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ABCD4
ATP binding cassette subfamily D member 4
ABCD4 encodes a lysosomal membrane protein that transports vitamin B12, enabling its conversion into cofactors required for amino acid and fatty acid metabolism. The ABCD4 gene provides instructions for producing a transporter protein located in the membrane surrounding lysosomes.
ABCD4 is located on the long (q) arm of chromosome 14, at band 14q24.3. Arm ratio per GRCh38 - banding schematic.
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Overview
ABCD4 is located on chromosome 14 and encodes a 606-amino acid protein belonging to the ATP-binding cassette (ABC) transporter family. This protein plays a specialised role in vitamin B12 metabolism by facilitating the vitamin's release from lysosomes. Lysosomes are membrane-bound organelles that house enzymes for degrading and repurposing cellular components.
The ABCD4 transporter enables subsequent processing of vitamin B12 into two active cofactors: adenosylcobalamin, which supports the breakdown of certain amino acids, fatty acids, and cholesterol, and methylcobalamin, which assists in converting homocysteine to methionine. Pathogenic variants in ABCD4 disrupt this transport function, leading to metabolic consequences that affect multiple organ systems. The gene is recognised on several NHS Genomic Medicine Service panels related to intellectual disability and inborn errors of metabolism.
What the gene does
The ABCD4 protein functions as a membrane transporter embedded in the lysosomal envelope. Lysosomes are intracellular structures containing enzymes that digest cellular waste and recycle nutrients. After vitamin B12 enters cells and reaches lysosomes, it must exit these compartments to undergo further metabolic processing.
ABCD4 accomplishes this transport in partnership with LMBD1, another lysosomal membrane protein. Together, these proteins form a functional complex that shuttles vitamin B12 across the lysosomal membrane into the cytoplasm. Once released, the vitamin undergoes enzymatic modifications to generate adenosylcobalamin. Adenosylcobalamin enables methylmalonyl coenzyme A mutase to catalyse degradation of specific substrates including branched-chain amino acids, odd-chain fatty acids, and cholesterol. Vitamin B12 is also converted to methylcobalamin, which catalyses homocysteine's transformation into methionine. Methionine is then used to synthesise proteins and generate S-adenosylmethionine, a crucial methyl donor for numerous cellular reactions.
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Chromosome location
ABCD4 is positioned at chromosomal band 14q24.3 on the long arm of chromosome 14. The precise genomic coordinates and exon structure contribute to the gene's regulation and splicing patterns, though detailed transcript architecture varies across databases.
Protein structure
The ABCD4 protein spans 606 amino acids and contains two functionally important regions. The ABC transmembrane type-1 domain extends from amino acids 39 to 332 and forms the channel through which vitamin B12 passes across the lysosomal membrane. The ABC transporter domain, located at amino acids 389 to 603, binds and hydrolyses ATP to provide the energy required for active transport. This modular architecture is characteristic of ABC transporter family members, which couple ATP hydrolysis to substrate movement across cellular membranes.
Key variants
Pathogenic variants in ABCD4 typically impair the protein's ability to transport vitamin B12 out of lysosomes. This disruption prevents adequate formation of adenosylcobalamin and methylcobalamin, resulting in accumulation of methylmalonic acid and homocysteine. The specific functional consequences depend on variant location and effect on protein folding, stability, or ATP-binding capacity.
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.1125del | p.Trp377fs | Pathogenic | ★★☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.1246_1247del | p.Ser416fs | Pathogenic | ★★☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.1588C>T | p.Gln530Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.423C>G | p.Asn141Lys | Pathogenic/Likely pathogenic | ★★☆☆ | Cobalamin C disease |
c.528C>A | p.Tyr176Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.542+1G>T | - | Pathogenic | ★★☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.1158dup | p.Phe387fs | Pathogenic | ★☆☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.1213_1214dup | p.Asp406fs | Pathogenic | ★☆☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.1276G>A | p.Gly426Ser | Pathogenic | ★☆☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
c.1294del | p.Arg432fs | Pathogenic | ★☆☆☆ | Methylmalonic acidemia with homocystinuria, type cblJ |
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
Variants in ABCD4 cause methylmalonic acidaemia with homocystinuria, cblJ type, a rare metabolic disorder marked by impaired neurodevelopment, visual system defects, haematological abnormalities, and nervous system dysfunction. The cblJ designation reflects the specific defect in vitamin B12 processing that distinguishes this form from other methylmalonic acidaemia subtypes caused by variants in different genes involved in cobalamin metabolism.
No disease links recorded for this gene in our reference set.
UK clinical status
ABCD4 appears on multiple NHS Genomic Medicine Service panels with green confidence ratings, indicating strong evidence for its clinical validity. The gene is included on the DDG2P panel, the Intellectual disability panel (version R29), the Likely inborn error of metabolism panel (version R98), and the Undiagnosed metabolic disorders panel. This panel membership reflects the gene's established role in metabolic disorders presenting with neurodevelopmental features.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What happens when ABCD4 does not function properly?
When ABCD4 function is impaired, vitamin B12 becomes trapped inside lysosomes and cannot be converted into the active cofactors adenosylcobalamin and methylcobalamin. This leads to accumulation of methylmalonic acid and homocysteine, which can affect neurological development, vision, and blood cell production.
How is ABCD4 different from other ABC transporter genes?
ABCD4 is specialised for transporting vitamin B12 across the lysosomal membrane, a unique function within the ABC transporter family. Most other ABC transporters move different substrates or operate in other cellular membranes, such as the peroxisomal membrane or the plasma membrane.
Is ABCD4-related methylmalonic acidaemia inherited?
Methylmalonic acidaemia with homocystinuria caused by ABCD4 variants follows an autosomal recessive inheritance pattern. This means an individual must inherit two pathogenic variants, one from each parent, to develop the condition. Parents who each carry one variant typically do not show symptoms.
References
- Deme JC, Hancock MA, Xia X. Purification and interaction analyses of two human lysosomal vitamin B12 transporters: LMBD1 and ABCD4. Molecular membrane biology. 2014. PMID: 25535791
- Coelho D, Kim JC, Miousse IR. Mutations in ABCD4 cause a new inborn error of vitamin B12 metabolism. Nature genetics. 2012. PMID: 22922874
- Kim JC, Lee NC, Hwu PW. Late onset of symptoms in an atypical patient with the cblJ inborn error of vitamin B12 metabolism: diagnosis and novel mutation revealed by exome sequencing. Molecular genetics and metabolism. 2012. PMID: 23141461