ABCA (ABC1)
The human ABCA subfamily contains 12 full transporters. These
transporters separate into two subgroups based primarily on phylogenetic
analysis and intron structure (Arnould et al., 2001). The first group of
transporters contains seven genes distributed among six chromosomes
(ABCA1, ABCA2, ABCA3, ABCA4, ABCA7, ABCA12, and ABCA13), whereas the
second group comprises five genes (ABCA5, ABCA6, ABCA8, ABCA9, and
ABCA10) on chromosome 17q24.3 (Broccardo et al., 2001). Several ABCA
sub-family genes are associated with human disease. The ABCA1 protein is
definitively involved in disorders of cholesterol transport and HDL
biosynthesis. Tangier Disease (TD), a rare genetic disorder of
lipoprotein metabolism, is caused by mutations in the ABCA1 gene
(Remaley et al., 1999; Rust et al., 1999). This disease presents with
deficient levels of HDL cholesterol and apoprotein A-I causing a myriad
of symptoms, including orange-yellow tonsils, anemia, thrombocytopenia,
peripheral neuropathy, hepatosplenomegaly, lymphadenopathy, and corneal
opacity. Tangier Disease can also be associated with an increased risk
of coronary artery disease (Maranghi et al., 2019). The ABCA4 protein
performs critical steps in the visual cycle by transporting vitamin A
derivatives in the outer segments of rod photoreceptor cells. Pathogenic
variants in ABCA4 cause Stargardt disease (STGD1), which is a
common early-onset maculopathy.