studies using model animals for MPS IV A, we isolated and performed molecular characterization of the mouse homolog of human GALNS. The 2.3-kb cDNA contains a 1560-bp open reading frame encoding 520 amino acid residues. The coding region has 84% similarity to the human GALNS cDNA at amino acid level. The mouse Galns gene was mapped by interspecific backcross analysis to the distal region of chromosome 8 where it co-segregates with Aprt. Northern blot analysis showed a wide expression of a single-copy gene, being higher especially in liver and kidney. The Galns gene was isolated from S129vJ genomic library and its genomic organization was characterized. The mouse Galns gene was about 50-kb long and organized into 14 exons and 13 introns. All intron-exon splice junctions conformed to the GT/AG consensus sequence except exon 8/intron 8 junction. Primer extension shows multiple transcription initiation sites between -44 and -75 although major transcription initiation site was observed at -90 bp from the ATG codon. The 5'-flanking region lacks canonical TATA and CAAT box sequences, but is G+C rich with 10 GC boxes (potential Sp1 binding sites), characteristic of a housekeeping gene promoter.

Combined adenine phosphoribosyltransferase and N-acetylgalactosamine-6-sulfate sulfatase deficiency

We describe a Czech patient with combined adenine phosphoribosyltransferase (APRT) deficiency (2,8-dihydroxyadenine urolithiasis) and N-acetylgalactosamine-6-sulfate sulfatase (GALNS) deficiency (mucopolysaccharidosis Type IVA, Morquio disease A). Adenine and its extremely insoluble derivative, 2,8-dihydroxyadenine, were identified in the urine, and APRT deficiency was confirmed in erythrocytes. There was excessive excretion of keratan sulfate in the urine, and GALNS deficiency was confirmed in leukocytes. GALNS and APRT are both located on chromosome 16q24.3, suggesting that the patient had a deletion involving both genes. PCR amplification of genomic DNA indicated that a novel junction was created by the fusion of sequences distal to GALNS exon 2 and proximal to APRT exon 3, and that the size of the deleted region was approximately 100 kb. The deletion breakpoints were localized within GALNS intron 2 and APRT intron 2. Several other genes, including the alpha subunit of cytochrome B (CYBA), which is deleted or mutated in the autosomal form of chronic granulomatous disease, are located in the 16q24.3 region, but PCR amplification showed that this gene was present in the proband. A patient with hemizygosity for GALNS deficiency and APRT deficiency has been reported from Japan recently. These findings indicate that: (i) APRT is located telomeric to GALNS; (ii) GALNS and APRT are transcribed in the same orientation (centromeric to telomeric); and (iii) combined APRT/GALNS deficiency may be more common than hitherto realized.

Clinical, biochemical and molecular findings in a two-generation Morquio A family

Mucopolysaccharidosis type IVA (Morquio A) is caused by a deficiency of N-acetylgalactosamine-6-sulfatase (GALNS), an enzyme capable of cleaving the sulfate group from both N-acetylgalactosamine-6-sulfate and galactose-6-sulfate. We describe here a two-generation Morquio A family with two distinct clinical phenotypes. The two probands from the second generation showed intermediate signs of the disease whereas their affected mother, aunt and two uncles had only very mild symptoms. Galactose-6-sulfatase (GALS) activity in leukocytes and fibroblasts of the affected family members was clearly deficient. Molecular genetic analysis of the GALNS gene revealed that two different point mutations segregate in the family, which correlated well with the clinical phenotype. The probands with intermediate symptoms were compound heterozygotes for the mutations R259Q and R94G, the latter one being inherited from the unaffected father. The mother and her affected siblings with the unusually mild phenotype were proven to be homozygous for the novel missense point mutation R259Q.

