rs118204437 — GALNS Arg386Cys

GALNS Arg386Cys — The Most Common Severe Mutation in Morquio Syndrome

Every cell in your body continuously breaks down and recycles old molecular scaffolding — including glycosaminoglycans¹¹ glycosaminoglycans
Long sugar-chain molecules embedded in cartilage, bone, and connective tissue that give these tissues their structural properties, such as keratan sulfate and chondroitin-6-sulfate. This recycling happens inside lysosomes, tiny cellular recycling compartments, and requires a precise sequence of enzymes. The GALNS gene encodes N-acetylgalactosamine-6-sulfate sulfatase²² N-acetylgalactosamine-6-sulfate sulfatase
EC 3.1.6.4 — an enzyme that removes sulfate groups from the keratan sulfate and chondroitin-6-sulfate sugar chains as they are broken down step by step, one of those critical enzymes. When GALNS is deficient, these glycosaminoglycans accumulate in lysosomes, distorting cells and progressively destroying cartilage and bone. The result is Mucopolysaccharidosis Type IVA (MPS IVA), also known as Morquio syndrome A.

The rs118204437 A allele (c.1156C>T on the coding strand; G>A on the plus genomic strand) changes arginine to cysteine at protein position 386. This is the single most frequently reported GALNS pathogenic variant worldwide, found in approximately 5% of all mutant GALNS alleles in the global literature — and in up to 32.5% of alleles from Latin American cohorts, consistent with a founder effect³³ founder effect
A founder effect occurs when a small ancestral population carrying a mutation gives rise to a larger population, concentrating that mutation at a higher frequency than in the general global population.

The Mechanism

The arginine at position 386 sits in the catalytic core of the GALNS enzyme⁴⁴ catalytic core of the GALNS enzyme
Structural analyses show Arg386 is a buried residue that stabilizes the substrate-binding pocket; its replacement by cysteine disrupts this architecture through changes in charge, polarity, and disulfide-bond potential. Biochemical studies confirm that the Arg386Cys substitution produces no detectable residual enzyme activity — placing it firmly in the severe phenotype category. Patients homozygous for R386C, or compound heterozygous for R386C paired with another null allele, typically present with the full severe form of MPS IVA: progressive skeletal dysplasia, short stature, joint hypermobility, corneal clouding, and cardiorespiratory involvement. A rat model was constructed by engineering the equivalent substitution (Arg388Cys) into the rodent GALNS gene, and it faithfully recapitulates the human skeletal and non-skeletal disease phenotype.

Because GALNS is encoded on the minus strand of chromosome 16, the coding-strand notation (c.1156C>T) and the plus-strand genomic notation (G>A at chr16:88,824,853) appear as complements of each other. Genome files from WGS or consumer genotyping report the plus-strand allele (G reference, A alternate).

The Evidence

Morrone et al. (2014)⁵⁵ Morrone et al. (2014)
Morrone A et al. Morquio A syndrome-associated mutations: a review of alterations in the GALNS gene and a new locus-specific database. Hum Mutat, 2014 curated 1,091 GALNS alleles from the global literature and identified Arg386Cys as the single most common, accounting for 5% of all alleles. It was distributed across 30 countries, with the highest concentration in Spain (25% of Spanish patient alleles), Argentina (11%), and China (9%). Its worldwide distribution and high relative frequency point to both ancient founder events and recurrent de-novo mutation at a CpG-adjacent site.

Tomatsu et al. (2005)⁶⁶ Tomatsu et al. (2005)
Tomatsu S et al. Mutation and polymorphism spectrum of the GALNS gene in mucopolysaccharidosis IVA (Morquio A). Hum Mutat, 2005 documented the broader GALNS mutation landscape in 148 unique alleles, confirming that Arg386Cys is one of the three mutations exceeding 5% prevalence, and that CpG-dinucleotide transitions (which include this mutation's mechanism) account for 26% of all GALNS pathogenic changes.

A large genotype-phenotype study by Yi et al. (2022)⁷⁷ Yi et al. (2022)
Yi Y et al. Investigation of GALNS variants and genotype-phenotype correlations in a large cohort of patients with mucopolysaccharidosis type IVA. J Inherit Metab Dis, 2022 analyzing 108 MPS IVA patients established that buried-residue missense variants (the class to which R386C belongs) drive severe phenotype in 92% of cases, providing a structural rationale for genotype-based prognosis.

ClinVar (Variation ID 700) lists 14 submissions from major genetics laboratories on four continents — Labcorp Genetics, Baylor Genetics, University of Padova, and Shriners Hospital — all independently classifying the variant as Pathogenic or Likely Pathogenic (2-star review status, no conflicts, last updated January 2026).

Practical Actions

MPS IVA has no cure, but the disease can be actively managed. The FDA approved elosulfase alfa (Vimizim), a recombinant form of GALNS enzyme, in February 2014. Administered weekly at 2 mg/kg IV, it reduces urinary keratan sulfate (a disease biomarker) by ~53% sustained over 5+ years of real-world follow-up, and a phase III randomized trial demonstrated a statistically significant improvement in six-minute walk test distance (+22.5 m, 95% CI 4–41 m) versus placebo. Early initiation before irreversible skeletal damage maximizes benefit.

Asymptomatic heterozygous carriers (GA genotype) require no personal clinical intervention, but genetic counseling is recommended before family planning. Each child of two confirmed carriers has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of being unaffected. Prenatal and preimplantation genetic testing are available once pathogenic variants are confirmed in both parents.

Interactions

MPS IVA is a monogenic recessive disease: the clinical outcome is determined primarily by whether a person carries two loss-of-function GALNS alleles (compound heterozygous or homozygous). Arg386Cys paired with a second null allele (frameshift, nonsense, or splice site) typically results in the same severe phenotype as homozygous R386C, because both copies of GALNS are non-functional. Compound heterozygosity with attenuated missense alleles (those retaining 1–13% residual activity) can produce intermediate or mild phenotypes. Assessment of the second allele is therefore critical for prognosis in affected individuals.