ABCB6 Mutations Cause Ocular Coloboma

Abstract

Ocular coloboma is a developmental defect of the eye and is due to abnormal or incomplete closure of the optic fissure. This disorder displays genetic and clinical heterogeneity. Using a positional cloning approach, we identified a mutation in the ATP-binding cassette (ABC) transporter ABCB6 in a Chinese family affected by autosomal-dominant coloboma. The Leu811Val mutation was identified in seven affected members of the family and was absent in six unaffected members from three generations. A LOD score of 3.2 at θ = 0 was calculated for the mutation identified in this family. Sequence analysis was performed on the ABCB6 exons from 116 sporadic cases of microphthalmia with coloboma (MAC), isolated coloboma, and aniridia, and an additional mutation (A57T) was identified in three patients with MAC. These two mutations were not present in the ethnically matched control populations. Immunostaining of transiently transfected, Myc-tagged ABCB6 in retinal pigment epithelial (RPE) cells showed that it localized to the endoplasmic reticulum and Golgi apparatus of RPE cells. RT-PCR of ABCB6 mRNA in human cell lines and tissue indicated that ABCB6 is expressed in the retinae and RPE cells. Using zebrafish, we show that abcb6 is expressed in the eye and CNS. Morpholino knockdown of abcb6 in zebrafish produces a phenotype characteristic of coloboma and replicates the clinical phenotype observed in our index cases. The knockdown phenotype can be corrected with coinjection of the wild-type, but not mutant, ABCB6 mRNA, suggesting that the phenotypes observed in zebrafish are due to insufficient abcb6 function. Our results demonstrate that ABCB6 mutations cause ocular coloboma.

Functional Coupling of ATP-binding Cassette Transporter Abcb6 to Cytochrome P450 Expression and Activity in Liver

“…This observation is relatively consistent with recent reports of growth and proliferation defects associated with loss of Abcb6 function in humans and in zebrafish (22). Although Abcb6 is capable of transporting heme precursors into the mitochondria (17,18) and plays a role in regulating porphyrin synthesis in vitro (17,26), Abcb6 deficiency in vivo did not have a significant impact on hepatic porphyrin or heme levels.…”

“…For example, retinal defects leading to ocular disease in humans has a strong association with loss of CYP1B1 expression (52,53). A similar association in retinal defects is also seen with loss of ABCB6 expression in humans (22). Although these results are correlative at present, they suggest a potential functional interaction between Abcb6 and Cyp1b1 in ocular disease.…”

“…In the human population, there are many incompletely understood incidences of a functional association between ABCB6 and human disease (19,22,51). However, the mechanistic association between loss of ABCB6 function and the pathogenesis of the disease is not clear.…”

Missense mutations in the ABCB6 transporter cause dominant familialpseudohyperkalemia

“…However, Tsuchida [37] questioned the mitochondrial localization of ABCB6, providing evidence for its localization in endoplasmic reticulum and Golgi membranes, rather than in mitochondria. The observation of extra-mitochondrial localization has been extended by the study of dominantly inherited ABCB6 mutations in patients with ocular coloboma [38].…”

Exome sequencing in developmental eye disease leads to identification of causal variants in GJA8, CRYGC, PAX6 and CYP1B1

“…Results from zebrafish rescue experiments indicated that these variants lead to loss of protein function. 31 However, in two more recent studies, several individuals homozygous for deleterious loss of function mutations, including frameshift and nonsense mutations, were found to have a newly identified rare Lan blood group antigen. There were no reports of coloboma or eye defects in individuals homozygous or heterozygous for these variants 32 .…”