Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease.

Together with its ß-subunit OSTM1, ClC-7 performs 2Cl-/H+ exchange across lysosomal membranes. Pathogenic variants in either gene cause lysosome-related pathologies, including osteopetrosis and lysosomal storage. CLCN7 variants can cause recessive or dominant disease. Different variants entail different sets of symptoms. Loss of ClC-7 causes osteopetrosis and mostly neuronal lysosomal storage. A recently reported de novo CLCN7 mutation (p.Tyr715Cys) causes widespread severe lysosome pathology (hypopigmentation, organomegaly, and delayed myelination and development, "HOD syndrome"), but no osteopetrosis. We now describe two additional HOD individuals with the previously described p.Tyr715Cys and a novel p.Lys285Thr mutation, respectively. Both mutations decreased ClC-7 inhibition by PI(3,5)P2 and affected residues lining its binding pocket, and shifted voltage-dependent gating to less positive potentials, an effect partially conferred to WT subunits in WT/mutant heteromers. This shift predicts augmented pH gradient-driven Cl- uptake into vesicles. Overexpressing either mutant induced large lysosome-related vacuoles. This effect depended on Cl-/H+-exchange, as shown using mutants carrying uncoupling mutations. Fibroblasts from the p.Y715C patient also displayed giant vacuoles. This was not observed with p.K285T fibroblasts probably due to residual PI(3,5)P2 sensitivity. The gain of function caused by the shifted voltage-dependence of either mutant likely is the main pathogenic factor. Loss of PI(3,5)P2 inhibition will further increase current amplitudes, but may not be a general feature of HOD. Overactivity of ClC-7 induces pathologically enlarged vacuoles in many tissues, which is distinct from lysosomal storage observed with the loss of ClC-7 function. Osteopetrosis results from a loss of ClC-7, but osteoclasts remain resilient to increased ClC-7 activity.

Quantification of N-acetyl-l-aspartate in dried blood spots: A simple and fast LC-MS/MS neonatal screening method for the diagnosis of Canavan disease.

BACKGROUND: Canavan disease is a devastating neurometabolic disorder caused by accumulation of N acetylaspartate in brain and body fluids due to genetic defects in the aspartoacylase gene (ASPA). New gene therapies are on the horizon but will require early presymptomatic diagnosis to be fully effective. METHODS: We therefore developed a fast and highly sensitive liquid chromatography mass spectrometry (LC-MS/MS)-based method for quantification of N-acetylaspartate in dried blood spots and established reference ranges for neonates and older controls. With this test, we investigated 45 samples of 25 Canavan patients including 8 with a neonatal sample. RESULTS: Measuring N-acetylaspartate concentration in dried blood with this novel test, all Canavan patients (with variable severity) were well separated from the control group (median; range: 5.7; 1.6-13.6 µmol/L [n = 45] vs 0.44; 0.24-0.99 µmol/L [n = 59] (p < 0.05)). There was also no overlap when comparing neonatal samples of Canavan patients (7.3; 5.1-9.9 µmol/L [n = 8]) and neonatal controls (0.93; 0.4-1.8 µmol/L [n = 784]) (p < 0.05). CONCLUSIONS: We have developed a new LC-MS/MS-based screening test for early postnatal diagnosis of Canavan disease that should be further evaluated in a population-based study once a promising treatment becomes available. The method meets the general requirements of newborn screening and should be appropriate for multiplexing with other screening approaches that combine chromatographic and mass spectrometry techniques.

Higher precision, first tier newborn screening for metachromatic leukodystrophy using 16:1-OH-sulfatide.

