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A4GALT
alpha 1,4-galactosyltransferase (P1PK blood group)
A4GALT encodes a glycosyltransferase enzyme responsible for synthesising the P1PK blood group antigens on red blood cells and other cell surfaces. The A4GALT gene provides instructions for producing alpha 1,4-galactosyltransferase, an enzyme that adds galactose sugar molecules to glycolipids on cell membranes.
A4GALT is located on the long (q) arm of chromosome 22, at band 22q13.2. Arm ratio per GRCh38 - banding schematic.
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Overview
A4GALT is located on chromosome 22 and encodes an enzyme belonging to the glycosyltransferase family. The protein functions primarily to transfer galactose residues onto specific glycolipid precursors, creating antigens that define the P1PK blood group system. These antigens appear on red blood cells and various tissue cell surfaces throughout the body.
Whilst the P1PK blood group has historically been of interest in transfusion medicine, natural variation in A4GALT activity is common in healthy populations. The gene demonstrates polymorphic variation that influences enzyme expression levels and subsequent antigen presentation, contributing to the diversity of blood group phenotypes observed across different ancestries.
What the gene does
The A4GALT enzyme catalyses the addition of galactose to globoside glycolipid structures through an alpha-1,4 linkage. This biochemical reaction occurs within the Golgi apparatus, where the enzyme transfers galactose from UDP-galactose donor molecules onto paragloboside acceptor substrates. The resulting product determines whether an individual expresses the P1 antigen phenotype.
Enzymatic activity depends on the presence of divalent metal cations and requires precise positioning of the donor and acceptor molecules within the catalytic site. The transferase demonstrates specificity for particular glycolipid structures, distinguishing it from other galactosyltransferases that modify different carbohydrate chains. Variation in enzyme activity levels, whether due to genetic differences or regulatory factors, directly influences the density of P1 antigens displayed on cell surfaces.
Beyond blood group determination, the glycolipids modified by A4GALT participate in cell-cell recognition processes and may serve as receptors for certain pathogens, though the physiological significance of P1PK antigens in non-transfusion contexts remains an area of ongoing investigation.
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Chromosome location
A4GALT resides on the long arm of chromosome 22 at position 22q13.2, a region near the telomere of the chromosome. The precise genomic coordinates place it within a gene-rich area that contains numerous other loci involved in diverse cellular functions. The gene structure has not been extensively characterised in publicly available databases regarding specific exon counts or alternative transcript isoforms.
Protein structure
The A4GALT protein comprises 353 amino acids and contains a DXD motif spanning amino acids 192-194. This short sequence motif is characteristic of many glycosyltransferases and plays a critical role in coordinating metal ion binding and positioning the UDP-galactose donor substrate during catalysis. The aspartate residues within the DXD motif chelate divalent cations, typically manganese or magnesium, which stabilise the nucleotide-sugar donor and facilitate the transfer reaction. The broader domain architecture beyond this catalytic motif has not been experimentally characterised in detail for this protein in the available structural data.
Key variants
Genetic variation in A4GALT contributes to the range of P1PK blood group phenotypes observed in human populations. Common polymorphisms can reduce or abolish enzyme activity, resulting in p or P2 phenotypes that lack certain P1PK antigens. These variants are generally benign and represent normal human diversity rather than pathogenic changes. Rare variants that completely eliminate enzyme function may result in the p phenotype, characterised by absence of P, P1, and Pk antigens, though individuals with this phenotype are typically healthy.
Sample of pathogenic variants
2 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 |
|---|---|---|---|---|
GRCh37/hg19 22q13.2(chr22:43059474-43145703)x1 | - | Pathogenic | - | not provided |
c.631C>G | p.Gln211Glu | Pathogenic | - | NOR polyagglutination syndrome |
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
A4GALT variation is not currently associated with recognised genetic disorders in clinical genetics practice. The primary significance of A4GALT polymorphisms lies in blood group serology and transfusion compatibility considerations. Individuals with rare phenotypes may produce antibodies against P1PK antigens they lack, which can occasionally complicate blood transfusion or, in pregnant individuals, contribute to haemolytic disease of the foetus and newborn in specific circumstances. However, these scenarios are uncommon and do not constitute inherited conditions in the conventional sense.
No disease links recorded for this gene in our reference set.
UK clinical status
Frequently asked questions
What is the P1PK blood group system?
The P1PK blood group system is a classification based on specific carbohydrate antigens present on red blood cells and other tissues. The A4GALT gene determines whether an individual expresses the P1 antigen, with common variants leading to P1-positive or P1-negative phenotypes.
Does A4GALT variation cause any health conditions?
Variation in A4GALT does not typically cause health conditions. Most polymorphisms represent normal human diversity in blood group expression. Rare individuals with complete absence of P1PK antigens are generally healthy, though they may require special consideration for blood transfusions.
Is A4GALT included in NHS genetic testing panels?
A4GALT is not currently included in NHS Genomic Medicine Service gene panels for inherited disease diagnosis. Testing for this gene is typically performed in blood bank settings for transfusion compatibility purposes rather than in clinical genetics contexts.