A compound heterozygous mutation in DPAGT1 results in a congenital disorder of glycosylation with a relatively mild phenotype

Abstract

Congenital disorders of glycosylation (CDG) are a large group of recessive multisystem disorders caused by impaired protein or lipid glycosylation. The CDG-I subgroup is characterized by protein N-glycosylation defects originating in the endoplasmic reticulum. The genetic defect is known for 17 different CDG-I subtypes. Patients in the few reported DPAGT1-CDG families exhibit severe intellectual disability (ID), epilepsy, microcephaly, severe hypotonia, facial dysmorphism and structural brain anomalies. In this study, we report a non-consanguineous family with two affected adults presenting with a relatively mild phenotype consisting of moderate ID, epilepsy, hypotonia, aggressive behavior and balance problems. Exome sequencing revealed a compound heterozygous missense mutation, c.85A>T (p.I29F) and c.503T>C (p.L168P), in the DPAGT1 gene. The affected amino acids are located in the first and fifth transmembrane domains of the protein. Isoelectric focusing and high-resolution mass spectrometry analyses of serum transferrin revealed glycosylation profiles that are consistent with a CDG-I defect. Our results show that the clinical spectrum of DPAGT1-CDG is much broader than appreciated so far.

Figure 1

Family structure and photographs of the patients. (a) Pedigree of PKMR13. Filled symbols indicate the affected individuals and single horizontal line indicates the marriage connection. The diagonal line indicates deceased individual. (b) Photographs of two affected (III:3 and III:4) individuals presenting with moderate ID, epilepsy, hypotonia, speech problems, aggressiveness and atypical facial dysmorphism.

Figure 2

Structure and location of the DPAGT1 gene on chromosome 11q23.3. The position of the two missense mutations, c.85A>T and c.503T>C is indicated in exons 1 and 4, respectively. Chromatograms revealing the complete segregation of both variations (shaded) with the phenotype in PKMR13.

Figure 3

Schematic representation of DPAGT1 protein and location of the mutations. (a) The white circles indicate hydrophilic residues, whereas gray circles indicate the hydrophobic residues of the protein. The mutations found in this study are indicated by red circles (p.I29F and p.L168P), located in the first and fifth transmembrane domains of the protein, respectively. The mutations that are already described in literature are shown by purple circles. (b) Positions of the amino-acid substitutions I29F and L168P (indicated by pink bar), which are highly conserved down to the Fruitfly, predicting the disruptive effect on the normal function of the protein.

Figure 4

Glycosylation analysis. (a) Transferrin isofocusing for CDG screening. Zero, 2, 4 indicate, respectively, the asialo-, disialo- and tetrasialo-transferrin isoforms. In patient III:3, an increase of asialo- and disialo-transferrin is visible as compared with the control. (b) C8-chip-QTOF mass spectrum of serum immunopurified transferrin. The gray bar indicates the transferrin protein.