Mild Feline Mucopolysaccharidosis Type VI

Abstract

The missense mutation, L476P, in the N-acetylgalactosamine 4-sulfatase (4S) gene, has previously been shown to be associated with a severe feline mucopolysaccharidosis type VI (MPS VI) phenotype. The present study describes a second mutation, D520N, in the same MPS VI cat colony, which is inherited independently of L476P and is associated with a clinically mild MPS VI phenotype in D520N/L476P compound heterozygous cats. Biochemical and clinical assessment of L476P homozygous, D520N/L476P compound heterozygous, and D520N homozygous cats demonstrated that the entire range of clinical phenotypes, from severe MPS VI, to mild MPS VI, to normal are clustered within a narrow range of residual 4S activity from 0.5% to 4.6% of normal levels. When overexpressed in CHO-KI cells, the secreted form of D520N 4S was inactivated in neutral pH conditions. In addition, intracellular D520N 4S protein was rapidly degraded and corresponded to 37%, 14.5%, and 0.67% of normal 4S protein levels in the microsomal, endosomal, and lysosomal compartments, respectively. However, the specific activity of lysosomal D520N 4S was elevated 22.5-fold when compared with wild-type 4S. These results suggest that the D520N mutation causes a rapid degradation of 4S protein. The effect of this is partially ameliorated as a result of a significant elevation in the specific activity of mutant D520N 4S reaching the lysosomal compartment. The mucopolysaccharidoses (MPS)1 are a group of lysosomal storage disorders that involve the deficiency of specific enzymes required for the degradation of glycosaminoglycans (GAG). MPS VI is characterized by a deficiency of N-acetylgalactosamine 4-sulfatase (4S), which leads to the lysosomal accumulation and urinary excretion of the GAG dermatan sulfate (DS) (1). The severe MPS VI phenotype is characterized by growth retardation, coarse facial features, joint stiffness, corneal clouding, skeletal deformities, and organ and soft tissue involvement. As a result of DS storage in the heart valve and lung, the normal function of these organs is often compromised, leading to early death in affected individuals. The central nervous system does not appear to be affected, even in individuals with clinically severe MPS VI (2).Patients with MPS VI are diagnosed by elevated levels of DS in their urine and a substantial reduction or lack of 4S activity in cells such as peripheral blood leukocytes and cultured skin fibroblasts. The clinical phenotype of MPS VI patients ranges from severe to relatively mild and has generally been shown to correlate with urinary DS and residual 4S activity levels (2). Generally, patients manifest symptomology associated with MPS VI when their 4S protein and activity levels are 5% or less of normal controls (3).Currently, there is no safe and effective treatment available for MPS patients other than surgical intervention to alleviate symptoms whenever possible. MPS VI is a good candidate for both enzyme and gene replacement therapy protocols for a number of reasons. First, the complication of targeting r...

Expression and Characterization of Human Recombinant and α-N-Actylglucosaminidase

“…Direct 32 P phosphorylation studies have demonstrated only the precur-cells but this relatively small amount of enzyme was sufficient to reduce the stored GAGs by 75%. This is in sor form of rf4S to be mannose-6-phosphorylated (31). As expected the mature 44-kDa form of rf4S was taken good agreement with the observation that less than 10% of normal activity seem to be sufficient for the up into the cells in the absence of M6P while in the presence of M6P the cell surface MPRs were blocked degradation of lysosomal storage material.…”

Mucopolysaccharidosis VI in cats – clarification regarding genetic testing

“…The published phenotypes and implications of the L476P (c.1427 T > C) variant in ARSB that causes a severe form of MPS VI have been recognized for many years [ 10 ]. However, a second variant of ARSB was identified within the same experimental feline colony [ 11 , 12 ]. Several laboratories now offer this MPS VI D520N “mild” DNA test to the cat breeding community.…”

“…A missense mutation (c.1558G > A), inherited independently from L476P and causing an amino acid substitution of the wild type aspartic acid to an asparagine at codon 520 (D520N) was identified in the same colony (Fig. 2 ) [ 11 , 12 ]. The phenotype of the D520N variant in combination with the L476P variant was dubbed MPS VI “mild”.…”

Mucopolysaccharidosis VI in a European Shorthair cat: Neurological presentation, computed tomography findings and genetic investigation

“…The GAGs are present in various connective tissues, in particular in bone and cartilage, and skeletal disease is the predominant abnormality associated with this disorder (Crawley et al, 2003). Previous studies described a missense mutation (c. 1558 G>A) with the amino acid substitution of the wild-type aspartic acid to an asparagine at codon 520 (D520N) in a colony of Siamese cats (Crawley et al, 1998;Yogalingam et al, 1998). The ARSB D520N variant alone has no evidence of association with the disease, but in combination with the L476P variant it can result in a mild form of MPS VI (Lyons et al, 2016).…”