Novel mutations of ABCB6 associated with autosomal dominant dyschromatosis universalis hereditaria

Dyschromatosis universalis hereditaria

Reticulate pigmentary dyschromatoses primarily include dyschromatosis universalis hereditaria (DUH), dyschromatosis symmetrica hereditaria (DSH) (Reticulate acropigmentation of Dohi), and unilateral dermatomal pigmentary dermatosis, which differ in their patterns of distribution. The disease was initially described by Ichikawa and Hiraga in Germany in 1933. The prevalence of DUH is 0.3 per 100,000 with a female preponderance. The skin lesions usually appear in infancy or early childhood and cease to progress beyond adolescence. The subtypes DUH 1 and DUH 3 are found to have autosomal dominant inheritance, which is the most common inheritance pattern, while DUH 2 has an autosomal recessive pattern. The most common gene involved in DUH is ABCB6, while the other genes include SASH 1, PER 3, and KITLG (DUH type 2). DUH is characterized by multiple irregular hyperpigmented macules interspersed with hypopigmented macules in a mottled pattern over the trunk and extremities. The face is involved in 50% of individuals. Rarely, it can also involve hairs, nails, mucous membranes, palms, and soles. Other varied presentations include localized forms, localization of lesions to sun-exposed areas, large macules, uniform palmar hypopigmentation, diffuse hyperpigmentation with spotty depigmented macules, and unilateral involvement. DUH has been reported to be associated with various cutaneous and systemic diseases. The authors have observed cases of DUH associated with hepatocellular carcinoma, solitary keratoacanthoma, and dermoid cyst. The various diagnostic modalities include dermoscopy, histopathology, electron microscopy, and targeted gene sequencing. Though various treatment modalities like NBUVB and lasers have been tried, no treatment is promising.

Structure and Mechanism of Human ABC Transporters

ABC transporters are essential for cellular physiology. Humans have 48 ABC genes organized into seven distinct families. Of these genes, 44 (in five distinct families) encode for membrane transporters, of which several are involved in drug resistance and disease pathways resulting from transporter dysfunction. Over the last decade, advances in structural biology have vastly expanded our mechanistic understanding of human ABC transporter function, revealing details of their molecular arrangement, regulation, and interactions, facilitated in large part by advances in cryo-EM that have rendered hitherto inaccessible targets amenable to high-resolution structural analysis. As a result, experimentally determined structures of multiple members of each of the five families of ABC transporters in humans are now available. Here we review this recent progress, highlighting the physiological relevance of human ABC transporters and mechanistic insights gleaned from their direct structure determination. We also discuss the impact and limitations of model systems and structure prediction methods in understanding human ABC transporters and discuss current challenges and future research directions.

Molecular insights into the human ABCB6 transporter

ABCB6 plays a crucial role in energy-dependent porphyrin transport, drug resistance, toxic metal resistance, porphyrin biosynthesis, protection against stress, and encoding a blood group system Langereis antigen. However, the mechanism underlying porphyrin transport is still unclear. Here, we determined the cryo-electron microscopy (cryo-EM) structures of nanodisc-reconstituted human ABCB6 trapped in an apo-state and an ATP-bound state at resolutions of 3.6 and 3.5 Å, respectively. Our structures reveal a unique loop in the transmembrane domain (TMD) of ABCB6, which divides the TMD into two cavities. It restrains the access of substrates in the inward-facing state and is removed by ATP-driven conformational change. No ligand cavities were observed in the nucleotide-bound state, indicating a state following substrate release but prior to ATP hydrolysis. Structural analyses and functional characterizations suggest an "ATP-switch" model and further reveal the conformational changes of the substrate-binding pockets triggered by the ATP-driven regulation.

Novel missense mutation of SASH1 in a Chinese family with dyschromatosis universalis hereditaria

BACKGROUND

Dyschromatosis universalis hereditaria (DUH) is a pigmentary dermatosis characterized by generalized mottled macules with hypopigmention and hyperpigmention. ABCB6 and SASH1 are recently reported pathogenic genes related to DUH, and the aim of this study was to identify the causative mutations in a Chinese family with DUH.

METHODS

Sanger sequencing was performed to investigate the clinical manifestation and molecular genetic basis of these familial cases of DUH, bioinformatics tools and multiple sequence alignment were used to analyse the pathogenicity of mutations.

RESULTS

A novel missense mutation, c.1529G>A, in the SASH1 gene was identified, and this mutation was not found in the National Center for Biotechnology Information Database of Short Genetic Variation, Online Mendelian Inheritance in Man, ClinVar, or 1000 Genomes Project databases. All in silico predictors suggested that the observed substitution mutation was deleterious. Furthermore, multiple sequence alignment of SASH1 revealed that the p.S510N mutation was highly conserved during evolution. In addition, we reviewed the previously reported DUH-related gene mutations in SASH1 and ABCB6.

