A frequent mild mutation in ALG6 may exacerbate the clinical severity of patients with congenital disorder of glycosylation Ia (CDG-Ia) caused by phosphomannomutase deficiency

Abstract

Single nucleotide polymorphisms occur throughout the human genome. A gene that causes one of the congenital disorders of glycosylation (CDG) has a mutation (911T-->C ) that changes a phenylalanine to serine at position 304 (F304S) of the alpha 1,3 glucosyl transferase. We show that this change reduces the ability of the gene product to rescue defective glycosylation of an alg6-deficient strain of Saccharomyces cerevisiae during rapid growth. This finding suggested that the mutation might affect glycosylation in humans. We therefore compared the frequency of this variant in 301 controls and in 101 CDG patients who carry known mutations in other genes involved in CDG, i.e. PMM2 (CDG-Ia; 91 patients) and MPI (CDG-Ib; 10 patients). The variant allele frequency is identical in both CDG patients (0.30) and controls (0.28). Importantly, the F304S genotype frequency in 55 CDG-Ia patients classified as mild/moderate (n = 28), or severe (n = 27) was significantly higher in severely affected patients (0.41) than in mild/moderately affected patients (0.21). Mortality (n = 9) was higher when F304S was present (n = 6). Severely affected patients with the PMM2 mutations F119L/R141H (n = 22) carry the F304S mutation more often (0.36) than mildly affected patients (0.18, n = 11) with this mutation. Clinical severity of mildly affected sibs with the same PMM2 mutations did not correlate with F304S genotype. Thus, the presence of the F304S allele may exacerbate the clinical outcome, especially in severely affected CDG patients. We speculate that this type of variant may be implicated in other multi-factorial disorders that involve N-glycosylation.

Muscle glycogenosis with low phosphorylase kinase activity: mutations in PHKA1, PHKG1 or six other candidate genes explain only a minority of cases

“…It is also possible that the PHKB amino-acid replacements found here predispose for the manifestation of muscle disease in combination with another genetic defect. 49 These possibilities cannot be tested in vitro and the small number of cases allows no statistically valid conclusion. The analysis of more muscle glycogenosis cases may clarify whether these or other PHKB sequence variants are indeed enriched in this patient population.…”

Insights into complexity of congenital disorders of glycosylation

“…Two point mutations c.391T>C in exon 5 and c.911T>C in exon 10 resulting in p.Y131H and p. F304S, respectively, although assumed being single nucleotide polymorphisms (rs35383149 and rs17856039) (31,32) are particularly interesting. It was shown that p.F304S polymorphism may exacerbate the clinical outcome of other CDGs, especially in severely aff ected patients (33). For the p. Y131H substitution it is still unclear whether the homozygous form c.391C/C is suffi cient to cause the disease or some additional genetic alteration has to be present (31).…”

Identification of a frequent variant inALG6, the cause of Congenital Disorder of Glycosylation-Ic

“…The Saccharomyces cerevisiae strain YG227 (Reiss et al, 1996) deficient in ALG6 (a kind gift from Markus Aebi) was grown in standard YPD and SC media (Sherman, 1991). The α1,3 glucosyltransferase complementation using patients ALG6 was done as described (Westphal et al, 2000a;Westphal et al, 2002).…”

“…Analysis of cell lines: Dermal fibroblast cultures were derived from control and patients, as described previously (Westphal et al, 2002). Metabolic labeling of various cell lines with [2-3 H] Mannose and HPLC analysis of the LLO chains was done as previously indicated.…”

“…Analysis of ALG6 mutations (GenBank: AF102851.1): Genomic and cDNA was prepared from documented CDG patient fibroblasts as previously indicated (Westphal et al, 2002). Genomic DNA from control volunteers, patients with documented Crohn's disease and colitis, patients thought to have mitochondrial disorders, and parents of the patient described in this report were prepared from leukocytes essentially as described previously (Westphal et al, 2001).…”