A novel common missense mutation G301C in the N-acetylgalactosamine-6-sulfate sulfatase gene in mucopolysaccharidosis IVA

Mucopolysaccharidosis IVA (MPS IVA) is an autosomal recessive lysosomal storage disorder caused by a genetic defect in N-acetylgalactosamine-6-sulfate sulfatase (GALNS). In previous studies, we have found two common mutations in Caucasians and Japanese, respectively. To characterize the mutational spectrum in various ethnic groups, mutations in the GALNS gene in Colombian MPS IVA patients were investigated, and genetic backgrounds were extensively analyzed to identify racial origin, based on mitochondrial DNA (mtDNA) lineages. Three novel missense mutations never identified previously in other populations and found in 16 out of 19 Colombian MPS IVA unrelated alleles account for 84.2% of the alleles in this study. The G301C and S162F mutations account for 68.4% and 10.5% of mutations, respectively, whereas the remaining F69V is limited to a single allele. The skewed prevalence of G301C in only Colombian patients and haplotype analysis by restriction fragment length polymorphisms in the GALNS gene suggest that G301C originated from a common ancestor. Investigation of the genetic background by means of mtDNA lineages indicate that all our patients are probably of native American descent.

Mucopolysaccharidosis IVA: a novel splice acceptor site mutation in intron 4 of the N-acetylgalactosamine-6-sulfate sulfatase gene in an Afghanistan girl with classical Morquio disease

We report here a novel splice site mutation in intron 4 of the gene for N-acetylgalactosamine-6-sulfate sulfatase (GALNS) in an Afghanistan girl with severe mucopolysaccharidosis IVA (classical Morquio disease). Direct sequencing revealed a homozygous G to A transition in the conserved splice acceptor site in intron 4 (cagG-->caaG: designated IVS 4(-I) G-->A) which eliminates 144 nucleotides of exon 5 in her GALNS transcript and introduces an immediate premature termination codon (at Trp 141 of exon 4). The IVS 4(-1) G-->A has not been seen in other populations and this is the first report of the molecular basis of classical Morquio disease in an Afghanistan patient.

Heteroallelic missense mutations of the galactosamine-6-sulfate sulfatase (GALNS) gene in a mild form of Morquio disease (MPS IVA)

Morquio disease (MPS IVA) is an autosomal recessive disorder caused by a deficiency of N-acetylgalactosamine-6-sulfate sulfatase (GALNS) activity. Patients commonly present in early infancy with growth failure, spondyloepiphyseal dysplasia, corneal opacification, and keratan sulfaturia, but milder forms have been described. We report on a patient who grew normally until age 5 years. Her keratan sulfaturia was not detected until adolescence, and she now has changes restricted largely to the axial skeleton. She has experienced only mildly impaired vision. At age 22, thin-layer chromatography of purified glycosaminoglycans showed some keratan sulfaturia. GALNS activity in fibroblast homogenate supernatants was 20 +/- 5% of controls (as compared to 5 +/- 3% of controls in severe MPS IVA, P < .003). Kinetic analysis of residual fibroblast GALNS activity in patient and parents revealed decreased K(m) and increased Vmax in the mother and daughter, but not in the father, compatible with compound heterozygosity. GALNS exons were amplified from 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 transversion 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 mutations, respectively) confirmed heterozygosity. In fibroblast transfection experiments, heterozygous R94C and F97V mutants independently expressed 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 MPS IVA. Our findings expand the clinical and biochemical phenotype of MPS IVA, but full delineation of the genotype-phenotype relationship requires further study of native and transfected mutant cell lines.

Mucopolysaccharidosis IVA: four new exonic mutations in patients with N-acetylgalactosamine-6-sulfate sulfatase deficiency

We report four new mutations in Japanese patients with mucopolysaccharidosis IVA (MPSIVA) who were heterozygous for a common double gene deletion. A nonsense mutation of CAG to TAG at codon 148 in exon 4 was identified, resulting in a change of Q to a stop codon and three missense mutations. V (GTC) to A (GCC) at codon 138 in exon 4, P (CCC) to S (TCC) at codon 151 in exon 5, and P (CCC) to L (CTC) at codon 151 in exon 5. Introduction of these mutations into the normal GALNS cDNA and transient expression in cultured fibroblasts resulted in a significant decrease in the enzyme activity. V138A and Q148X mutations result in changes of restriction site, which were analyzed by restriction-enzyme assay. P151S and P151L mutations that did not alter the restriction site were detected by direct sequencing or allele specific oligohybridization. Detection of the double gene deletion was initially done using Southern blots and was confirmed by PCR. Haplotypes were determined using seven polymorphisms to the GALNS locus in families with the double gene deletion. Haplotype analysis showed that the common double gene deletion occurred on a single haplotype, except for some variation in a VNTR-like polymorphism. This finding is consistent with a common founder for all individuals with this mutation.

