Characterization of the 415G>A (E139K) PMM2 mutation in carbohydrate-deficient glycoprotein syndrome type Ia disrupting a splicing enhancer resulting in exon 5 skipping

Carbohydrate-deficient glycoprotein syndrome type Ia (CDGS) is an autosomal recessive disorder, characterized by a central nervous system dysfunction and multiorgan failure associated with defective N-glycosylation and phosphomannomutase (PMM) deficiency related to mutations in the PMM2 gene. A total of 26 different missense mutations and one single base pair deletion have already been described. We found by sequencing and restriction analysis, in two unrelated French patients with CDG type Ia a compound heterozygosity for two mutations in exon 5: a new mutation 415G>A (E139K) and the most frequent mutation 425G>A (R141H ). The 415G>A mutation disrupted a splicing enhancer sequence: (GAR)n-(GAR)n resulting in exon 5 skipping. We studied the activity of these mutant proteins expressed in E Coli. Compared to the normal PMM protein activity, the R141H and transcript without exon 5 expressed a protein with undetectable specific activity when the E139K mutant protein expressed a residual activity of 25%. The E139K mutant protein could be expressed at a sufficient level in vivo to confer residual activity compatible with life in these patients when absence of residual PMM activity is likely lethal.