Alternative titles; symbols
HGNC Approved Gene Symbol: HGSNAT
SNOMEDCT: 75238000; ICD10CM: E76.22;
Cytogenetic location: 8p11.21-p11.1 Genomic coordinates (GRCh38) : 8:43,140,464-43,202,855 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 8p11.21-p11.1 | Mucopolysaccharidosis type IIIC (Sanfilippo C) | 252930 | Autosomal recessive | 3 |
| Retinitis pigmentosa 73 | 616544 | Autosomal recessive | 3 |
The HGSNAT gene encodes heparan acetyl-CoA:alpha-glucosaminide N-acetyltransferase (EC 2.3.1.78), an enzyme that catalyzes acetylation of the terminal glucosamine residues of heparan sulfate prior to its hydrolysis by alpha-N-acetyl glucosaminidase (summary by Klein et al., 1978). The enzyme is sometimes referred to as N-acetyltransferase (Fan et al., 2006).
Fan et al. (2006) identified the gene encoding heparan acetyl-CoA:alpha-glucosaminide N-acetyltransferase, the enzyme defective in mucopolysaccharidosis IIIC, also known as Sanfilippo syndrome type C (252930) (Klein et al., 1978). In a proteomics study of mouse lysosomal membrane proteins, Fan et al. (2006) identified an unknown protein whose human homolog, TMEM76, was encoded by a gene that maps to 8p11.1. They expressed a full-length mouse expressed sequence tag (EST) in human MPS IIIC fibroblasts and observed that its protein product localized to the lysosome and corrected the enzymatic defect. Fan et al. (2006) used the mouse sequence to identify the full-length human homolog, HGSNAT. The deduced C-terminal portion of HGSNAT is highly conserved across species, including plants and bacteria; however, the N-terminal portion, extending to transmembrane domain B, is found only in metazoans. Neither of these regions has homology with functional domains identified elsewhere, including those from proteins known to bind CoA or acetyl-CoA; therefore, HGSNAT represents the human member of a new family of enzymes.
By examining candidate genes in the region of chromosome 8 associated with MPS IIIC, Hrebicek et al. (2006) identified HGSNAT, which they called TMEM76. The deduced 656-amino acid protein with a calculated molecular mass of 73 kD contains an N-terminal signal peptide, 4 putative N-glycosylation sites, and 11 transmembrane domains. Topology modeling predicted the N terminus to be inside the lysosome and the C terminus to be cytosolic. Hrebicek et al. (2006) also identified a splice variant lacking exons 9 and 10 that encodes a protein with an in-frame deletion of 64 amino acids in transmembrane domains 3 and 4. This variant was considered likely to encode an inactive enzyme since it was detected in the RNA of 3 MPS IIIC patients with nearly complete loss of N-acetyltransferase activity. Northern blot analysis detected ubiquitous expression of 4.5- and 2.1-kb transcripts. Highest expression was in leukocytes, heart, lung, placenta, and liver, and much lower expression was found in thymus, colon, and brain. RT-PCR detected TMEM76 in normal human skin, fibroblasts, white blood cells, and skeletal muscle.
Haer-Wigman et al. (2015) performed RT-PCR analysis of HGSNAT RNA derived from normal human retina and leukocytes. Both tissues yielded the expected product, but the authors noted that obtaining a product from leukocytes required an additional set of amplification, indicating that HGSNAT expression levels in the retina are much higher than those in blood leukocytes. RT-PCR analysis of total RNA from mouse eye showed Hgsnat expression as early as embryonic day 14, with equally high expression levels after birth, at postnatal days 0 and 30.
N-acetyltransferase is a lysosomal membrane enzyme whose function is to acetylate the nonreducing, terminal alpha-glucosamine residue of intralysosomal heparin or heparan sulfate, converting it into a substrate for luminal alpha-N-acetylglucosaminidase (Fan et al., 2006). Therefore, N-acetyltransferase catalyzes the only synthetic reaction known to occur in the lysosome. To do this, the enzyme uses a cytosolic cofactor, acetyl-coenzyme A (acetyl-CoA). Thus, its substrate and cofactor are separated by the lysosomal membrane. The mechanism by which this spatial problem is overcome is controversial. One model suggests a ping-pong mechanism involving an initial acetylation reaction of the enzyme in the cytosol, followed by a transfer of the acetyl group to the intralysosomal heparin-alpha-glucosamine residue (Bame and Rome, 1986; Ausseil et al., 2006). An alternative model proposes that the enzyme operates via a random order ternary complex mechanism; that is, there is no acetylated enzyme intermediate (Meikle et al., 1995). The enzyme, which proved difficult to purify, is believed to be a dimer of 120 kD subunits containing asn-linked oligosaccharides. It is also postulated that the 120-kD subunit may be only the catalytic subunit of a protein complex whose other unidentified members are also required for functionality.