Newborn screening (NBS) for metachromatic leukodystrophy (MLD) is based on first-tier measurement of sulfatides in dried blood spots (DBS) followed by second-tier measurement of arylsulfatase A in the same DBS. This approach is very precise with 0-1 false positives per ∼30,000 newborns tested. Recent data reported here shows that the sulfatide molecular species with an α-hydroxyl, 16­carbon, mono-unsaturated fatty acyl group (16:1-OH-sulfatide) is superior to the original biomarker 16:0-sulfatide in reducing the number of first-tier false positives. This result is consistent across 4 MLD NBS centers. By measuring 16:1-OH-sulfatide alone or together with 16:0-sulfatide, the estimated false positive rate is 0.048% and is reduced essentially to zero with second-tier arylsulfatase A activity assay. The false negative rate is predicted to be extremely low based on the demonstration that 40 out of 40 newborn DBS from clinically-confirmed MLD patients are detected with these methods. The work shows that NBS for MLD is extremely precise and ready for deployment. Furthermore, it can be multiplexed with several other inborn errors of metabolism already tested in NBS centers worldwide.

Repurposing SGLT2 inhibitors: Treatment of renal proximal tubulopathy in Fanconi-Bickel syndrome with empagliflozin.

Renal proximal tubulopathy in Fanconi-Bickel syndrome is caused by impaired basolateral glucose transport via GLUT2 and consequently, intracellular accumulation of glucose and glycogen. SGLT2 inhibitors act on apical glucose reabsorption of renal proximal tubular cells. The purpose of this study was to retrospectively describe the first experiences with repurposing the SGLT2 inhibitor empagliflozin to treat the generalized tubulopathy in Fanconi-Bickel syndrome. A case series was conducted of seven persons from five families (five males, two females; three children, who were 14y5m, 2y9m, and 1y6m old) with genetically confirmed Fanconi-Bickel syndrome, off-label treated with empagliflozin. Median (range) age at start of empagliflozin was 27 years (1y6m - 61y) and duration of follow-up under empagliflozin treatment was 169 days (57-344). Under empagliflozin (up to 25 mg/d), biochemical parameters of tubular cell integrity (urinary N-acetyl-glucosaminidase) and/or tubular functions (including urinary α1-microglobulin) improved in all persons with Fanconi-Bickel syndrome, albeit to varying degrees. Clinically, supplementations (i.e., phosphate, alkali, carnitine, and alfacalcidol) could be completely discontinued in the three children, whereas results in the four adult patients were more variable and not as significant. Empagliflozin was well-tolerated and no symptomatic hypoglycemia was observed. In conclusion, SGLT2 inhibitors such as empagliflozin shift the metabolic block in Fanconi-Bickel syndrome, that is, they intervene specifically in the underlying pathophysiology and can thus attenuate renal proximal tubulopathy, especially when started in early childhood.

Treatment Outcomes for Maple Syrup Urine Disease Detected by Newborn Screening.

OBJECTIVE: Maple syrup urine disease (MSUD), a life-threatening metabolic disorder, is included in newborn screening (NBS) programs worldwide. The study aims to evaluate the impact of NBS on the long-term outcome of MSUD patients. METHODS: We performed a prospective, national, multicenter, observational study. RESULTS: In the studied NBS cohort (N = 33; 22 classic MSUD [cMSUD], 11 variant MSUD [vMSUD]; median age at last visit 10.4 years), 32 (97%) patients survived, 58% of them had normal cognitive functions (median IQ 87). Initial peak leucine increased linearly with age in cMSUD (median: 1712 µmol/L), but not in vMSUD. Global IQ correlated inversely with the initial peak leucine concentration (P = .04; ß = -0.0081) and the frequency of decompensations (P = .02; ß = -9.133). A cluster analysis identified 2 subgroups differing in their long-term metabolic control (median leucine concentration: 162 vs 278 µmol/L; P < .001). In cMSUD, lower leucine concentrations were associated with a higher IQ (95.5 vs 80; P = .008). Liver transplantation (median age 5.8 years) was not associated with better cognitive outcome. NBS is highly sensitive for cMSUD, but vMSUD might be missed (N = 2 missed by NBS). CONCLUSIONS: NBS and the early start of treatment improve survival and long-term outcome in individuals with cMSUD. Disease severity is an important modifier of outcome; however, the time to NBS report and the quality of long-term metabolic control had an independent impact on cognitive outcome, highlighting the importance of an early diagnosis and the quality of treatment.