Homozygosity for the R1268Q Mutation in MRP6, the Pseudoxanthoma Elasticum Gene, Is Not Disease-Causing

doi:10.1006/bbrc.2000.3101

Biochemical and Biophysical Research Communications

Volume 274, Issue 2, 2 August 2000, Pages 297-301

https://doi.org/10.1006/bbrc.2000.3101 Get rights and content

Abstract

Pseudoxanthoma elasticum (PXE) is an inherited systemic disorder of connective tissue, characterized by progressive calcification of the elastic fibers in the eye, the skin, and the cardiovascular system, resulting in decreased vision, skin lesions, and life-threatening vascular disease, with highly variable phenotypic expression. The PXE locus has been mapped to chromosome 16p13.1, and was recently further refined to a 500 kb-region, containing two pseudogenes and four candidate genes. In a comprehensive mutational screening, we were able to exclude the responsibility of pM5, UNK, and MRP1 genes, candidate on the basis of their genetic localization. Conversely, we have found pathogenetic mutations in the MRP6 gene, in patients affected with PXE, indicating that human MRP6, which encodes a 1503 amino-acids membrane protein, member of the human ATP binding cassette (ABC) transporters superfamily, is the gene responsible for PXE. In one large PXE pedigree for which we had identified a nonsense mutation (R1141X), we came across a G to A transition at position 3803 of the MRP6 cDNA sequence (R1268Q). Astonishingly, this latter variant was found at the homozygous state in the proband's unaffected husband. We investigated the R1268Q mutation, and found the Q1268 allele at a relatively high frequency (0.19) in a Caucasian control population (n = 62 subjects). Genotype frequencies were in Hardy-Weinberg equilibrium, and three healthy volunteers were homozygous for the Q1268 allele. These data indicate that the R1268Q variant in the MRP6 gene does not cause PXE per se. Further studies will elucidate if it may play a role when found in compound heterozygotes.

References (18)

O. Le Saux et al.
Genomics
(1999)
D. Quaglino et al.
Biochim. Biophys. Acta
(2000)
D.P. Germain et al.
Am. J. Hum. Genet.
(1998)
K. Neldner
Clin. Dermatol.
(1988)
F.M. Pope
Br. J. Dermatol.
(1975)
B. Struk et al.
Hum. Mol. Genet.
(1997)
S. van Soest et al.
Genome Res.
(1997)
L. Cai et al.
J. Mol. Med.
(2000)

There are more references available in the full text version of this article.

Cited by (36)