Hrebicek et al. (2006) performed functional expression of human HGSNAT and the mouse ortholog and demonstrated that it is the gene that encodes the lysosomal N-acetyltransferase. HGSNAT did not show structural similarity to any known prokaryotic or eukaryotic N-acetyltransferase or to other lysosomal proteins, on the basis of sequence homology searches. The authors suggested that this enzyme belongs to a new structural class of proteins that transport the activated acetyl residues across the cell membrane.
Fan et al. (2006) demonstrated that the HGSNAT gene contains 18 exons.
Fan et al. (2006) identified the HGSNAT gene on chromosome 8p11.1 by comparison of mouse and human sequences.
Mucopolysaccharidosis IIIC
Mucopolysaccharidosis IIIC (MPS3C), or Sanfilippo syndrome type C (252930), is an autosomal recessive disorder characterized by the lysosomal storage of heparin and heparan sulfate. The hallmark of the Sanfilippo syndrome is severe central nervous system involvement but only mild somatic disease. Fan et al. (2006) performed mutation analyses of 2 MPS IIIC cell lines and identified a splice junction mutation that accounted for 3 mutant alleles and a single-basepair insertion accounting for the fourth.
Hrebicek et al. (2006) identified the HGSNAT gene as the site of mutations causing MPS IIIC. Among 30 probands, they found 4 nonsense mutations, 3 frameshift mutations due to deletions or a duplication, 6 splice site mutations, and 14 missense mutations.
In 3 unrelated Portuguese patients with MPS IIIC, Coutinho et al. (2008) identified 2 different mutations in the HGSNAT gene (610453.0006 and 610453.0007).
Ruijter et al. (2008) reported a high frequency of 2 HGSNAT mutations in the Dutch population (R344C; 610453.0008 and S518F; 610453.0009), which occurred in 22 and 29.3% of mutant alleles, respectively.
Feldhammer et al. (2009) stated that 50 pathogenic mutations in the HGSNAT gene had been reported to date, and the authors identified 10 novel mutations. A review of published mutations showed that they span the entire HGSNAT gene, and there were no obvious genotype/phenotype correlations.
Canals et al. (2011) identified 9 different pathogenic mutations in the HGSNAT gene, including 7 novel mutations, in 11 patients with MPS IIIC, including 7 of Spanish origin, 1 from Argentina, and 3 from Morocco. The most common mutation was 372-2A-G (610453.0007), which was found in 4 Spanish patients, with a frequency of 50% (7 of 14 alleles) for the Spanish patients. The second most common mutation was 234+1G-A (610453.0010), which was found in 1 Spanish and 2 Moroccan patients. Haplotype analysis indicated a founder effect for both of these mutations. Each of the 7 novel mutations was found in only 1 patient. In vitro functional expression assays in COS-7 cells showed that missense mutations had practically no residual enzyme activity (range, 0-1.19%).
Retinitis Pigmentosa 73
In 6 affected individuals from 2 Ashkenazi Jewish families and 1 Dutch family with nonsyndromic retinitis pigmentosa (RP73; 616544), Haer-Wigman et al. (2015) identified homozygosity for mutations in the HGSNAT gene (610453.0011-610453.0013). The mutations segregated with disease and were not found in 211 Ashkenazi Jewish controls. Comprehensive examination of affected individuals from all 3 families revealed no extraocular abnormalities, apart from mild hearing impairment at age 59 years in 1 patient.
In a cell line from a patient with MPS IIIC (MPS3C; 252930), Fan et al. (2006) identified a homozygous G-to-A substitution in the first nucleotide of intron 4 of the HGSNAT gene, resulting in deletion of exon 4. The data indicated that the patient was homozygous for the splice junction mutation.
