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ALG5
ALG5 dolichyl-phosphate beta-glucosyltransferase
ALG5 encodes an enzyme that attaches glucose molecules to growing sugar chains during protein glycosylation, a process critical for cellular function. The ALG5 gene provides instructions for making dolichyl-phosphate beta-glucosyltransferase, an enzyme that catalyses a specific step in the assembly of glycan structures.
ALG5 is located on the long (q) arm of chromosome 13, at band 13q13.3. Arm ratio per GRCh38 - banding schematic.
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Overview
The ALG5 gene is located on chromosome 13 and encodes dolichyl-phosphate beta-glucosyltransferase, an enzyme integral to the protein glycosylation pathway. Glycosylation refers to the process by which carbohydrate groups are covalently attached to proteins, a modification that influences protein stability, localisation, and biological activity. The ALG5 enzyme operates within the endoplasmic reticulum, where it transfers a glucose residue from dolichyl-phosphate-glucose to a lipid-linked oligosaccharide precursor. This precursor oligosaccharide is subsequently transferred en bloc to newly synthesised proteins, enabling proper folding and quality control. Pathogenic variants in ALG5 impair this glycosylation process, resulting in congenital disorders of glycosylation that can manifest with diverse clinical features including developmental delay, neurological abnormalities, and organ dysfunction. Understanding ALG5 function provides insight into fundamental cellular biochemistry and the molecular basis of inherited metabolic conditions.
What the gene does
The ALG5 protein functions as a dolichyl-phosphate beta-glucosyltransferase, catalysing the addition of the first of three glucose residues to the growing dolichol-linked oligosaccharide in the endoplasmic reticulum lumen. This enzymatic reaction is essential for N-linked glycosylation, one of the most common post-translational modifications in eukaryotic cells. The substrate for ALG5 is a lipid-linked heptasaccharide structure assembled on the cytoplasmic face of the endoplasmic reticulum membrane and subsequently flipped into the lumen. ALG5 transfers glucose from dolichyl-phosphate-glucose to the mannose residue at a specific position on this oligosaccharide, forming a glucosylated intermediate that serves as substrate for subsequent glycosyltransferases.
The glucosylation steps facilitated by ALG5 and related enzymes are critical for the function of calnexin and calreticulin, lectin chaperones that recognise mono-glucosylated glycans and assist in protein folding. Without proper glucose addition, newly synthesised glycoproteins cannot engage effectively with the endoplasmic reticulum quality-control machinery, leading to misfolding and potential aggregation. This quality-control system ensures that only correctly folded proteins exit the endoplasmic reticulum and proceed through the secretory pathway to their final cellular destinations.
ALG5 belongs to the larger family of glycosyltransferases, enzymes that catalyse the transfer of sugar moieties from activated donor molecules to specific acceptor substrates. The enzyme exhibits specificity for both its sugar donor, dolichyl-phosphate-glucose, and its acceptor substrate, the lipid-linked oligosaccharide intermediate. Proper coordination of ALG5 activity with upstream and downstream enzymes in the glycosylation pathway is essential for efficient oligosaccharide assembly. Disruption of ALG5 function creates a bottleneck in this assembly line, reducing the availability of fully assembled oligosaccharides and thereby compromising the glycosylation of numerous cellular proteins that depend on this modification for stability and function.
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Chromosome location
The ALG5 gene is situated on the long arm of chromosome 13 at cytogenetic band 13q13.3. This chromosomal region contains multiple genes involved in diverse cellular processes. The genomic structure of ALG5 includes several exons that are transcribed and spliced to generate the mature messenger RNA encoding the 324-amino-acid protein. The precise chromosomal location of ALG5 at 13q13.3 has been confirmed through genomic mapping studies and is consistent across human reference genome assemblies. This region of chromosome 13 does not harbour particularly high densities of disease-associated genes compared to some other chromosomal segments, though genes in this vicinity contribute to various metabolic and structural cellular functions. Knowledge of the gene's chromosomal position aids in genetic testing strategies and helps contextualise findings from chromosomal microarray or sequencing studies that may identify deletions or rearrangements affecting this locus.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein. The ALG5 protein comprises 324 amino acids and is predicted to be an integral membrane protein residing in the endoplasmic reticulum, consistent with its role in processing lipid-linked oligosaccharides. Membrane topology predictions suggest the presence of transmembrane helices that anchor the protein within the endoplasmic reticulum membrane, positioning catalytic residues appropriately for interaction with both the lipid-linked substrate and the dolichyl-phosphate-glucose donor. The enzyme's active site is presumed to face the endoplasmic reticulum lumen, where the later steps of oligosaccharide assembly occur. Structural studies of related glycosyltransferases indicate that these enzymes typically adopt a conserved fold that accommodates both donor and acceptor substrates, facilitating efficient catalysis. Although high-resolution structural data for ALG5 itself remain limited, comparison with homologous enzymes provides insight into probable catalytic mechanisms and substrate recognition motifs. The compact size of the protein and its integration into the membrane environment reflect the specialised function of adding a single glucose residue during a complex multi-step biosynthetic pathway.
