Novel variants and clinical symptoms in four new ALG3-CDG patients, review of the literature, and identification of AAGRP-ALG3 as a novel ALG3 variant with alanine and glycine-rich N-terminus.

ALG3-CDG is one of the very rare types of congenital disorder of glycosylation (CDG) caused by variants in the ER-mannosyltransferase ALG3. Here, we summarize the clinical, biochemical, and genetic data of four new ALG3-CDG patients, who were identified by a type I pattern of serum transferrin and the accumulation of Man5 GlcNAc2 -PP-dolichol in LLO analysis. Additional clinical symptoms observed in our patients comprise sensorineural hearing loss, right-descending aorta, obstructive cardiomyopathy, macroglossia, and muscular hypertonia. We add four new biochemically confirmed variants to the list of ALG3-CDG inducing variants: c.350G>C (p.R117P), c.1263G>A (p.W421*), c.1037A>G (p.N346S), and the intron variant c.296+4A>G. Furthermore, in Patient 1 an additional open-reading frame of 141 bp (AAGRP) in the coding region of ALG3 was identified. Additionally, we show that control cells synthesize, to a minor degree, a hybrid protein composed of the polypeptide AAGRP and ALG3 (AAGRP-ALG3), while in Patient 1 expression of this hybrid protein is significantly increased due to the homozygous variant c.160_196del (g.165C>T). By reviewing the literature and combining our findings with previously published data, we further expand the knowledge of this rare glycosylation defect.

Fatal outcome after heart surgery in PMM2-CDG due to a rare homozygous gene variant with double effects.

Variants in Phosphomannomutase 2 (PMM2) lead to PMM2-CDG, the most frequent congenital disorder of glycosylation (CDG). We here describe the disease course of a ten-month old patient who presented with the classical PMM2-CDG symptoms as cerebellar hypoplasia, retinitis pigmentosa, seizures, short stature, hepato- and splenomegaly, anaemia, recurrent vomiting and inverted mamillae. A severe form of tetralogy of Fallot was diagnosed and corrective surgery was performed at the age of 10 months. At the end of the cardiopulmonary bypass, a sudden oedematous reaction of the myocardium accompanied by biventricular pump failure was observed immediately after heparin antagonization with protamine sulfate. The patient died seven days after surgery, since myocardial function did not recover on ECMO support. We here describe the first patient carrying the homozygous variant g.18313A > T in the PMM2 gene (NG_009209.1) that either can lead to c.394A > T (p.I132F) or even loss of 100 bp due to exon 5 skipping (c.348_447del; p.G117Rfs*4) which is comparable to a null allele. Proliferation and doubling time of the patient's fibroblasts were affected. In addition, we show that the induction of cellular stress by elevating the cell culture temperature to 40 °C led to a decrease of the patients' PMM2 transcript as well as PMM2 protein levels and subsequently to a significant loss of residual activity. We assume that metabolic stressful processes occurring after cardiac surgery led to the drop of the patient's PMM activity below a life-sustaining niveau which paved the way for the fatal outcome.