Mucopolysaccharidosis IVA: submicroscopic deletion of 16q24.3 and a novel R386C mutation of N-acetylgalactosamine-6-sulfate sulfatase gene in a classical Morquio disease

The N-acetylgalactosamine-6-sulfate sulfatase (GALNS) gene, which is responsible for autosomal recessive mucopolysaccharidosis IVA (MPSIVA), has been assigned to the long arm of chromosome 16, subregion 24.3, an area where the adenine phosophoribosyltransferase (APRT) gene and renal dipeptidase (DPEP I) gene are also localized. Molecular genetic studies on a severely affected patient with MPSIVA (Morquio disease), without karyotypic abnormality, revealed a partial submicroscopic deletion of 16q24.3 and a single point mutation on the other allele, with no functional GALNS activity. The patient, his mother, and siblings were hemizygous for GALNS and APRT loci, evidenced by informative RFLP and gene dosage analyses combined with a fluorescence in situ hybridization, utilizing a partial genomic clone of GALNS, but heterozygosity was retained at the DPEP I locus and proximal D16S7. Haplotyping of the family members revealed recombinational events between DPEP I locus and three other polymorphic loci on the paternal chromosome, localizing GALNS gene on the proximal side to DPEP I gene. As estimated from the genetic distance between two flanking markers of proximal D16S7 and distal DPEP I locus, size of the deletion was less than 3Mb. Mother of the boy and two older siblings were asymptomatic, despite this interstitial deletion of the Giemsa-light G band. The remaining paternal allele had no gene rearrangement but GALNS activity was not encoded as Arginine at 386 was replaced with Cysteine (R386C), suggesting this alteration accounts for the severe phenotype. Allelic loss of APRT is frequently observed in cancer tissues, thereby suggesting that the tumor suppressor gene locates near the APRT locus. No family member has evidence of any malignant disease. This study is apparently the first documentation of interstitial deletion of 16q24.3, involving GALNS and APRT genes.

Expression, purification and characterization of recombinant human N-acetylgalactosamine-6-sulphatase

Full-length cDNA sequences encoding human N-acetylgalactosamine-6-sulphatase were stably expressed in Chinese hamster ovary cells under the transcriptional control of the human polypeptide chain elongation factor 1 alpha gene promoter. A clonal cell line overexpressing recombinant N-acetylgalactosamine-6-sulphatase to a level of approx. 3 mg/l of culture medium was isolated. The secreted precursor enzyme was purified to homogeneity by a two-column procedure with an overall yield of 53% of the activity. The physical and catalytic parameters of the recombinant enzyme were similar to those of the mature form isolated from liver. On SDS/PAGE and gel filtration, recombinant N-acetylgalactosamine-6-sulphatase had a native molecular mass of 58-60 kDa. Recombinant N-acetylgalactosamine-6-sulphatase was endocytosed by mucopolysaccharidosis IVA fibroblasts via the mannose-6-phosphate receptor-mediated pathway and was efficiently localized to lysosomes.

Mucopolysaccharidosis IVA: identification of a common missense mutation I113F in the N-Acetylgalactosamine-6-sulfate sulfatase gene

Mucopolysaccharidosis IVA is an autosomal recessive lysosomal storage disorder caused by a deficiency of N-acetylgalactosamine-6-sulfate sulfatase. The recent isolation and characterization of cDNA and genomic sequences encoding GALNS has facilitated identification of the molecular lesions that cause MPS IVA. We identified a common missense mutation among Caucasian MPS IVA patients. The mutation was originally detected by SSCP, and successive sequencing revealed an A-->T transversion at nt 393. This substitution altered the isoleucine at position 113 to phenylalanine (I113F) in the 622 amino acid GALNS protein and was associated with a severe phenotype in a homozygote. Compound heterozygotes with one I113F-allele mutation have a wide range of clinical phenotypes. Transfection experiments in GALNS-deficient fibroblasts revealed that the mutation drastically reduces the enzyme activity of GALNS. Allele-specific oligonucleotide or SSCP analysis indicated that this mutation accounted for 22.5% (9/40) of unrelated MPS IVA chromosomes from 23 Caucasian patients, including 6 consanguineous cases. Of interest, the I1e 113-->Phe substitution occurred in only Caucasian MPS IVA patients and in none of the GALNS alleles of 20 Japanese patients. These findings identify a frequent missense mutation among MPS IVA patients of Caucasian ancestry, that results in severe MPS IVA when homoallelic, and will facilitate molecular diagnosis of most such patients and identification of heterozygous carriers. In addition to this common mutation, 10 different point mutations and 2 small deletions were detected, suggesting allelic heterogeneity in GALNS gene.