CONCLUSION

Although the affected family members had identical mutations, differences in the clinical manifestations of these family members were observed, which reveals the complexity of the phenotype-influencing factors in DUH. Our findings reveal the mutation responsible for DUH in this family and broaden the mutational spectrum of the SASH1 gene.

Identification of a Novel Mutation in SASH1 Gene in a Chinese Family With Dyschromatosis Universalis Hereditaria and Genotype-Phenotype Correlation Analysis

Dyschromatosis universalis hereditaria (DUH) is a rare genodermatosis characterized by mottled hyperpigmented and hypopigmented macules. SASH1 and ABCB6 have been identified as the causative genes for this disorder. We performed whole exome sequencing on a Chinese family with DUH and genotype-phenotype correlation analysis in DUH and lentiginous phenotype patients. A novel heterozygous missense mutation p.Q518P in SASH1 gene was detected in this family. A majority of patients with SASH1 mutations presented as a distinct clinical phenotype clearly different from that in patients with ABCB6 mutations. Our findings further enrich the reservoir of SASH1 mutations in DUH. The clinical phenotypic difference between SASH1 and ABCB6 variants is suggestive of a close phenotype-genotype link in DUH.

The human ABCB6 protein is the functional homologue of HMT-1 proteins mediating cadmium detoxification

ABCB6 belongs to the family of ATP-binding cassette (ABC) transporters, which transport various molecules across extra- and intra-cellular membranes, bearing significant impact on human disease and pharmacology. Although mutations in the ABCB6 gene have been linked to a variety of pathophysiological conditions ranging from transfusion incompatibility to pigmentation defects, its precise cellular localization and function is not understood. In particular, the intracellular localization of ABCB6 has been a matter of debate, with conflicting reports suggesting mitochondrial or endolysosomal expression. ABCB6 shows significant sequence identity to HMT-1 (heavy metal tolerance factor 1) proteins, whose evolutionarily conserved role is to confer tolerance to heavy metals through the intracellular sequestration of metal complexes. Here, we show that the cadmium-sensitive phenotype of Schizosaccharomyces pombe and Caenorhabditis elegans strains defective for HMT-1 is rescued by the human ABCB6 protein. Overexpression of ABCB6 conferred tolerance to cadmium and As(III) (As2O3), but not to As(V) (Na2HAsO4), Sb(V), Hg(II), or Zn(II). Inactivating mutations of ABCB6 abolished vacuolar sequestration of cadmium, effectively suppressing the cadmium tolerance phenotype. Modulation of ABCB6 expression levels in human glioblastoma cells resulted in a concomitant change in cadmium sensitivity. Our findings reveal ABCB6 as a functional homologue of the HMT-1 proteins, linking endolysosomal ABCB6 to the highly conserved mechanism of intracellular cadmium detoxification.

Genetics of anophthalmia and microphthalmia. Part 1: Non-syndromic anophthalmia/microphthalmia

Eye formation is the result of coordinated induction and differentiation processes during embryogenesis. Disruption of any one of these events has the potential to cause ocular growth and structural defects, such as anophthalmia and microphthalmia (A/M). A/M can be isolated or occur with systemic anomalies, when they may form part of a recognizable syndrome. Their etiology includes genetic and environmental factors; several hundred genes involved in ocular development have been identified in humans or animal models. In humans, around 30 genes have been repeatedly implicated in A/M families, although many other genes have been described in single cases or families, and some genetic syndromes include eye anomalies occasionally as part of a wider phenotype. As a result of this broad genetic heterogeneity, with one or two notable exceptions, each gene explains only a small percentage of cases. Given the overlapping phenotypes, these genes can be most efficiently tested on panels or by whole exome/genome sequencing for the purposes of molecular diagnosis. However, despite whole exome/genome testing more than half of patients currently remain without a molecular diagnosis. The proportion of undiagnosed cases is even higher in those individuals with unilateral or milder phenotypes. Furthermore, even when a strong gene candidate is available for a patient, issues of incomplete penetrance and germinal mosaicism make diagnosis and genetic counseling challenging. In this review, we present the main genes implicated in non-syndromic human A/M phenotypes and, for practical purposes, classify them according to the most frequent or predominant phenotype each is associated with. Our intention is that this will allow clinicians to rank and prioritize their molecular analyses and interpretations according to the phenotypes of their patients.