In a cell line from a patient with MPS IIIC (252930), Fan et al. (2006) identified compound heterozygosity for a splice junction mutation (610453.0001) and an insertion of a single G after nucleotide 1344A, resulting in a frameshift after gly448 and the generation of a premature stop codon 21 codons downstream.
In a patient with MPS IIIC (252930) from the United Kingdom, Hrebicek et al. (2006) identified homozygosity for a 932C-T transition in exon 9 of the HGSNAT gene, resulting in a pro311-to-leu (P311L) substitution.
In a patient with MPS IIIC (252930) from the Czech Republic, Hrebicek et al. (2006) identified compound heterozygosity for mutation in the HGSNAT gene: a 1046T-G transversion in exon 10, resulting in a leu349-to-ter (L349X) substitution, and a 1529T-A transversion in exon 14, resulting in a met510-to-lys (M510K) substitution (610453.0005).
For discussion of the met510-to-lys (M510K) mutation in the HGSNAT gene that was found in compound heterozygous state in a patient with mucopolysaccharidosis type IIIC (MPS3C; 252930) by Hrebicek et al. (2006), see 610453.0004.
In 3 unrelated Portuguese patients with MPS IIIC (252930), Coutinho et al. (2008) identified a 1-bp insertion (525dupT) in exon 5 of the HGSNAT gene, resulting in a frameshift and premature protein truncation. Two patients were homozygous for the mutation, and the third was compound heterozygous with a splice site mutation (610453.0007), which was predicted to result in nonsense-mediated mRNA decay.
In a Portuguese patient with MPS IIIC (252930), Coutinho et al. (2008) identified compound heterozygosity for an A-to-G transition (372-2A-G) and the 525dupT (610453.0006) mutation in the HGSNAT gene. The splice site mutation was predicted to result in the skipping of exon 4 and nonsense-mediated mRNA decay.
Canals et al. (2011) identified the 372-2A-G mutation in 4 Spanish patients with MPS IIIC. Three were homozygous for the mutation and 1 was heterozygous. Haplotype analysis indicated a founder effect. Transcript analysis showed that the mutation resulted in 2 transcripts: 1 with skipping of exon 4, creating a frameshift and a premature stop codon, and the other with an in-frame deletion of 4 amino acids. Only 1 of the transcripts was subject to nonsense-mediated mRNA decay.
Ruijter et al. (2008) identified a 1030C-T transition in exon 11 of the HGSNAT gene, resulting in an arg344-to-cys (R344C) substitution, in 22% of mutant alleles among 29 patients with MPS IIIC (252930). All of the patients were of Dutch origin.
Feldhammer et al. (2009) identified the R344C mutation in patients from Germany and Singapore.
By in vitro functional expression studies in COS-7 cells, Canals et al. (2011) demonstrated that the mutant R344C protein had almost no residual enzyme activity.
Ruijter et al. (2008) identified a 1553C-T transition in exon 16 of the HGSNAT gene, resulting in a ser518-to-phe (S518F) substitution, in 29.3% of mutant alleles among 29 patients with MPS IIIC (252930). The mutation occurred only in patients of Dutch origin, suggesting a founder effect.
Feldhammer et al. (2009) identified the S528F mutation in patients of German origin.
By in vitro functional expression studies in COS-7 cells, Canals et al. (2011) demonstrated that the mutant S518F protein had almost no residual enzyme activity.
In a Spanish patient and 2 Moroccan patients with MPS IIIC (252930), Canals et al. (2011) identified a homozygous G-to-A transition in intron 2 (234+1G-A) of the HGSNAT gene. Transcript analysis indicated that the mutation resulted in the skipping of exon 2, but not nonsense-mediated mRNA decay. Haplotype analysis indicated a founder effect.
In 3 affected individuals from 2 consanguineous Israeli families of Ashkenazi Jewish descent with nonsyndromic retinitis pigmentosa (RP73; 616544), Haer-Wigman et al. (2015) identified homozygosity for a c.370A-T transversion (c.370A-T, NM_152419.2) in exon 3 of the HGSNAT gene, resulting in an arg124-to-trp (R124W) substitution at a moderately conserved residue. The mutation segregated with disease in both families and was not found in 211 Ashkenazi Jewish, 250 Israeli, or 5,036 mostly Dutch controls, or in the 1000 Genomes Project, Exome Variant Server, or dbSNP databases. RT-PCR of patient and control leukocytes demonstrated that transcripts lacking exon 3 were only detected in patients, suggesting that the c.370A-T variant causes partial skipping of exon 3 and thus a frameshift, predicted to result in a truncated protein (Cys79ValfsTer20) lacking all transmembrane domains and the C terminus. HGSNAT activity in patient blood leukocytes was less than half that of healthy controls, but was in the upper range of that found in patients with mucopolysaccharidosis type IIIC (252930). Comprehensive physical examination for extraocular features was negative except for mild hearing impairment in 1 of the RP patients.