Mucopolysaccharidosis type IVA: common double deletion in the N-acetylgalactosamine-6-sulfatase gene (GALNS)

Mucopolysaccharidosis IVA (MPS IVA) is an autosomal recessive disorder caused by a deficiency in N-acetylgalactosamine-6-sulfatase (GALNS). We found two separate deletions of nearly 8.0 and 6.0 kb in the GALNS gene, including some exons. There are Alu repetitive elements near the breakpoints of the 8.0-kb deletion, and this deletion resulted from an Alu-Alu recombination. The other 6.0-kb deletion involved illegitimate recombinational events between incomplete short direct repeats of 8 bp at deletion breakpoints. The same rearrangement has been observed in a heteroallelic state in four unrelated patients. This is the first documentation of a common double deletion a gene that is not a member of a gene cluster.

Mucopolysaccharidosis IVA: screening and identification of mutations of the N-acetylgalactosamine-6-sulfate sulfatase gene

Mutations causing mucopolysaccharidosis IVA in 15 Japanese and one Caucasian patient were characterized. To screen these mutations, we used a combination of single strand conformation polymorphism analysis and heteroduplex analysis for PCR products of targeted cDNA or genomic DNA. Various small mutations were identified in 23 of 26 alleles, while the other six alleles had large rearrangements. Cycle sequencing of PCR products revealed 15 different mutations, including 12 missense, one nonsense, one frame shift (2 bp deletion) and one splice site mutation, in accord with the broad range of clinical phenotypes. Two alleles have different mutations in the same nucleotide position of exon 3 (R94C, CGC-->TGC; R94G, CGC-->GGC), diagnosed by sequencing and by allelic-specific oligohybridization (ASO). One allele had two amino acid changes, E450V in exon 12 and V488M in exon 13, thereby indicating a double point mutation. All 16 mutations reported were confirmed by restriction enzyme assay or by allelic-specific oligohybridization. Transfection of mutagenized cDNAs into patients' fibroblasts showed that all mutations caused completely deficient or markedly decreased N-acetylgalactosamine-6-sulfate sulfatase (GALNS) activity, thereby indicating that these mutations were responsible for the enzyme deficiency.

Polymerase chain reaction detection of two novel human N-acetylgalactosamine-6-sulfate sulfatase gene polymorphisms by single-strand conformation polymorphism analysis or by StyI and StuI cleavages

We describe two common single-base polymorphisms of the N-acetylgalactosamine-6-sulfate sulfatase gene after StyI or StuI restriction sites. These polymorphisms were readily detected by single-strand conformation polymorphism (SSCP), using the polymerase chain reaction (PCR).

Morquio A syndrome: cloning, sequence, and structure of the human N-acetylgalactosamine 6-sulfatase (GALNS) gene

Deficiency of the lysosomal enzyme, N-acetylgalactosamine 6-sulfatase (GALNS;EC 3.1.6.4), results in the storage of the glycosaminoglycans, keratan sulfate and chondroitin 6-sulfate, which leads to the lysosomal storage disorder Morquio A syndrome. Four overlapping genomic clones derived from a chromosome 16-specific gridded cosmid library containing the entire GALNS gene were isolated. The structure of the gene and the sequence of the exon/intron boundaries and the 5' promoter region were determined. The GALNS gene is split into 14 exons spanning approximately 40 kb. The potential promoter for GALNS lacks a TATA box but contains GC box consensus sequences, consistent with its role as a housekeeping gene. The GALNS gene contains an Alu repeat in intron 5 and a VNTR-like sequence in intron 6.

