Spliceosome malfunction causes neurodevelopmental disorders with overlapping features

Pre-mRNA splicing is a highly coordinated process. While its dysregulation has been linked to neurological deficits, our understanding of the underlying molecular and cellular mechanisms remains limited. We implicated pathogenic variants in U2AF2 and PRPF19, encoding spliceosome subunits in neurodevelopmental disorders (NDDs), by identifying 46 unrelated individuals with 23 de novo U2AF2 missense variants (including 7 recurrent variants in 30 individuals) and 6 individuals with de novo PRPF19 variants. Eight U2AF2 variants dysregulated splicing of a model substrate. Neuritogenesis was reduced in human neurons differentiated from human pluripotent stem cells carrying two U2AF2 hyper-recurrent variants. Neural loss of function (LoF) of the Drosophila orthologs U2af50 and Prp19 led to lethality, abnormal mushroom body (MB) patterning, and social deficits, which were differentially rescued by wild-type and mutant U2AF2 or PRPF19. Transcriptome profiling revealed splicing substrates or effectors (including Rbfox1, a third splicing factor), which rescued MB defects in U2af50-deficient flies. Upon reanalysis of negative clinical exomes followed by data sharing, we further identified 6 patients with NDD who carried RBFOX1 missense variants which, by in vitro testing, showed LoF. Our study implicates 3 splicing factors as NDD-causative genes and establishes a genetic network with hierarchy underlying human brain development and function.

Understanding the role of SGLT2 inhibitors in glycogen storage disease type Ib: the experience of one UK centre

BACKGROUND

Glycogen storage disease type Ib (GSD Ib) is a severe disorder of carbohydrate metabolism due to bi-allelic variants in SLC37A4. It is associated with neutropaenia and neutrophil dysfunction, which has recently been attributed to the accumulation of 1,5-anhydroglucitol-6-phosphate (1,5AG6P) within neutrophils. Treatment with sodium-glucose co-transporter-2 (SGLT2) inhibitors, such as empagliflozin, is a novel therapy that reduces 1,5-anhydroglucitol (1,5AG) in plasma.

RESULTS

We report our experience in treating 8 paediatric GSD Ib patients with empagliflozin with a cumulative treatment time greater than 12 years. Treatment with a median dose of 5 mg (0.22 mg/kg height weight) of empagliflozin resulted in improvement in bowel health, growth, and laboratory parameters. Plasma 1,5AG levels reduced by a median of 78%. Baseline 1,5AG levels in our cohort were higher than in adult patients with GSD Ib. Hypoglycaemia on empagliflozin treatment occurred in 50% of our cohort.

CONCLUSION

We report the largest single centre cohort of GSD Ib patients treated with empagliflozin to date. Treatment with SGLT2 inhibitors is a novel and favourable treatment option for neutropaenia and neutrophil dysfunction in GSD Ib. We suggest a low starting dose of empagliflozin with careful titration due to the risk of hypoglycaemia. The interpretation of 1,5AG levels and their role in treatment monitoring is yet to be established, and requires ongoing research.

Pathogenic variants in glutamyl-tRNAGln amidotransferase subunits cause a lethal mitochondrial cardiomyopathy disorder

Mitochondrial protein synthesis requires charging a mitochondrial tRNA with its amino acid. Here, the authors describe pathogenic variants in the GatCAB protein complex genes required for the generation of glutaminyl-mt-tRNA^Gln, that impairs mitochondrial translation and presents with cardiomyopathy.

Mitochondrial protein synthesis requires charging mt-tRNAs with their cognate amino acids by mitochondrial aminoacyl-tRNA synthetases, with the exception of glutaminyl mt-tRNA (mt-tRNA^Gln). mt-tRNA^Gln is indirectly charged by a transamidation reaction involving the GatCAB aminoacyl-tRNA amidotransferase complex. Defects involving the mitochondrial protein synthesis machinery cause a broad spectrum of disorders, with often fatal outcome. Here, we describe nine patients from five families with genetic defects in a GatCAB complex subunit, including QRSL1, GATB, and GATC, each showing a lethal metabolic cardiomyopathy syndrome. Functional studies reveal combined respiratory chain enzyme deficiencies and mitochondrial dysfunction. Aminoacylation of mt-tRNA^Gln and mitochondrial protein translation are deficient in patients’ fibroblasts cultured in the absence of glutamine but restore in high glutamine. Lentiviral rescue experiments and modeling in S. cerevisiae homologs confirm pathogenicity. Our study completes a decade of investigations on mitochondrial aminoacylation disorders, starting with DARS2 and ending with the GatCAB complex.