Identification through exome sequencing of the first PMM2-CDG individual of Mexican mestizo origin

PMM2 -CDG T237M Mutation in a Patient with Cerebral Palsy-Like Phenotypes Reported from South India

Congenital disorder of glycosylation (CDG) is an autosomal recessively inherited disorder. Hypotonia, stroke-like episodes, and peripheral neuropathy are also associated with the condition that typically develops during infancy. The patient, a 12-year-old girl born to healthy consanguineous parents, was diagnosed with cerebral palsy as a child. The affected patient has hypotonia, inadequate speech, strabismus, and developmental delay with mild mental retardation, which are key symptoms of CDG. Whole-exome sequencing (WES) identified the known missense pathogenic variant PMM2 c.710 C > T, p.T237M in the patient coding for the phosphomannomutase 2 (PMM2) confirming molecular testing of CDG. The patient's parents carried heterozygous PMM2 c.710 C > T variants. This study highlights the importance of WES in patients with a developmental disability or other neurological conditions, which is also useful in screening risk factors in couples with infertility or miscarriage issues.

ALG1-CDG Caused by Non-functional Alternative Splicing Involving a Novel Pathogenic Complex Allele

This study reports on a Mexican mestizo patient with a multi-systemic syndrome including neurological involvement and a type I serum transferrin profile. Clinical exome sequencing revealed complex alleles in ALG1, the encoding gene for the chitobiosyldiphosphodolichol beta-mannosyltransferase that participates in the formation of the dolichol-pyrophosphate-GlcNAc2Man5, a lipid-linked glycan intermediate during N-glycan synthesis. The identified complex alleles were NM_019109.5(ALG1): c.[208 + 16_208 + 19dup; 208 + 25G > T] and NM_019109.5(ALG1): c.[208 + 16_208 + 19dup; 1312C > T]. Although both alleles carried the benign variant c.208 + 16_208 + 19dup, one allele carried a known ALG1 pathogenic variant (c.1312C > T), while the other carried a new uncharacterized variant (c.208 + 25G > T) causing non-functional alternative splicing that, in conjunction with the benign variant, defines the pathogenic protein effect (p.N70S_S71ins9). The presence in the patient’s serum of the pathognomonic N-linked mannose-deprived tetrasaccharide marker for ALG1-CDG (Neu5Acα2,6Galβ1,4-GlcNAcβ1,4GlcNAc) further supported this diagnosis. This is the first report of an ALG1-CDG patient from Latin America.

Non-functional alternative splicing caused by a Latino pathogenic variant in a case of PMM2-CDG

We report on a Mexican mestizo with a multisystemic syndrome including neurological involvement and a type I serum transferrin isoelectric focusing (Tf IEF) pattern. Diagnosis of PMM2-CDG was obtained by clinical exome sequencing (CES) that revealed compound heterozygous variants in PMM2, the encoding gene for the phosphomannomutase 2 (PMM2). This enzyme catalyzes the conversion of mannose-6-P to mannose-1-P required for the synthesis of GDP-Man and Dol-P-Man, donor substrates for glycosylation reactions. The identified variants were c.422G>A (R141H) and c.178G>T, the former being the most frequent PMM2 pathogenic mutation and the latter a previously uncharacterized variant restricted to the Latino population with conflicting interpretations of pathogenicity and that we here report causes leaky non-functional alternative splicing (p.V60Cfs*3).