Key variants
Pathogenic variants in ALG5 disrupt the enzyme's ability to transfer glucose to the lipid-linked oligosaccharide, impairing N-linked glycosylation. Research suggests that most disease-causing variants are missense changes that alter critical amino acids within the catalytic or substrate-binding regions, though nonsense and frameshift variants have also been reported. The inheritance pattern for ALG5-related conditions is autosomal recessive, meaning that affected individuals typically carry two pathogenic variants, one inherited from each parent. Carriers, who possess one pathogenic variant and one functional copy, generally do not exhibit clinical features, as the remaining functional allele provides sufficient enzyme activity for normal glycosylation under typical physiological conditions.
The spectrum of variants identified in ALG5 includes changes that reduce but do not completely abolish enzyme activity, as well as variants that result in profound loss of function. Genotype-phenotype correlations are emerging, with some evidence suggesting that variants retaining residual enzyme activity may be associated with milder or later-onset phenotypes. Diagnostic laboratories assess ALG5 variants using a combination of computational prediction tools, biochemical assays measuring glycosylation patterns, and functional studies in cellular models. Glycosylation analysis of serum transferrin is a common screening test that can reveal abnormal glycan profiles indicative of congenital disorders of glycosylation, prompting further gene-specific testing to identify the underlying genetic cause.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.289C>T | p.Arg97Ter | Pathogenic | ★☆☆☆ | Polycystic kidney disease 7 |
c.774G>A | p.Trp258Ter | Pathogenic | ★☆☆☆ | Polycystic kidney disease 7 |
c.235C>T | p.Arg79Trp | Pathogenic | - | Polycystic kidney disease 7 |
c.623G>A | p.Arg208His | Pathogenic | - | Polycystic kidney disease 7 |
c.635G>A | p.Arg212His | Pathogenic | - | Polycystic kidney disease 7 |
c.703_704del | p.Gln235fs | Pathogenic | - | Polycystic kidney disease 7 |
c.773G>A | p.Trp258Ter | Pathogenic | - | Polycystic kidney disease 7 |
c.115C>T | p.Arg39Ter | Likely pathogenic | ★☆☆☆ | Polycystic kidney disease 7 |
c.239-2A>G | - | Likely pathogenic | ★☆☆☆ | Polycystic kidney disease 7 |
c.672G>A | p.Trp224Ter | Likely pathogenic | ★☆☆☆ | Polycystic kidney disease 7 |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Pathogenic variants in ALG5 cause a subtype of congenital disorder of glycosylation, historically designated CDG-Ic. Congenital disorders of glycosylation encompass a group of inherited metabolic conditions characterised by defective glycosylation of proteins and lipids. Individuals with ALG5-related congenital disorder of glycosylation may present with a range of clinical features, including developmental delay, intellectual disability, hypotonia, seizures, and hepatic dysfunction. The severity and specific manifestations can vary considerably between affected individuals, even among those sharing identical ALG5 variants, reflecting the complexity of glycosylation pathways and potential modifying genetic or environmental factors.
Because glycosylation is a ubiquitous cellular process affecting numerous proteins across diverse tissues, ALG5 deficiency has multi-system consequences. Neurological involvement is common, with affected children often experiencing motor and cognitive delays. Additional features may include coagulation abnormalities, feeding difficulties, and structural abnormalities detectable on brain imaging. The diagnosis of ALG5-related congenital disorder of glycosylation typically involves biochemical testing to detect abnormal glycosylation patterns, followed by molecular genetic testing to confirm pathogenic variants in the ALG5 gene. Early identification enables appropriate clinical management and genetic counselling for families regarding recurrence risk in future pregnancies.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic ALG5 variants typically follow autosomal dominant inheritance.
Each child has a 50% chance of inheriting the pathogenic variant, regardless of sex.
