A genetic and clinical study of individuals with nonsyndromic retinopathy consequent upon sequence variants in HGSNAT, the gene associated with Sanfilippo C mucopolysaccharidosis

Abstract

Pathogenic variants in the gene HGSNAT (heparan-α-glucosaminide N-acetyltransferase) have been reported to underlie two distinct recessive conditions, depending on the specific genotype, mucopolysaccharidosis type IIIC (MPSIIIC)-a severe childhood-onset lysosomal storage disorder, and adult-onset nonsyndromic retinitis pigmentosa (RP). Here we describe the largest cohort to-date of HGSNAT-associated nonsyndromic RP patients, and describe their retinal phenotype, leukocyte enzymatic activity, and likely pathogenic genotypes. We identified biallelic HGSNAT variants in 17 individuals (15 families) as the likely cause of their RP. None showed any other symptoms of MPSIIIC. All had a mild but significant reduction of HGSNAT enzyme activity in leukocytes. The retinal condition was generally of late-onset, showing progressive degeneration of a concentric area of paramacular retina, with preservation but reduced electroretinogram responses. Symptoms, electrophysiology, and imaging suggest the rod photoreceptor to be the cell initially compromised. HGSNAT enzymatic testing was useful in resolving diagnostic dilemmas in compatible patients. We identified seven novel sequence variants [p.(Arg239Cys); p.(Ser296Leu); p.(Phe428Cys); p.(Gly248Ala); p.(Gly418Arg), c.1543-2A>C; c.1708delA], three of which were considered to be retina-disease-specific alleles. The most prevalent retina-disease-specific allele p.(Ala615Thr) was observed heterozygously or homozygously in 8 and 5 individuals respectively (7 and 4 families). Two siblings in one family, while identical for the HGSNAT locus, but discordant for retinal disease, suggest the influence of trans-acting genetic or environmental modifying factors.

Keywords: HGSNAT; inherited retinal disease; retinopathy.

FIGURE 1

Retinal imaging—color, autofluorescence (Optos), foveal optical coherence tomography (OCT) (Heidelberg Spectralis) for patients MEH3, MEH5, MEH9, MEH10 (a)–(d) and for (e) affected (MEH12) and (f) unaffected (MEH13) siblings HOM for p.Ala615Th, and for patients NEI2 and NEI4 (g and h)

FIGURE 2

Full‐field ERG and PERG findings in cases MEH1 (a), MEH4 (b), MEH5 (c) and MEH12 (d) compared with a representative unaffected control subject (e). Full‐field ERGs include the dark‐adapted (DA) ERGs (flash strengths 0.01 and 10.0 cd s/m²; DA 0.01 and DA 10.0) and light‐adapted (LA) ERGs for a flash strength of 3.0 cd s/m² (LA 3.0; 30 and 2 Hz). The PERG is recorded to an alternating chequerboard (15 × 11°). The full‐field ERGs show evidence of similar rod and cone dysfunction (a), rod > cone dysfunction (b and d) or a loss of rod function with preserved peripheral cone system function (c). PERG P50 shows reduction, in keeping with severe (a, b) or relatively mild (c, d) macular dysfunction. There was a high degree of interocular symmetry and recordings are shown from the right eye only. Patient traces are superimposed to demonstrate reproducibility. Broken lines replace blink artifacts for clarity

FIGURE 3

Multiple organism sequence alignment of regions spanning the R239, G248, S296, G418, and F428 amino acids of the HGSNAT protein, showing their conservation, highly conserved in red and moderately conserved in blue. Alignment was performed using https://www.uniprot.org/align/. Protein sequences used for alignment are Q68QP4–2 in the human, KZBY75 in the chimp, H9EWF5 in the Rhesus macaque, F1NBK1in the cat, M3X793 in the mouse, Q3UDW8 in the chicken, F1Q893 in the zebrafish, and A0A0A1XN23 in the Melon fruit fly

FIGURE 4

Schematic representation of the HGSNAT gene showing the localization and distribution of variants associated with MPSIIIC and with nonsyndromic retinitis pigmentosa (RP). Exonic variants are represented below and intronic variants above. Each vertical dot represents a unique variant. Blue dots represent the 71 variants associated with MPSIIIC described to date (Martins et al., 2019). Red dots represent the 8 reported variants associated with nonsyndromic RP in HGSNAT of which 6 were novel (see text). Green dots represent the 12 variants associated with nonsyndromic retinitis pigmentosa in HGSNAT described in this study, 7 of which are novel (see Table 2)