ABCD4 gene

ATP binding cassette subfamily D member 4

The ABCD4 gene provides instructions for making a protein that is involved in the conversion of vitamin B12 (also known as cobalamin) into one of two molecules, adenosylcobalamin (AdoCbl) or methylcobalamin (MeCbl). AdoCbl is required for the normal function of an enzyme known as methylmalonyl CoA mutase. This enzyme helps break down certain protein building blocks (amino acids), fats (lipids), and cholesterol. AdoCbl is called a cofactor because it helps methylmalonyl CoA mutase carry out its function. MeCbl is also a cofactor, but for an enzyme known as methionine synthase. This enzyme converts the amino acid homocysteine to another amino acid, methionine. The body uses methionine to make proteins and other important compounds.

The ABCD4 protein is found in the membrane that surrounds cell structures called lysosomes. Lysosomes are compartments within cells in which enzymes digest and recycle materials. In the lysosomal membrane, the ABCD4 protein interacts with another protein called LMBD1 (produced from the LMBRD1 gene). Together, these two proteins transport vitamin B12 out of lysosomes, making it available for further processing into AdoCbl and MeCbl.

At least five mutations in the ABCD4 gene have been found to cause methylmalonic acidemia with homocystinuria, cblJ type, one form of a disorder that causes developmental delay, eye defects, neurological problems, and blood abnormalities. ABCD4 gene mutations involved in this condition lead to production of an abnormal ABCD4 protein that is unable to function. A shortage of functional ABCD4 protein prevents the release of vitamin B12 from lysosomes, so the vitamin is unavailable for the production of AdoCbl and MeCbl. Because both of these cofactors are missing, the enzymes that require them (methylmalonyl CoA mutase and methionine synthase) do not function normally. As a result, certain amino acids, lipids, and cholesterol are not broken down and homocysteine cannot be converted to methionine. This dual defect results in a buildup of toxic compounds as well as homocysteine, and a decrease in the production of methionine within the body. This combination of imbalances leads to the signs and symptoms of methylmalonic acidemia with homocystinuria.

Cytogenetic Location: 14q24.3, which is the long (q) arm of chromosome 14 at position 24.3

Molecular Location: base pairs 74,285,277 to 74,303,064 on chromosome 14 (Homo sapiens Annotation Release 109, GRCh38.p12) (NCBI)

Cytogenetic Location: 14q24.3, which is the long (q) arm of chromosome 14 at position 24.3
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