Identification of the Gene Encoding the Enzyme Deficient in Mucopolysaccharidosis IIIC (Sanfilippo Disease Type C)

Abstract

Mucopolysaccharidosis IIIC (MPS IIIC), or Sanfilippo C, represents the only MPS disorder in which the responsible gene has not been identified; however, the gene has been localized to the pericentromeric region of chromosome 8. In an ongoing proteomics study of mouse lysosomal membrane proteins, we identified an unknown protein whose human homolog, TMEM76, was encoded by a gene that maps to 8p11.1. A full-length mouse expressed sequence tag was expressed in human MPS IIIC fibroblasts, and its protein product localized to the lysosome and corrected the enzymatic defect. The mouse sequence was used to identify the full-length human homolog (HGSNAT), which encodes a protein with no homology to other proteins of known function but is highly conserved among plants and bacteria. Mutational analyses of two MPS IIIC cell lines identified a splice-junction mutation that accounted for three mutant alleles, and a single base-pair insertion accounted for the fourth.

Sanfilippo syndrome type C: mutation spectrum in the heparan sulfate acetyl-CoA: α-glucosaminide N-acetyltransferase (HGSNAT) gene

“…Eight insertions and deletions affecting exons 5, 7, 11, 13, 17, and 18 of the HGSNAT gene (Table 1) have been already reported. In particular: mutations c.1034_1049del, c.1336_1372dup, and c.1750delG in patients from the Czech Republic, Poland, and Finland, respectively, were reported in our original publication, whereas Fedele et al [2007] reported mutations c.682_740del, c.739delA, and c.1669_1674delinsACAT, present in patients of Italian origin, and Fan et al [2006] found c.1345dupG in a Canadian patient. Interestingly, the single base deletion, c.739delA, predicted to result in a frameshift and the appearance of a premature termination codon (PTC) 87 bp downstream from the deletion, was also found in homozygous fashion in a patient from North Africa and in combination with p.R506X in a Belgian patient.…”

“…Below we summarize all previously identified mutations and present them with a unified numbering system based on the sequence of GenBank entries NM_152419.2 and NG_009552.1 as previously reported [Fan et al, 2006;Ruijter et al, 2008;Fedele et al, 2007]. We also report 10 novel mutations, including those identified in patient families from Pakistan, the United States, the UK, Germany, Greece, Turkey, and Belarus.…”

“…The gene was cloned only recently by two independent laboratories [Fan et al, 2006;Hřebíček et al, 2006]. Based on our previous studies that defined the chromosomal localization of the MPS IIIC allele [Ausseil et al, 2004] and molecular properties of HGSNAT [Hopwood et al, 1983;Pohlmann et al, 1981;Ausseil et al, 2006], we searched the centromeric region of human chromosome 8 for a gene encoding a protein with multiple transmembrane domains and a molecular weight of 100 kDa, which allowed us to exclude the majority of the genes with exception of the gene initially called TMEM76 [Hřebíček et al, 2006].…”

“…According to predictions made by empirical computer algorithms, the Cterminus of the enzyme is exposed to the cytoplasm and contains conserved Tyr-X-X-Y (where Y is a hydrophobic bulk residue) and Leu-Leu sequence motifs involved in the interaction with the adaptor proteins responsible for the lysosomal targeting of membrane proteins [Bonifacino and Traub, 2003]. Significantly, Fan et al [2006], who independently identified the HGSNAT gene by large-scale proteomic study of the lysosomal compartment, have come to the same structure of the transmembrane domains.…”

Co-morbidity of Sanfilippo Syndrome type C and d-2-hydroxyglutaric aciduria

“…Over time, airway obstruction and/or pulmonary infection can lead to cardiopulmonary arrest. A subtype, Sanfilippo type C syndrome is caused by deficiency of alpha-glucosamine N-acetyltransferase encoded by HGSNAT [4]. No disease-modifying treatment is presently available.…”