Is X-linked, infantile onset ALG13-related developmental and epileptic encephalopathy a congenital disorder of glycosylation?

Datta Alexandre et al. have generated an excellent review of the phenotype associated with X-linked, infantile onset disease due to ALG13 gene variants in 38 patients.1 The work is quite comprehensive. However, the link between this genetic encephalopathic epilepsy and a congenital disorder of glycosylation (CDG) remains elusive.

ALG13-CDG is an X-linked inborn error of metabolism, presenting with a multisystem phenotype and severe central nervous system symptoms primarily in girls and only a few boys. This is surprising for an X-linked disorder. Also, nearly all patients have normal transferrin glycosylation, which is the routinely used biochemical screening test for CDGs.² The absence of secretory glycan abnormalities in some CDGs, like this one, is not yet fully understood.³

There may be an answer to this enigma. It centers around two important concepts in the field of glycobiology, namely the inherent lethality of severe CDGs due to null mutations in nonredundant glycosylation genes and the concept of cell autonomous toxicity. ALG13 was first identified as a cause of CDG in 2012 in a boy with a de novo, and likely pathogenic, variant c.280A>G, p.(Lys94Glu). He presented with seizures, microcephaly, delayed visual maturation, extrapyramidal signs, hepatomegaly, and a bleeding tendency and died at 1 year of age. This patient manifested evidence of abnormal transferrin glycosylation in plasma, but he stands alone. Of the 38 patients in the paper by Datta Alexandre et al.,¹ 12 had normal transferrin screening and 25 either were not tested or did not have the test reported. Results of one individual were inconclusive. None of these individuals had clearly abnormal transferrin testing indicative of a CDG. Recently, a similar clinical and biochemical analysis of 29 ALG13 patients, who mostly carried the frequent N107S mutation, showed normal glycosylation by either mass spectrometry of transferrin or transferrin isoelectric focusing. However, mutating this universally conserved amino acid in yeast does impair glycosylation.³

Perhaps the reason for seeing normal glycosylation in ALG13 patients is that alternate testing methods need to be utilized. In addition, all of these individuals are hypomorphic from a genetic viewpoint, namely, that the AGL13 gene product is present but not entirely functional.

Because ALG13-CDG is an X-linked disorder, skewed X-inactivation might explain the normal biochemical test results in females; however, this could not be confirmed. Perhaps the disease has a mosaic pattern in liver, where some genetically intact cells compensate for the glycosylation abnormality expected to be seen in blood.

Based on the high prevalence in females, we should also consider that ALG13 pathogenic variants do not lead to loss of function, but rather to gain of function. In this case, glycan abnormalities might be different from a “loss of glycans.” In that case, we are probably not using the right biochemical method to identify the abnormalities affecting glycosylation. Higher resolution methods for free glycan analysis may be better to look for subtle changes, compatible with the biochemical diagnosis of CDG in females with ALG13 defect.

The jury is still out on whether this severe X-linked, infantile onset disease due to ALG13 gene mutations in females is a classic CDG. A better delineation of the biochemical perturbations will probably require the utilization of mass spectrometry analysis of cellular glycoproteins and a suitable human cell line that expresses the ALG13 gene and can be used to compare controls with mutants.