Lysosomal Storage Disorders: Molecular Basis and Therapeutic Approaches

Expanding the phenotypic and genotypic spectrum of patients with HGSNAT-related retinopathy

BACKGROUND

Variants in HGSNAT have historically been associated with syndromic mucopolysaccharidosis type IIIC (MPSIIIC) but more recent studies demonstrate cases of HGSNAT-related non-syndromic retinitis pigmentosa. We describe and expand the genotypic and phenotypic spectrum of this disease.

MATERIALS AND METHODS

This is a retrospective, observational, case series of 11 patients with pericentral retinitis pigmentosa due to variants in HGSNAT gene without a syndromic diagnosis of MPSIIIC. We reviewed ophthalmologic data extracted from medical records, genetic testing, color fundus photos, fundus autofluorescence (FAF), and optical coherence tomography (OCT).

RESULTS

Of the 11 patients, the mean age was 52 years (range: 26-78). The mean age of ophthalmologic symptoms onset was 45 years (range: 15-72). The visual acuity varied from 20/20 to 20/80 (mean 20/30 median 20/20). We described five novel variants in HGSNAT: c.715del (p.Arg239Alafs *37), c.118 G>A (p.Asp40Asn), c.1218_1220delinsTAT, c.1297A>G (p.Asn433Asp), and c.1726 G>T (p.Gly576*).

CONCLUSIONS

HGSNAT has high phenotypic heterogeneity. Data from our cohort showed that all patients who had at least one variant of c.1843 G>A (p.Ala615Thr) presented with the onset of ocular symptoms after the fourth decade of life. The two patients with onset of ocular symptoms before the fourth decade did not carry this variant. This may suggest that c.1843 G>A variant is associated with a later onset of retinopathy.

Newborn Screening of 6 Lysosomal Storage Disorders by Tandem Mass Spectrometry

This study was designed to screen 6 lysosomal storage diseases (LSDs) in neonates using tandem mass spectrometry (MS/MS), and establish cutoff values for these LSDs with 3000 dried blood spots (DBS) samples. Cutoff values for α-L-iduronidase (IDUA), α-galactosidase (GLA), acid beta glucosidase (ABG), β-galactocerebrosidase (GALC), acid sphingomyelinase (ASM), and acid alpha glucosidase (GAA) were as follows: GLA, > 2.06 μmol/L·h; ABG, > 1.78 μmol/L·h; ASM, > 0.99 μmol/L·h; IDUA, > 1.33 μmol/L·h; GALC, > 0.84 μmol/L·h; and GAA, > 2.06 μmol/L·h. There were 30 positives in initial MS/MS screening test, and 15 samples were still positive with repeat testing. Their parents/guardians were recontacted and DBS samples were collected again for test. Only 1 child showed abnormal GAA enzyme activity after recontacting process, and was diagnosed with Pompe disease after genetic screening. Eventually, cutoff values of 6 specific enzyme activities were established and MS/MS is effective for early LSDs screening.