UK clinical status
Within the NHS Genomic Medicine Service, the ALG5 gene is included on the Cystic Kidney Disease panel with a green classification. Green-rated genes have a definitive evidence base linking variants to the panel's phenotypic scope, meaning that testing ALG5 in appropriate clinical contexts is considered diagnostically robust. Although ALG5 is primarily associated with congenital disorders of glycosylation affecting multiple systems, renal manifestations have been documented in some individuals with pathogenic ALG5 variants, justifying the gene's inclusion on panels evaluating kidney pathology. The presence of ALG5 on this NHS panel ensures that patients presenting with cystic kidney disease or related renal phenotypes will have this gene analysed as part of comprehensive genomic testing. Clinicians utilise PanelApp classifications to guide test selection and interpretation, with green genes representing high-confidence associations that inform clinical decision-making and cascade testing within families.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
No specific dietary or lifestyle interventions have been established to prevent or modify the course of ALG5-related congenital disorders of glycosylation. Because the underlying defect involves a fundamental enzymatic step in glycosylation, general nutritional support and symptomatic management form the cornerstone of care. Some research in related glycosylation disorders has explored the potential role of dietary supplements such as mannose in bypassing certain enzymatic defects, though such approaches are condition-specific and have not been shown to be effective for ALG5 deficiency.
Families managing a child with an ALG5-related condition typically work closely with multidisciplinary clinical teams, including metabolic specialists, neurologists, and dietitians, to optimise nutritional intake and address feeding difficulties that may arise. Maintaining adequate hydration and caloric intake is important, particularly in individuals with hepatic involvement or failure to thrive. Physical and occupational therapy may support motor development and daily functioning. While no lifestyle modification can reverse the genetic defect, supportive care tailored to each individual's clinical presentation can improve quality of life and developmental outcomes. Families are encouraged to discuss any proposed dietary changes or supplements with their healthcare provider to ensure safety and appropriateness for the individual's specific medical needs.
Supplement considerations
There is no conclusive evidence that any dietary supplement can prevent or treat ALG5-related congenital disorder of glycosylation. The enzyme deficiency caused by pathogenic ALG5 variants cannot be corrected through nutritional supplementation, as the problem lies in the protein's structure and catalytic activity rather than in substrate availability. Some experimental therapies have been investigated in other forms of congenital disorders of glycosylation, such as oral mannose for MPI-CDG, but these approaches target specific metabolic defects that differ from the ALG5 enzymatic step.
Parents and carers considering vitamin or mineral supplements for an affected child should consult with the child's metabolic or genetics specialist before initiating any supplementation. Standard multivitamins may be recommended as part of general nutritional support, particularly if dietary intake is inadequate due to feeding difficulties. However, there is no evidence that megadoses of specific nutrients or alternative supplements alter the course of ALG5 deficiency. Unproven supplements may carry risks, including interactions with prescribed medications or exacerbation of metabolic imbalances. Evidence-based management focuses on symptomatic and supportive care rather than supplementation aimed at correcting the glycosylation defect. Any consideration of novel therapeutic agents should occur within the context of clinical research protocols, ensuring appropriate monitoring and evaluation of efficacy and safety.
Frequently asked questions
What does the ALG5 gene do?
The ALG5 gene encodes an enzyme that adds a glucose molecule to a lipid-linked sugar chain during protein glycosylation in the endoplasmic reticulum. This step is essential for proper protein folding and quality control within cells.
How is ALG5-related congenital disorder of glycosylation inherited?
ALG5-related conditions follow an autosomal recessive inheritance pattern. Affected individuals inherit two pathogenic variants, one from each parent. Parents who are carriers typically do not have symptoms.
What symptoms might occur with ALG5 variants?
Individuals with pathogenic ALG5 variants may experience developmental delay, intellectual disability, hypotonia, seizures, and liver dysfunction. The severity and range of features can vary considerably among affected individuals.
Can ALG5 variants be detected before symptoms appear?
ALG5 variants can be identified through genetic testing, including newborn screening programmes in some settings or carrier screening for prospective parents. Biochemical tests measuring glycosylation patterns may also suggest a disorder before genetic confirmation.
Is there a cure for ALG5-related glycosylation disorder?
There is currently no cure for ALG5-related congenital disorder of glycosylation. Management focuses on supportive care, addressing symptoms, and optimising development through multidisciplinary clinical support.
Why is ALG5 included on the NHS Cystic Kidney Disease panel?
ALG5 is included on the NHS Cystic Kidney Disease panel because some individuals with pathogenic variants in this gene have presented with renal manifestations, including cystic kidney features, alongside other systemic symptoms.