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ISPD
CDP-L-ribitol pyrophosphorylase A
ISPD is located on the short (p) arm of chromosome 7, at band 7p21.2. Arm ratio per GRCh38 - banding schematic.
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Overview
The ISPD gene, located on chromosome 7, encodes CDP-L-ribitol pyrophosphorylase A, an enzyme essential for a specific type of protein modification called glycosylation. This process involves attaching sugar-like molecules to proteins, which is crucial for their stability and function throughout the body. ISPD participates in the biosynthesis pathway that creates ribitol-phosphate chains, which are then transferred onto a protein called alpha-dystroglycan. This modification is particularly important in muscle tissue and the developing nervous system. Pathogenic variants in ISPD follow an autosomal recessive inheritance pattern, meaning that two altered copies of the gene must be present for disease manifestations to occur.
What the gene does
The ISPD enzyme catalyses the formation of CDP-ribitol from ribitol 5-phosphate and cytidine triphosphate, a reaction that generates an activated sugar donor molecule required for protein glycosylation. This activity is part of a multi-step biochemical pathway that builds complex carbohydrate structures on alpha-dystroglycan, a protein that connects the cellular skeleton inside muscle cells to the extracellular matrix outside the cell. Proper glycosylation of alpha-dystroglycan is essential for maintaining the structural integrity of muscle fibres during contraction and for anchoring neurons in the developing brain. When ISPD function is impaired, the glycosylation pathway cannot proceed normally, resulting in under-glycosylated alpha-dystroglycan that fails to bind its extracellular partners effectively. This disruption particularly affects tissues with high mechanical demands, such as skeletal and cardiac muscle, and regions of active neural migration during foetal development.
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Chromosome location
ISPD is located on the short arm of chromosome 7 at position 21.2, designated as 7p21.2. This chromosomal region contains multiple genes involved in development and metabolism. The precise genomic coordinates and exon structure contribute to the gene's regulation and the potential impact of different variant types.
Protein structure
The ISPD protein comprises 451 amino acids. Structural analysis indicates a disordered region spanning amino acids 1-29 at the N-terminus, which may provide flexibility for protein-protein interactions or regulatory modifications. The remainder of the protein is predicted to adopt a structured fold that accommodates the enzyme's catalytic activity, though detailed crystallographic data for specific functional domains has not been extensively characterised in published literature.
Key variants
Pathogenic variants in ISPD include missense changes that impair enzyme activity, nonsense variants that introduce premature stop codons, and deletions that disrupt the reading frame. The severity of clinical features often correlates with the degree of residual enzyme function, with null variants typically associated with more severe phenotypes. Carrier status for one altered ISPD copy generally does not cause symptoms, as the second functional copy provides sufficient enzyme activity for normal glycosylation.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Biallelic pathogenic variants in ISPD are associated with a spectrum of conditions collectively termed ISPD-related disorders, which fall under the broader category of congenital disorders of glycosylation. These conditions include forms of congenital muscular dystrophy characterised by muscle weakness and wasting from birth or early infancy, often accompanied by structural brain abnormalities such as abnormal folding of the cerebral cortex or underdevelopment of the cerebellum. Some individuals present with limb contractures present at birth, known as arthrogryposis, while others may develop progressive muscle weakness resembling limb-girdle muscular dystrophy. Eye abnormalities and intellectual disability are also reported in a subset of affected individuals, reflecting the widespread importance of proper glycosylation across multiple organ systems.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic ISPD variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
ISPD holds green (high-evidence) status on numerous NHS Genomic Medicine Service panels, reflecting its well-established role in inherited disease. These include the Congenital muscular dystrophy panel, Congenital disorders of glycosylation panel, Intellectual disability panel, and Cerebellar hypoplasia panel. The gene also appears on specialty panels for Arthrogryposis, Limb girdle muscular dystrophies, Malformations of cortical development, Structural eye disease, and Likely inborn error of metabolism. This broad representation across clinical panels underscores the diverse presentations associated with ISPD variants and the importance of considering this gene in differential diagnoses for individuals with complex neuromuscular phenotypes.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
How is ISPD-related disease inherited?
ISPD-related conditions follow autosomal recessive inheritance. An affected individual inherits one pathogenic variant from each parent. Parents who each carry one variant typically do not show symptoms but have a 25% chance with each pregnancy of having an affected child.
What types of specialists manage ISPD-related conditions?
Management typically involves a multidisciplinary team that may include neurologists, geneticists, neuromuscular specialists, physiotherapists, and occupational therapists. The specific team composition depends on the individual's clinical features and needs.
Can carrier screening detect ISPD variants?
Yes, carrier screening can identify individuals who carry one pathogenic ISPD variant. This information is most relevant for family planning, particularly if both partners are found to be carriers, as this would indicate a 25% chance of an affected child with each pregnancy.