Functional data in the mouse and other model organisms
As nearly all human ABC genes have a one-to-one ortholog in the mouse genome, murine knockouts and other modified alleles represent an excellent source of functional information in animal models for ABC gene diseases. Mutant alleles have been generated for all murine ABC genes allowing for phenotypic screens. Specific phenotypes are reported for 46 of the 53 murine ABC genes (Table 2 ). The reported phenotypes for mouse ABC genes are highly diverse. They include abnormal lipid, cholesterol, and glucose levels, development of specific organ systems such as the eye, adrenal gland, lung, liver, thyroid, male reproductive tract, heart, spleen, thymus, arteries, lymphocytes, and brain. In mice, disruption (homozygous knockout) of the Abca3 , Abcb7 ,Abcb10 , Abce1 , and Abcf1 genes results in embryonic lethality, and Cftr /Abcc7 , Abcc9 , and thetrac allele in Abcg5 are associated with premature lethality (Chase et al., 2010).
Many mouse ABC gene alleles recapitulate features of human Mendelian disorders or known human ABC gene function. ABCA1 mutations in humans cause Tangier disease characterized by the defective formation of HDL particles, and Abca1 -/- mice also have HDL deficiency. ABCA3 transports lipids into the lung’s lamellar bodies, and mutations cause severe neonatal lung surfactant deficiency;Abca3 -/- mice have abnormal lung development and morphology. ABCA4 flips retinoid-lipid complexes in photoreceptor disks (Molday, Zhong, & Quazi, 2009). ABCA4 mutations cause Stargardt disease and related maculopathies, characterized by excessive lipofuscin (A2E) accumulation (Allikmets et al., 1997).Abca4 -/- mice also accumulate lipofuscin/A2E, have abnormal dark adaptation, and thinning of the outer nuclear layer (Mata et al., 2001). ABCA12 is expressed in the lamellar granules of the skin, and mutations cause two forms of recessive congenital ichthyosis (Kelsell et al., 2005; Lefevre et al., 2003).Abca12 -/- mice also have abnormal scaly skin. Interestingly, Abca12 -/- mice also have lung surfactant deficiency, lamellar body abnormalities, and alveolar collapse, demonstrating an essential function in lipid transport in the mouse lung.
ABCB1 encodes P-glycoprotein, an efflux transporter found overexpressed in many chemotherapy multidrug-resistant tumor cell lines and plays a vital role in eliminating xenobiotics.ABCB1 -/- collie dogs have sensitivity to ivermectin toxicity (Mealey, Bentjen, Gay, & Cantor, 2001), andAbcb1a /Abcb1b -/- mice have an abnormal distribution of compounds in the brain (Mason, Pariante, & Thomas, 2008). The TAP1 and TAP2 proteins (ABCB2 and ABCB3) together form a transporter for peptides subsequently loaded onto class I HLA molecules, and mutations in either gene cause immunodeficiency in humans, abnormal T cell levels, and antigen presentation in the mouse (de la Salle et al., 1994; Van Kaer, Ashton-Rickardt, Ploegh, & Tonegawa, 1992). ABCB4 transports phospholipids in bile and protein defects lead to recessive intrahepatic cholestasis in both humans and mice.Abcb4 -/- mice also have extensive bone developmental abnormalities (Dixon et al., 2000; Hochrath et al., 2013).
ABCC2 transports organic anions, including bile salts, and recessive mutations cause Dubin-Johnson syndrome, a hereditary hyperbilirubinemia (Wada et al., 1998). A rat model, the TR rat, andAbcc2 -/- mice have abnormal liver weight and physiology, bile secretion, and composition (Vlaming et al., 2006).ABCC6 mutations cause pseudoxanthoma elasticum, a calcification disorder, and ABCC6 facilitates the release of nucleoside triphosphates, the source of pyrophosphate, and inhibitor of calcification (Bergen et al., 2000; Jansen et al., 2013; Le Saux et al., 2000; K. Moitra et al., 2017). Mouse Abcc6 -/- mice have abnormal skin calcification and morphology (Gorgels et al., 2005). CFTR/ABCC7 encodes a chloride ion channel that plays a crucial role in exocrine secretion in the lung, intestine, pancreas, vas deferens, and skin, and recessive mutations cause cystic fibrosis (Anguiano et al., 1992; Quinton, 1999). Multiple Cftr mutations on different mouse backgrounds recapitulate nearly all the features of the human disease (O’Neal et al., 1993; Zeiher et al., 1995). ABCC8 and ABCC9 encode the sulfonylurea receptors SUR and SUR2, and mutations in ABCC8 (Chutkow et al., 2001) in humans cause non-insulin-dependent diabetes (Thomas et al., 1995). Knockouts of either Abcc8 or Abcc9 cause hypoglycemia and insulin abnormalities, and Abcc9 -/- mice also die prematurely (Chutkow et al., 2001; Seghers, Nakazaki, DeMayo, Aguilar-Bryan, & Bryan, 2000).
