CSGALNACT1-congenital disorder of glycosylation: A mild skeletal dysplasia with advanced bone age

Abstract

Congenital disorders of glycosylation (CDGs) comprise a large number of inherited metabolic defects that affect the biosynthesis and attachment of glycans. CDGs manifest as a broad spectrum of disease, most often including neurodevelopmental and skeletal abnormalities and skin laxity. Two patients with biallelic CSGALNACT1 variants and a mild skeletal dysplasia have been described previously. We investigated two unrelated patients presenting with short stature with advanced bone age, facial dysmorphism, and mild language delay, in whom trio-exome sequencing identified novel biallelic CSGALNACT1 variants: compound heterozygosity for c.1294G>T (p.Asp432Tyr) and the deletion of exon 4 that includes the start codon in one patient, and homozygosity for c.791A>G (p.Asn264Ser) in the other patient. CSGALNACT1 encodes CSGalNAcT-1, a key enzyme in the biosynthesis of sulfated glycosaminoglycans chondroitin and dermatan sulfate. Biochemical studies demonstrated significantly reduced CSGalNAcT-1 activity of the novel missense variants, as reported previously for the p.Pro384Arg variant. Altered levels of chondroitin, dermatan, and heparan sulfate moieties were observed in patients' fibroblasts compared to controls. Our data indicate that biallelic loss-of-function mutations in CSGALNACT1 disturb glycosaminoglycan synthesis and cause a mild skeletal dysplasia with advanced bone age, CSGALNACT1-CDG.

Keywords: CSGALNACT1-CDG; CSGalNAcT-1; advanced bone age; cartilage and brain development; glycosaminoglycan; joint laxity; macrocephaly; proteoglycan; short stature.

Figure 1

Radiological features in two unrelated patients with CSGALNACT1‐CDG. (a–g) Neonatal skeletal radiographs in P2 showing: (a, b) advanced carpotarsal bone age; (a–e, g) short and plump long bones, narrow chest, and coronal clefting of vertebrae; and (f) trident‐shaped acetabula, and monkey wrench appearance of the proximal femur. (h–l) Follow‐up skeletal radiographs in P2 showing scoliosis and pectus excavatum: (h, i, l) age 4 years and 10 months; (j, k) age 3 years and 10 months. (m–o) Mildly advanced bone age in P3 at the age of 7 months (m, n); vanishing by the age of 9 years and 9 months (o)

Figure 2

Clinical features in two unrelated patients with CSGALNACT1‐CDG. (a) P2 at the age of 2 days presenting with relative macrocephaly with frontal bossing, midface hypoplasia, anteverted nares, downslanting palpebral fissures, ankyloglossia, dysplastic ears, rhizomelia, narrow chest, brachydactyly, and single palmar crease. (b) Aggravation of the phenotype with age. P2 at the age of 5 years showing marked disproportionate stature, macrocephaly, pectus excavatum, and skin laxity. (c) P3 at the age of 10 years with camptodactyly, mild skeletal anomalies and facial gestalt similar to P2 with short and downslanting palpebral fissures, midface hypoplasia, flat nasal bridge, prominent nasal tip, and dysplastic external ears

Figure 3

Identification and characterization of CSGALNACT1 variants. (a) Simplified pedigrees showing segregation of identified CSGALNACT1 variants. Individuals carrying biallelic CSGALNACT1 variants are indicated by solid symbols, whereas unaffected individuals are indicated as open symbols. (b) Schematic of the human CSGALNACT1 locus with exons displayed as gray (untranslated) and black (translated) boxes and the localization of reported and newly identified CSGALNACT1 variants on complementary DNA and protein level indicated. (c) The two newly identified and one previously reported CSGALNACT1 missense variants affect invariantly conserved amino acids. (d) Schematic illustration of GAG synthesis with emphasis on CSGALNACT1. (e) In vitro GalNAc‐transferase activities of p.Asp432Tyr‐CSGalNAcT‐1 and of p.Asn264Ser‐CSGalNAcT‐1 from cell lysates were significantly decreased towards chondroitin as the acceptor compared to that of wild‐type (WT)‐CSGalNAcT‐1. Values are the means ± standard error (n = 3). *p < .0005 and **p < .0001 versus WT were calculated by the analysis of variance (ANOVA) Dunnett test. Expression levels of the recombinant CSGalNAcT‐1 proteins were estimated by comparing the chemiluminescence intensity using the standard curve generated with concentration‐defined 3×FLAG‐bovine alkaline phosphatase. Two clones of variants p.Asn264Ser are shown. (f) CS/DS‐stub antibodies showed a significantly reduced binding to patients’ fibroblasts as compared to the control subject, indicating that the number of CS/DS chains were reduced in the fibroblast cells from P1 and P2. *p < .0005 versus control (Cont) were calculated by the ANOVA Dunnett test