Wide clinical spectrum in ALG8-CDG: clues from molecular findings suggest an explanation for a milder phenotype in the first-described patient

BACKGROUND: Congenital disorders of glycosylation (CDG) includes ALG8 deficiency, a protein N-glycosylation defect with a broad clinical spectrum. If most of the 15 previously reported patients present an early-onset multisystem severe disease and early death, three patients including the cas princeps, present long-term survival and less severe symptoms. METHODS: In order to further characterize ALG8-CDG, two new ALG8 patients are described and mRNA analyses of the ALG8-CDG cas princeps were effected. RESULTS: One new patient exhibited a hepato-intestinal and neurological phenotype with two novel variants (c.91A > C p.Thr31Pro; c.139dup p.Thr47Asnfs*12). The other new patient, homozygous for a known variant (c.845C > T p.Ala282Val), presented a neurological phenotype with epilepsy, intellectual disability and retinis pigmentosa. The cas princeps ALG8-CDG patient was reported to have two heterozygous frameshift variants predicted to be without activity. We now described a novel ALG8 transcript variant in this patient and the 3D model of the putative encoded protein reveals no major difference with that of the normal ALG8 protein. CONCLUSION: The description of the two new ALG8 patients affirms that ALG8-CDG is a severe disease. In the cas princeps, as the originally described frameshift variants are degraded, the novel variant is promoted and could explain a milder phenotype