In 3 Dutch sibs with nonsyndromic retinitis pigmentosa (RP73; 616544), in whom general physical examination did not reveal any additional symptoms, Haer-Wigman et al. (2015) identified homozygosity for a c.1843G-A transition (c.1843G-A, NM_152419.2) in exon 18 of the HGSNAT gene, resulting in an ala615-to-thr (A615T) substitution, as well as heterozygosity for a c.398G-C transversion in exon 4, resulting in a gly133-to-ala (G133A; 610453.0011) substitution, both at highly conserved residues. The G133A mutation was not found in 211 Ashkenazi Jewish, 250 Israeli, or 5,036 mostly Dutch controls, or in the 1000 Genomes Project, Exome Variant Server, or dbSNP databases. However, the authors noted that the A615T mutation is a rare variant with a mean allele frequency of 0.59% in the European American population of the Exome Variant Server, and a frequency of 0.56% in an exome variant database of 5,036 primarily Dutch individuals. In addition, A615T had previously been reported in patients with mucopolysaccharidosis type IIIC (MPS3C; 252930), both times homozygously and in combination with another homozygous variant, W403C (610453.0014), by Hrebicek et al. (2006) and Feldhammer et al. (2009). Haer-Wigman et al. (2015) stated that it had been shown that A615T results in moderately reduced HGSNAT activity (50 to 70% of wildtype; Fedele and Hopwood, 2010), and suggested that although it was unlikely that the homozygous presence of this variant alone could cause retinal dystrophy, it might act as a modifier in patients with RP due to other genes. HGSNAT activity in blood leukocytes from the 3 Dutch sibs with RP was less than half that of healthy controls, but was slightly higher than that found in patients with MPS3C.
By functional analysis of the A615T and W403C mutations that had been found together in homozygosity in patients with MPS3C, Fedele and Hopwood (2010) found that the mutations function together to abolish HGSNAT activity.
For discussion of the c.398G-C transversion (c.398G-C, NM_152419.2) in the HGSNAT gene, resulting in a gly122-to-ala (G133A) substitution, that was found in compound heterozygous state in 3 Dutch sibs with retinitis pigmentosa (RP73; 616544) by Haer-Wigman et al. (2015), see 610453.0012.
For discussion of the homozygous c.1209G-T transversion (c.1209G-T, NM_152419.2) in the HGSNAT gene, resulting in a trp403-to-cys substitution (W403C), that was originally found in compound heterozygous state in patients with mucopolysaccharidosis type IIIC (MPS3C; 252930) by Hrebicek et al. (2006), see 610453.0012.