ABCD1 mutations in humans cause the X-linked adrenoleukodystrophy; Abcd1 -/- mice display abnormal myelination, brain cell morphology, astrocytosis, axon degeneration, and impaired coordination. Interestingly,Abcd2 -/- mice manifest neuronal and axon degeneration, ataxia, hyperactivity, tremors, abnormal microglial cell morphology, posture, and coordination, suggesting that this protein also functions in the brain (Ferrer et al., 2005). A family with a bile acid synthesis defect and peroxisomal abnormalities displayed mutations in ABCD3(?). In addition, Abcd3 -/- mice display increased liver size, abnormal peroxisomes, and a deficit in mature C24 bile acids (Ferdinandusse et al., 2015). ABCG2 is a uric acid transporter, and common variants in the ABCG2 gene are associated with gout, a disorder of blood uric acid levels (Dehghan et al., 2008).Abcg2 -/- mice display sensitivity to phytotoxins and increased blood and urine uric acid (Ichida et al., 2012; Jonker et al., 2002). The ABCG5 and ABCG8 genes encode half-transporters that function as a heterodimer and transport phytosterols. Mutations in these genes cause recessive sitosterolemia in humans and abnormal blood lipid and cholesterol levels in the mouse (Chase et al., 2010; Klett et al., 2004).
Many mouse ABC gene mutations are associated with phenotypes likely to be caused by a defect in cellular transport or provide clues to human gene function. For example, the human ABCA2 gene is highly expressed in the brain and closely related to the known lipid transporters ABCA1, ABCA3, and ABCA4. Disruption of the mouseAbca2 gene leads to multiple behavioral and neurological phenotypes and decreased body weight (Sakai et al., 2007). ABCA7 is also a lipid transporter, and mouse Abca7 disruption mice exhibit reduced HDL cholesterol, adipose tissue, and kidney size (Kim et al., 2005). There is an ABCA5 -related gene cluster in all mammals, but the function of these genes is unknown.Abca5 -/- mice display abnormal liver morphology and physiology and develop an adult lethal dilated cardiomyopathy-like heart phenotype. The protein locates primarily in the lysosomes (Kubo et al., 2005). Abca6 knockout mice display decreased circulating serum albumin, decreased total protein, and increased hematocrit.Abca8b -/- mice show diminished cholesterol, HDL cholesterol, LDL cholesterol, and triglyceride levels, and humans withABCA8 mutations and low HDL cholesterol have been described (Trigueros-Motos et al., 2017). Expression of the Abca9 gene is principally in the brain (Piehler, Kaminski, Wenzel, Langmann, & Schmitz, 2002).
The human ABCG1 and ABCG4 genes are evolutionarily related to each other and the sterol transporters ABCG5 and ABCG8, but their function in humans is unknown. The mouse Abcg1 knockout results in the accumulation of neutral lipids and phospholipids in hepatocytes and macrophages and plays a role in loading cholesterol onto HDL particles (Kennedy et al., 2005). Loss of Abcg1 can affect the immune function of macrophages (Wojcik et al., 2008). ABCG4 is highly expressed in the brain, and the Abcg4 -/-mouse has abnormal sterol efflux in the brain (Annilo et al., 2001; Wang et al., 2008).
Several ABCB family half transporters proteins localize to the mitochondria, and their functions were elucidated from mouse models. InAbcb6 -/- mice, the gene is expressed in the mitochondria and transport coproporphyrin III into the mitochondria (Ulrich et al., 2012). The ABCB8 protein localizes to the mitochondria, and Abcb8 disruption leads to defects in mitochondrial iron export, cytosolic Fe/S protein levels, and cardiomyopathy (Ichikawa et al., 2012). In addition, disruption of Abcb10 leads to a lack of heme biosynthesis and erythropoiesis (Yamamoto et al., 2014).
In summary, for nearly all the human ABC genes causing Mendelian disorders, the mouse knockout strains recapitulate one or more phenotypes and serve as valuable models for further studies and development of therapeutics. In addition, the disruption of mouse ABC genes of unknown function has ascribed multiple cellular transport properties to several transporters, leading to extensive new knowledge on the full spectrum of ABC transporter efflux properties.