ABCB6 Resides in Melanosomes and Regulates Early Steps of Melanogenesis Required for PMEL Amyloid Matrix Formation

Genetically inheritable pigmentation defects provide a unique opportunity to reveal the function of proteins contributing to melanogenesis. Dyschromatosis universalis hereditaria (DUH) is a rare pigmentary genodermatosis associated with mutations in the ABCB6 gene. Here we use optical and electron microscopy imaging combined with biochemical tools to investigate the localization and function of ABCB6 in pigment cells. We show that ABCB6 localizes to the membrane of early melanosomes and lysosomes of the human melanocytic cell line MNT-1. Depletion of ABCB6 by siRNA impaired PMEL amyloidogenesis in early melanosomes and induced aberrant accumulation of multilamellar aggregates in pigmented melanosomes. PMEL fibril formation and normal maturation of pigmented melanosomes could be restored by the overexpression of wild-type ABCB6 but not by variants containing an inactivating catalytic mutation (K629M) or the G579E DUH mutation. In line with the impairment of PMEL matrix formation in the absence of ABCB6, morphological analysis of the retinal pigment epithelium of ABCB6 knockout mice revealed a significant decrease of melanosome numbers. Our study extends the localization of ABCB6 to melanosomes, suggesting a potential link between the function of ABCB6 and the etiology of DUH to amyloid formation in pigment cells.

Erythrocytes lacking the Langereis blood group protein ABCB6 are resistant to the malaria parasite Plasmodium falciparum

The ATP-binding cassette transporter ABCB6 was recently discovered to encode the Langereis (Lan) blood group antigen. Lan null individuals are asymptomatic, and the function of ABCB6 in mature erythrocytes is not understood. Here, we assessed ABCB6 as a host factor for Plasmodium falciparum malaria parasites during erythrocyte invasion. We show that Lan null erythrocytes are highly resistant to invasion by P. falciparum, in a strain-transcendent manner. Although both Lan null and Jr(a-) erythrocytes harbor excess porphyrin, only Lan null erythrocytes exhibit a P. falciparum invasion defect. Further, the zoonotic parasite P. knowlesi invades Lan null and control cells with similar efficiency, suggesting that ABCB6 may mediate P. falciparum invasion through species-specific molecular interactions. Using tandem mass tag-based proteomics, we find that the only consistent difference in membrane proteins between Lan null and control cells is absence of ABCB6. Our results demonstrate that a newly identified naturally occurring blood group variant is associated with resistance to Plasmodium falciparum.

Elizabeth Egan and colleagues demonstrate that host ATP binding cassette transporter ABCB6, which encodes the Langereis blood group antigen, promotes erythrocyte invasion by the malaria parasite Plasmodium falciparum. This study suggests that asymptomatic Langereis null individuals may be better protected from malaria.

Differential Diagnosis of Two Chinese Families with Dyschromatoses by Targeted Gene Sequencing

Background:

The dyschromatoses are a group of disorders characterized by simultaneous hyperpigmented macules together with hypopigmented macules. Dyschromatosis universalis hereditaria (DUH) and dyschromatosis symmetrica hereditaria are two major types. While clinical and histological presentations are similar in these two diseases, genetic diagnosis is critical in the differential diagnosis of these entities.

Methods:

Three patients initially diagnosed with DUH were included. The gene test was carried out by targeted gene sequencing. All mutations detected on ADAR1 and ABCB6 genes were analyzed according to the frequency in control database, the mutation types, and the published evidence to determine the pathogenicity.

Results:

Family pedigree and clinical presentations were reported in 3 patients from two Chinese families. All patients have prominent cutaneous dyschromatoses involving the whole body without systemic complications. Different pathogenic genes in these patients with similar phenotype were identified: One novel mutation on ADAR1 (c. 1325C>G) and one recurrent mutation in ABCB6 (c. 1270T>C), which successfully distinguished two diseases with the similar phenotype.

Conclusion:

Targeted gene sequencing is an effective tool for genetic diagnosis in pigmentary skin diseases.

Dyschromatosis Universalis Hereditaria with Oral Leukokeratosis--A Case of Mistaken Identity and Review of the Literature

Dyschromatosis universalis hereditaria (DUH) is a rare pigmentary genodermatosis characterized by reticulated hyper- and hypopigmented macules distributed over the trunk and extremities in otherwise healthy patients. DUH presents in a fashion similar to that of a variety of reticulate and pigmentary dermatoses, some of which are associated with precancerous entities and other comorbidities. It is therefore imperative that the clinician recognize and differentiate these disorders so that appropriate screening and counseling can be offered to the patient. We report a case of DUH in a 13-year-old boy presenting with oral leukokeratosis, with a review of the literature exploring the differential diagnoses.