Mucopolysaccharidosis IV A: molecular cloning of the human N-acetylgalactosamine-6-sulfatase gene (GALNS) and analysis of the 5'-flanking region

Mucopolysaccharidosis IV A (MPS IV A) is the result of a genetic deficiency in a lysosomal hydrolase, N-acetylgalactosamine-6-sulfatase (GALNS). To investigate MPS IV A patients at the level of the genome, we analyzed the structure of the human GALNS-encoding gene. From the genomic library of a normal subject in lambda EMBL3, we isolated five overlapping clones covering the coding region of the GALNS cDNA and determined the structural organization. The gene is about 50 kb long and contains 14 exons. The 5'-flanking region lacks a canonical TATA box and CCAAT sequences, but is G+C-rich (70.5%), with four GC boxes, characteristic of a housekeeping gene promoter. The transcription initiation site was determined by primer extension analysis, using RNA from human liver and HeLa cells. Transcription was found to initiate at a few sites, the major ones being 58 and 22 bp upstream of the translation initiation codon. The 5'-flanking region had promoter activity by transient expression, determined using a CAT assay. In addition, this region retained promoter activity, even in reverse orientation. The region -98 to -1 upstream of the ATG codon was defined by deletion analysis to be a minimal promoter. One GC box in this region is likely to be a binding site of a regulatory element.

Mucopolysaccharidosis IV A: assignment of the human N-acetylgalactosamine-6-sulfate sulfatase (GALNS) gene to chromosome 16q24

Plasmid clones of three independent genomic fragments of the gene for human N-acetylgalactosamine-6-sulfate sulfatase (GALNS; EC 3.1.6.4) were utilized in a fluorescence in situ suppression hybridization study to assign the locus to chromosome 16q24. Enzyme assay for GALNS in a patient with del(16)(q22.1) confirmed this finding.

The morquio A syndrome (mucopolysaccharidosis IVA) gene maps to 16q24.3

The gene for N-acetylgalactosamine-6-sulfatase, the deficiency of which results in Morquio A syndrome (mucopolysaccharidosis type IVA), was assigned to chromosome 16 at band q24.3 by fluorescence in situ hybridization. Localization of this band was confirmed by PCR analysis of a somatic cell hybrid panel used for fine mapping of chromosome 16.

Mucopolysaccharidosis type IVA. N-acetylgalactosamine-6-sulfate sulfatase exonic point mutations in classical Morquio and mild cases

Mucopolysaccharidosis type IVA (MPS IVA) results from a genetic deficiency of N-acetylgalactosamine-6-sulfate (Gal-NAc6S) sulfatase. We have identified two different exonic mutations causing GalNAc6S sulfatase deficiency in two unrelated Japanese families, in one patient with classical Morquio disease, and in two brothers with a mild form of MPS IVA. The nucleotide sequence of the full-length cDNA derived from a patient with classical Morquio disease revealed a two-base deletion at nucleotide position 1343-1344 (1344-1345 or 1345-1346) that altered the reading frame (designated 1342delCA). This mutation, inherited from the proband's consanguineous parents, was revealed by TaqI restriction analysis of a cDNA fragment amplified by the polymerase chain reaction. In the proband with the mild form of the disease, a C to G transversion at nucleotide 667 predicted the substitution of Lys for Asn204 (N204K). Since a new AluI site was created by the N204K mutation, restriction analysis indicated that the affected brothers were homozygous for this mutation, as confirmed by the finding that both their parents had this lesion. Transient expression in GalNAc6S sulfatase deficient fibroblasts of these two mutant alleles showed completely deficient or markedly decreased enzyme activities, thereby indicating that these two mutations were responsible for the enzyme deficiency.

Morquio disease: isolation, characterization and expression of full-length cDNA for human N-acetylgalactosamine-6-sulfate sulfatase

We cloned and sequenced a full-length cDNA of human placental N-acetylgalactosamine-6-sulfate sulfatase, the enzyme deficient in Morquio disease. The 2339-nucleotide sequence contained 1566 nucleotides which encoded a polypeptide of 522 amino acid residues. The deduced amino acid sequence was composed of a 26-amino acid N-terminal signal peptide and a mature polypeptide of 496 amino acid residues including two potential asparagine-linked glycosylation sites. Expression of the cDNA in transfected deficient fibroblasts resulted in higher production of this sulfatase activity than in untransfected deficient fibroblasts. The cDNA clone was hybridized to only a 2.3-kilobase species of RNA in human fibroblasts. The amino acid sequence of N-acetylgalactosamine-6-sulfate sulfatase showed a high degree of homology with those of other sulfatases such as human arylsulfatases A, B or C, glucosamine-6-sulfatase, iduronate-2-sulfatase and sea urchin arylsulfatase.