A novel variant in ALG1 gene associated with congenital disorder of glycosylation: A case report and short literature review

BACKGROUND

The congenital disorder of glycosylation associated with ALG1 (ALG1-CDG) is a rare autosomal recessive disease. Due to the deficiency of β1,4 mannosyltransferase caused by pathogenic variants in ALG1 gene, the assembly and processing of glycans in the protein glycosylation pathway are impaired, resulting in a broad clinical spectrum with multi-organ involvement. To raise awareness of clinicians for its manifestations and genotype, we here reported a new patient with a novel variant in ALG1 gene and reviewed the literature to study the genotype-phenotype correlation.

METHOD

Clinical characteristics were collected, and clinical exome sequencing was used to identify the causative variants. MutationTaster, PyMol, and FoldX were used to predict the pathogenicity, changes in 3D model molecular structure of protein, and changes of free energy caused by novel variants.

RESULTS

The proband was a 13-month-old Chinese Han male characterized by epileptic seizures, psychomotor development delay, muscular hypotonia, liver and cardiac involvement. Clinical exome sequencing revealed the biallelic compound heterozygosity variants, a previously reported variant c.434G>A (p.G145N, paternal) and a novel variant c.314T>A (p.V105N, maternal). The literature review found that in severe phenotypes, the incidences of clinical manifestations were significantly higher than that in mild phenotypes, including congenital nephrotic syndrome, agammaglobulinemia, and severe hydrops. Homozygous c.773C>T was a strongly pathogenic variant associated with a severe phenotype. When heterozygous for c.773C>T, patients with another variant leading to substitution in amino acids within the strongly conserved regions (c.866A>T, c.1025A>C, c.1182C>G) may cause a more severe phenotype than those within less-conserved regions (c.434G>A, c.450C>G, c.765G>A, c.1287T>A). c.1129A>G, c.1076C>T, and c.1287T>A were more likely to be associated with a mild phenotype. The assessment of disease phenotypes requires a combination of genotype and clinical manifestations.

CONCLUSIONS

The case reported herein adds to the mutations identified in ALG1-CDG and a review of this literature expands the study of the phenotypic and genotypic spectrum of this disorder.

Podocyte-Specific Sialylation-Deficient Mice Serve as a Model for Human FSGS

BACKGROUND

The etiology of steroid-resistant nephrotic syndrome, which manifests as FSGS, is not completely understood. Aberrant glycosylation is an often underestimated factor for pathologic processes, and structural changes in the glomerular endothelial glycocalyx have been correlated with models of nephrotic syndrome. Glycans are frequently capped by sialic acid (Sia), and sialylation's crucial role for kidney function is well known. Human podocytes are highly sialylated; however, sialylation's role in podocyte homeostasis remains unclear.

METHODS

We generated a podocyte-specific sialylation-deficient mouse model (PCmas^-/- ) by targeting CMP-Sia synthetase, and used histologic and ultrastructural analysis to decipher the phenotype. We applied CRISPR/Cas9 technology to generate immortalized sialylation-deficient podocytes (asialo-podocytes) for functional studies.

RESULTS

Progressive loss of sialylation in PCmas^-/- mice resulted in onset of proteinuria around postnatal day 28, accompanied by foot process effacement and loss of slit diaphragms. Podocyte injury led to severe glomerular defects, including expanded capillary lumen, mesangial hypercellularity, synechiae formation, and podocyte loss. In vivo, loss of sialylation resulted in mislocalization of slit diaphragm components, whereas podocalyxin localization was preserved. In vitro, asialo-podocytes were viable, able to proliferate and differentiate, but showed impaired adhesion to collagen IV.

CONCLUSIONS

Loss of cell-surface sialylation in mice resulted in disturbance of podocyte homeostasis and FSGS development. Impaired podocyte adhesion to the glomerular basement membrane most likely contributed to disease development. Our data support the notion that loss of sialylation might be part of the complex process causing FSGS. Sialylation, such as through a Sia supplementation therapy, might provide a new therapeutic strategy to cure or delay FSGS and potentially other glomerulopathies.

Congenital nephrotic syndrome in an infant with ALG1-congenital disorder of glycosylation

Congenital nephrotic syndrome (NS) in the newborn is most frequently related to mutations in genes specific for structural integrity of the glomerular basement membrane and associated filtration structures within the kidney, resulting in massive leakage of plasma proteins into the urine. Occurrence of congenital NS in a multi-system syndrome is less common. We describe the case of an infant with deteriorating neurological status, seizures, edema, and proteinuria who was found to have a mutation in gene ALG1 and a renal biopsy consistent with congenital NS. Furthermore, we briefly review rare existing case reports documenting congenital NS in patients with mutations in ALG1, and treatment strategies, including novel use of peritoneal dialysis.

Genetic causes of focal segmental glomerulosclerosis: implications for clinical practice

Focal segmental glomerulosclerosis (FSGS) is a common cause of steroid-resistant nephrotic syndrome in children and adults. Although FSGS is considered a podocyte disease, the aetiology is diverse. In recent years, many inheritable genetic forms of FSGS have been described, caused by mutations in proteins that are important for podocyte function. In the present commentary, we review these genetic causes of FSGS and describe their prevalence in familial and sporadic FSGS. In routine clinical practice, the decision to perform the costly DNA analysis should be based on the assessment if the results affect the care of the individual patient with respect to the evaluation of extra-renal manifestations, treatment decisions, transplantation and genetic counselling.