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.