Role of the N-terminal transmembrane domain in the endo-lysosomal targeting and function of the human ABCB6 protein

ATP-binding cassette, subfamily B (ABCB) 6 is a homodimeric ATP-binding cassette (ABC) transporter present in the plasma membrane and in the intracellular organelles. The intracellular localization of ABCB6 has been a matter of debate, as it has been suggested to reside in the mitochondria and the endo-lysosomal system. Using a variety of imaging modalities, including confocal microscopy and EM, we confirm the endo-lysosomal localization of ABCB6 and show that the protein is internalized from the plasma membrane through endocytosis, to be distributed to multivesicular bodies and lysosomes. In addition to the canonical nucleotide-binding domain (NBD) and transmembrane domain (TMD), ABCB6 contains a unique N-terminal TMD (TMD₀), which does not show sequence homology to known proteins. We investigated the functional role of these domains through the molecular dissection of ABCB6. We find that the folding, dimerization, membrane insertion and ATP binding/hydrolysis of the core–ABCB6 complex devoid of TMD₀ are preserved. However, in contrast with the full-length transporter, the core–ABCB6 construct is retained at the plasma membrane and does not appear in Rab5-positive endosomes. TMD₀ is directly targeted to the lysosomes, without passage to the plasma membrane. Collectively, our results reveal that TMD₀ represents an independently folding unit, which is dispensable for catalysis, but has a crucial role in the lysosomal targeting of ABCB6.

The intracellular localization of ATP-binding cassette, sub family B (ABCB) 6 is a matter of debate. We show that ABCB6 is internalized from the plasma membrane to multivesicular bodies and lysosomes. Molecular dissection of the ABCB6 protein reveals a role of its N-terminal domain in targeting.

Dyschromatosis universalis hereditaria with renal failure

Dyschromatosis universalis hereditaria (DUH) is a rare autosomal dominant inherited dermatosis which usually appears during childhood and is characterized by dyspigmentation, with both hypopigmented and hyperpigmented macules. We report a case of DUH with unexplained childhood-onset renal failure. The association between DUH and renal failure is yet to be proven by further studies.

Functional coupling of ATP-binding cassette transporter Abcb6 to cytochrome P450 expression and activity in liver

Although endogenous mechanisms that negatively regulate cytochrome P450 (P450) monooxygenases in response to physiological and pathophysiological signals are not well understood, they are thought to result from alterations in the level of endogenous metabolites, involved in maintaining homeostasis. Here we show that homeostatic changes in hepatic metabolite profile in Abcb6 (mitochondrial ATP-binding cassette transporter B6) deficiency results in suppression of a specific subset of hepatic P450 activity. Abcb6 null mice are more susceptible to pentobarbital-induced sleep and zoxazolamine-induced paralysis, secondary to decreased expression and activity of Cyp3a11 and Cyp2b10. The knock-out mice also show decrease in both basal and xeno-inducible expression and activity of a subset of hepatic P450s that appear to be related to changes in hepatic metabolite profile. These data, together with the observation that liver extracts from Abcb6-deficient mice suppress P450 expression in human primary hepatocytes, suggest that this mouse model may provide an opportunity to understand the physiological signals and the mechanisms involved in negative regulation of P450s.

Screening the expression of ABCB6 in erythrocytes reveals an unexpectedly high frequency of Lan mutations in healthy individuals

Lan is a high-incidence blood group antigen expressed in more than 99.9% of the population. Identification of the human ABC transporter ABCB6 as the molecular basis of Lan has opened the way for studies assessing the relation of ABCB6 function and expression to health and disease. To date, 34 ABCB6 sequence variants have been described in association with reduced ABCB6 expression based on the genotyping of stored blood showing weak or no reactivity with anti-Lan antibodies. In the present study we examined the red blood cell (RBC) surface expression of ABCB6 by quantitative flow cytometry in a cohort of 47 healthy individuals. Sequencing of the entire coding region of the ABCB6 gene in low RBC ABCB6 expressors identified a new allele (IVS9+1G>A, affecting a putative splice site at the boundary of exon 9) and two nonsynonymous SNPs listed in the SNP database (R192Q (rs150221689) and G588 S (rs145526996)). The R192Q mutation showed co-segregation with reduced RBC ABCB6 expression in a family, and we found the G588 S mutation in a compound heterozygous individual with undetectable ABCB6 expression, suggesting that both mutations result in weak or no expression of ABCB6 on RBCs. Analysis of the intracellular expression pattern in HeLa cells by confocal microscopy indicated that these mutations do not compromise overall expression or the endolysosomal localization of ABCB6. Genotyping of two large cohorts, containing 235 and 1039 unrelated volunteers, confirmed the high allele frequency of Lan-mutations. Our results suggest that genetic variants linked to lower or absent cell surface expression of ABCB6/Langereis may be more common than previously thought.