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SLC4A4
solute carrier family 4 member 4
SLC4A4 is located on the long (q) arm of chromosome 4, at band 4q13.3. Arm ratio per GRCh38 - banding schematic.
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Overview
SLC4A4 belongs to the solute carrier family 4, a group of genes encoding membrane transport proteins involved in moving ions across cellular boundaries. The protein produced by SLC4A4 functions as an electrogenic sodium-bicarbonate cotransporter, primarily expressed in the kidneys and eyes. This transporter plays an essential role in reclaiming bicarbonate from the renal tubules, preventing excessive acid accumulation in the blood.
Pathogenic changes in SLC4A4 follow an autosomal recessive inheritance pattern, meaning affected individuals carry two altered copies of the gene. These variants impair the protein's ability to transport bicarbonate effectively, resulting in a metabolic acidosis characterised by reduced blood pH. The condition typically presents in infancy or early childhood with growth delays, ocular features, and biochemical abnormalities.
What the gene does
The SLC4A4 protein operates as an electrogenic sodium-bicarbonate cotransporter, moving sodium ions and bicarbonate ions together across the cell membrane with a net charge transfer. In the proximal tubules of the kidney, the transporter sits on the basolateral surface of epithelial cells, where it moves bicarbonate from inside the cell into the bloodstream whilst simultaneously importing sodium. This process is fundamental to the kidney's role in maintaining systemic acid-base equilibrium.
The protein's activity is regulated through interactions with carbonic anhydrase enzymes, which catalyse the conversion between carbon dioxide and bicarbonate. The SLC4A4 transporter also contributes to fluid and electrolyte homeostasis by coupling bicarbonate movement to sodium flux, creating an electrochemical gradient that drives further ion transport processes. In the eye, the protein supports proper development and maintenance of ocular structures, though the precise mechanisms remain under investigation. Disruption of these transport functions leads to bicarbonate wasting in urine and systemic acidification.
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Chromosome location
SLC4A4 is located on chromosome 4 at position q13.3, within the long arm of the chromosome. The gene spans a substantial genomic region and encodes multiple transcript variants through alternative splicing, producing protein isoforms with differing tissue distributions. The chromosomal region 4q13.3 contains several genes involved in transport and metabolic functions.
Protein structure
The SLC4A4 protein comprises 1079 amino acids organised into transmembrane and cytoplasmic domains that facilitate its function as a bicarbonate transporter. The N-terminal region includes a segment required for interaction with AHCYL1 spanning amino acids 1-62, which may play a role in regulatory processes. Three disordered regions are present at amino acids 39-78, 238-265, and 1012-1079, providing flexibility that may be important for conformational changes during transport cycles.
The protein contains an interaction region with carbonic anhydrase 4 (CA4) at amino acids 748-779, enabling functional coupling between bicarbonate generation and transport. Two CA2-binding regions at amino acids 1002-1004 and 1030-1033 facilitate association with carbonic anhydrase 2, another enzyme critical for bicarbonate production. A short motif required for basolateral targeting at amino acids 1057-1059 ensures proper localisation of the transporter to the appropriate membrane surface in polarised epithelial cells.
Key variants
Pathogenic variants in SLC4A4 are distributed across the gene and typically result in loss of transporter function. Most disease-causing changes are missense variants that alter single amino acids within functionally important regions, disrupting either the transport mechanism itself or the protein's ability to reach the cell membrane. Nonsense and frameshift variants also occur, generally producing truncated proteins lacking essential domains.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Variants in SLC4A4 cause proximal renal tubular acidosis, an autosomal recessive condition characterised by the kidney's inability to reclaim bicarbonate from the tubular fluid. Affected individuals typically present in infancy or early childhood with failure to thrive, vomiting, and biochemical evidence of metabolic acidosis with normal anion gap. Ocular manifestations are common, including band keratopathy, cataracts, and glaucoma, which can lead to visual impairment if untreated. Some individuals also develop dental abnormalities and short stature as secondary features of chronic acidosis.
- Proximal renal tubular acidosis (SLC4A4) Dedicated page coming soon
Inheritance pattern
Conditions caused by pathogenic SLC4A4 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
SLC4A4 is included on multiple NHS Genomic Medicine Service gene panels, reflecting its clinical importance in inherited renal and ocular conditions. The gene holds green classification status on the Renal tubulopathies panel (R198), indicating strong evidence for its role in kidney disease. It also appears with green status on the Intellectual disability panel (R29) and the Structural eye disease panel (R36), recognising the broader phenotypic spectrum associated with SLC4A4 variants. Additionally, SLC4A4 is listed in the Developmental Disorders Genotype-to-Phenotype database (DDG2P) with green classification.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
How is SLC4A4-related renal tubular acidosis inherited?
The condition follows an autosomal recessive inheritance pattern. An affected individual inherits one altered copy of SLC4A4 from each parent, who are typically unaffected carriers. Each child of two carrier parents has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of inheriting two working copies.
What is the difference between SLC4A4 and other genes causing renal tubular acidosis?
SLC4A4 encodes a sodium-bicarbonate cotransporter in the proximal tubule, whilst other genes associated with renal tubular acidosis affect different segments of the kidney or distinct transport mechanisms. For example, some genes affect distal tubule function or involve different ion channels, leading to variations in biochemical presentation and associated features.
Can SLC4A4 variants affect only the eyes without kidney involvement?
Most individuals with pathogenic SLC4A4 variants present with both renal and ocular features, as the protein functions in both organ systems. However, the severity of kidney versus eye involvement can vary between affected individuals, and phenotypic expression may depend on the specific variants present and other genetic or environmental factors.