Ausseil, J., Landry, K., Seyrantepe, V., Trudel, S., Mazur, A., Lapointe, F., Pshezhetsky, A. V. An acetylated 120-kDa lysosomal transmembrane protein is absent from mucopolysaccharidosis IIIC fibroblasts: a candidate molecule for MPS IIIC. Molec. Genet. Metab. 87: 22-31, 2006. [PubMed: 16293432] [Full Text: https://doi.org/10.1016/j.ymgme.2005.09.021]
Bame, K. J., Rome, L. H. Genetic evidence for transmembrane acetylation by lysosomes. Science 233: 1087-1089, 1986. [PubMed: 3090688] [Full Text: https://doi.org/10.1126/science.3090688]
Canals, I., Elalaoui, S. C., Pineda, M., Delgadillo, V., Szlago, M., Jaouad, I. C., Sefiani, A., Chabas, A., Coll, M. J., Grinberg, D., Vilageliu, L. Molecular analysis of Sanfilippo syndrome type C in Spain: seven novel HGSNAT mutations and characterization of the mutant alleles. Clin. Genet. 80: 367-374, 2011. [PubMed: 20825431] [Full Text: https://doi.org/10.1111/j.1399-0004.2010.01525.x]
Coutinho, M. F., Lacerda, L., Prata, M. J., Ribeiro, H., Lopes, L., Ferreira, C., Alves, S. Molecular characterization of Portuguese patients with mucopolysaccharidosis IIIC: two novel mutations in the HGSNAT gene. (Letter) Clin. Genet. 74: 194-195, 2008. [PubMed: 18518886] [Full Text: https://doi.org/10.1111/j.1399-0004.2008.01040.x]
Fan, X., Zhang, H., Zhang, S., Bagshaw, R. D., Tropak, M. B., Callahan, J. W., Mahuran, D. J. Identification of the gene encoding the enzyme deficient in mucopolysaccharidosis IIIC (Sanfilippo disease type C). Am. J. Hum. Genet. 79: 738-744, 2006. [PubMed: 16960811] [Full Text: https://doi.org/10.1086/508068]
Fedele, A. O., Hopwood, J. J. Functional analysis of the HGSNAT gene in patients with mucopolysaccharidosis IIIC (Sanfilippo C syndrome). Hum. Mutat. 31: E1574-1586, 2010. Note: Electronic Article. [PubMed: 20583299] [Full Text: https://doi.org/10.1002/humu.21286]
Feldhammer, M., Durand, S., Mrazova, L., Boucher, R.-M., Laframboise, R., Steinfeld, R., Wraith, J. E., Michelakakis, H., van Diggelen, O. P., Hrebicek, M., Kmoch, S., Pshezhetsky, A. V. Sanfilippo syndrome type C: mutation spectrum in the heparan sulfate acetyl-CoA: alpha-glucosaminide N-acetyltransferase (HGSNAT) gene. Hum. Mutat. 30: 918-925, 2009. [PubMed: 19479962] [Full Text: https://doi.org/10.1002/humu.20986]
Haer-Wigman, L., Newman, H., Leibu, R., Bax, N. M., Baris, H. N., Rizel, L., Banin, E., Massarweh, A., Roosing, S., Lefeber, D. J., Zonneveld-Vrieling, M. N., Isakov, O., Shomron, N., Sharon, D., Den Hollander, A. I., Hoyng, C. B., Cremers, F. P. M., Ben-Yosef, T. Non-syndromic retinitis pigmentosa due to mutations in the mucopolysaccharidosis type IIIC gene, heparan-alpha-glucosaminide N-acetyltransferase (HGSNAT). Hum. Molec. Genet. 24: 3742-3751, 2015. [PubMed: 25859010] [Full Text: https://doi.org/10.1093/hmg/ddv118]
Hrebicek, M., Mrazova, L., Seyrantepe, V., Durand, S., Roslin, N. M., Noskova, L., Hartmannova, H., Ivanek, R., Cizkova, A., Poupetova, H., Sikora, J., Urinovska, J., and 18 others. Mutations in TMEM76 cause mucopolysaccharidosis IIIC (Sanfilippo C syndrome). Am. J. Hum. Genet. 79: 807-819, 2006. [PubMed: 17033958] [Full Text: https://doi.org/10.1086/508294]
Klein, U., Kresse, H., von Figura, K. Sanfilippo syndrome type C: deficiency of acetyl-CoA: alpha-glucosaminide N-acetyltransferase in skin fibroblasts. Proc. Nat. Acad. Sci. 75: 5185-5189, 1978. [PubMed: 33384] [Full Text: https://doi.org/10.1073/pnas.75.10.5185]
Meikle, P. J., Whittle, A. M., Hopwood, J. J. Human acetyl-coenzyme A:alpha-glucosaminide N-acetyltransferase: kinetic characterization and mechanistic interpretation. Biochem. J. 308: 327-333, 1995. [PubMed: 7755582] [Full Text: https://doi.org/10.1042/bj3080327]
Ruijter, G. J. G., Valstar, M. J., van de Kamp, J. M., van der Helm, R. M., Durand, S., van Diggelen, O. P., Wevers, R. A., Poorthuis, B. J., Pshezhetsky, A. V., Wijburg, F. A. Clinical and genetic spectrum of Sanfilippo type C (MPS IIIC) disease in The (sic) Netherlands. Molec. Genet. Metab. 93: 104-111, 2008. [PubMed: 18024218] [Full Text: https://doi.org/10.1016/j.ymgme.2007.09.011]