Galactokinase deficiency: lessons from the GalNet registry.

PURPOSE: Galactokinase (GALK1) deficiency is a rare hereditary galactose metabolism disorder. Beyond cataract, the phenotypic spectrum is questionable. Data from affected patients included in the Galactosemias Network registry were collected to better characterize the phenotype. METHODS: Observational study collecting medical data of 53 not previously reported GALK1 deficient patients from 17 centers in 11 countries from December 2014 to April 2020. RESULTS: Neonatal or childhood cataract was reported in 15 and 4 patients respectively. The occurrence of neonatal hypoglycemia and infection were comparable with the general population, whereas bleeding diathesis (8.1% versus 2.17-5.9%) and encephalopathy (3.9% versus 0.3%) were reported more often. Elevated transaminases were seen in 25.5%. Cognitive delay was reported in 5 patients. Urinary galactitol was elevated in all patients at diagnosis; five showed unexpected Gal-1-P increase. Most patients showed enzyme activities ≤1%. Eleven different genotypes were described, including six unpublished variants. The majority was homozygous for NM_000154.1:c.82C>A (p.Pro28Thr). Thirty-five patients were diagnosed following newborn screening, which was clearly beneficial. CONCLUSION: The phenotype of GALK1 deficiency may include neonatal elevation of transaminases, bleeding diathesis, and encephalopathy in addition to cataract. Potential complications beyond the neonatal period are not systematically surveyed and a better delineation is needed.

Small aminothiol compounds improve the function of Arg to Cys variant proteins: effect on the human cystathionine ß-synthase p.R336C.

The key regulatory point of L-methionine (Met) and L-homocysteine (Hcy) degradation is catalyzed by cystathionine beta-synthase (CBS). CBS deficiency is caused by mutations in CBS gene, often resulting in protein misfolding. The prevalence of CBS deficiency in Qatar is 1/1800, ∼200-fold higher than the worldwide prevalence of 1/344 000. Almost all patients bear the CBS p.R336C variant. More than 20 years ago, it was shown in vitro that two unrelated protein variants with a substitution of an arginine (Arg) residue by cysteine (Cys) could be rescued by cysteamine (mercaptoethylamine), likely via formation of a disulfide between Cys and cysteamine, functionally mimicking the wild-type (WT) Arg side-chain. Based on these findings, we aimed to study whether cysteamine was able to improve the function of p.R336C CBS variant. Additionally, we tested the effect of mercaptoethylguanidine (MEG), a compound with a guanidino and a thiol function that may resemble Arg structure better than cysteamine. Three purified recombinant CBS proteins (p.R336C, p.R336H and WT) were pre-incubated with cysteamine, MEG or Cys (as negative control), and CBS activity and stability were measured. Pre-incubation with cysteamine and MEG increased the enzymatic activity of the p.R336C protein, which was absent upon pre-incubation with Cys. The WT and the p.R336H variant enzyme activity presented no increase with any of the tested compounds. Our results show that cysteamine and MEG are able to specifically improve the function of the CBS p.R336C variant, suggesting that any Arg-to-Cys substitution accessible to these small molecules may be converted back to a moiety resembling Arg.

Brain function in classic galactosemia, a galactosemia network (GalNet) members review.

Classic galactosemia (CG, OMIM #230400, ORPHA: 79,239) is a hereditary disorder of galactose metabolism that, despite treatment with galactose restriction, affects brain function in 85% of the patients. Problems with cognitive function, neuropsychological/social emotional difficulties, neurological symptoms, and abnormalities in neuroimaging and electrophysiological assessments are frequently reported in this group of patients, with an enormous individual variability. In this review, we describe the role of impaired galactose metabolism on brain dysfunction based on state of the art knowledge. Several proposed disease mechanisms are discussed, as well as the time of damage and potential treatment options. Furthermore, we combine data from longitudinal, cross-sectional and retrospective studies with the observations of specialist teams treating this disease to depict the brain disease course over time. Based on current data and insights, the majority of patients do not exhibit cognitive decline. A subset of patients, often with early onset cerebral and cerebellar volume loss, can nevertheless experience neurological worsening. While a large number of patients with CG suffer from anxiety and depression, the increased complaints about memory loss, anxiety and depression at an older age are likely multifactorial in origin.