Angioid streaks in pseudoxanthoma elasticum: Role of the p.R1268Q mutation in the ABCC6 gene
2011, Journal of Investigative Dermatology
Microvascular involvement in pseudoxanthoma elasticum: Capillaroscopic findings
2004, Presse Medicale
Le but de cette étude est de décrire pour la première fois l’angio-architecture capillaire au cours du pseudoxanthome élastique, maladie génétique rare liée à des mutations du gène ABCC6, et dont l’atteinte calcifiante systémique est en particulier responsable des complications cardio-vasculaires qui font toute la gravité de l’affection.
Sept patients atteints de pseudoxanthome élastique cliniquement et histologiquement certain, ont été examinés en capillaroscopie, l’absence de connectivite ou de diabète associé ayant été préalablement vérifiée.
L’ensemble des malades avait une microangiopathie caractérisée par une densité capillaire normale, un œdème péricapillaire fréquent, un excès de tortuosité des anses, avec présence d’un nombre significativement élevé de dystrophies mineures et, à des degrés variables, un ralentissement du flux sanguin capillaire matérialisé par un phénomène de sludge.
Cette étude descriptive montre qu’il existe une microangiopathie au cours du pseudoxanthome élastique. Celle-ci n’est cependant pas spécifique et une étude contrôlée en aveugle est nécessaire pour confirmer ces résultats. La mise en évidence de corrélations génotype/phénotype dans cette maladie pourrait donner une place à l’examen capillaroscopique dans la stratégie diagnostique chez les patients jeunes ou dans l’évaluation pronostique de l’atteinte cardio-vasculaire.
The aim of this study was to describe, for the first time, the capillary angio-architecture during pseudoxanthoma elasticum, a rare genetic disease related to mutations in the ABCC6 gene, of which the systemic calcifying involvement is responsible notably for very severe cardiovascular complications.
Seven patients suffering from clinically and histologically documented confirmed pseudoxanthoma elasticum were examined with capillaroscopy, the absence of concomitant connective tissue disease or diabetes having been checked beforehand.
All the patients exhibited a microangiopathy, characterised by normal capillary density, frequent pericapillary oedema, excessively coiled fibres with a significantly increased number of minor dystrophies and, to varying degrees, a slowing down of capillary blood flow demonstrated by a sludge phenomenon.
This descriptive study shows that a microangiopathy exists during pseudoxanthoma elasticum. However, the latter is not specific and a double blind controlled study is required to confirm these results. The discovery of genotype/phenotype correlations in this disease would provide a place for capillaroscopy in the diagnostic strategy in young patients or in the assessment of the cardiovascular involvement.
Subcellular localization and N-glycosylation of human ABCC6, expressed in MDCKII cells
2003, Biochemical and Biophysical Research Communications
Mutations in the gene coding for a human ABC transporter protein, ABCC6 (MRP6), are responsible for the development of pseudoxanthoma elasticum. Here, we demonstrate that human ABCC6, when expressed by retroviral transduction in polarized mammalian (MDCKII) cells, is exclusively localized to the basolateral membrane. The human ABCC6 in MDCKII cells was found to be glycosylated, in contrast to the underglycosylated form of the protein, as expressed in Sf9 cells. In order to localize the major glycosylation site(s) in ABCC6, we applied limited proteolysis on the fully glycosylated and underglycosylated forms, followed by immunodetection with region-specific antibodies for ABCC6. Our results indicate that Asn15, which is located in the extracellular N-terminal region of human ABCC6, is the only N-glycosylation site in this protein. The polarized mammalian expression system characterized here provides a useful tool for further examination of routing, glycosylation, and function of the normal and pathological variants of human ABCC6.
Pseudoxanthoma elasticum: A clinical, histopathological, and molecular update
2003, Survey of Ophthalmology
Pseudoxanthoma elasticum is an autosomally inherited disorder that is associated with the accumulation of mineralized and fragmented elastic fibers in the skin, Bruch's membrane in the retina, and vessel walls. The ophthalmic and dermatologic expression of pseudoxanthoma elasticum and its vascular complications are heterogeneous, with considerable variation in phenotype, progression, and mode of inheritance. Using linkage analysis and mutation detection techniques, mutations in the ABCC6 gene were recently implicated in the etiology of pseudoxanthoma elasticum. ABCC6 encodes the sixth member of the ATP-binding cassette transporter and multidrug resistance protein family (MRP6). In humans, this transmembrane protein is highly expressed in the liver and kidney. Lower expression was found in tissues affected by pseudoxanthoma elasticum, including skin, retina, and vessel walls. So far, the substrates transported by the ABCC6 protein and its physiological role in the etiology of pseudoxanthoma elasticum are not known. A functional transport study of rat MRP6 suggests that small peptides such as the endothelin receptor antagonist BQ123 are transported by MRP6. Similar molecules transported by ABCC6 in humans may be essential for extracellular matrix deposition or turnover of connective tissue at specific sites in the body. One of these sites is Bruch's membrane. This review is an update on etiology of pseudoxanthoma elasticum, including its clinical and genetic features, pathogenesis, and biomolecular basis.
Loss of ATP-dependent transport activity in pseudoxanthoma elasticum-associated mutants of human ABCC6 (MRP6)
2002, Journal of Biological Chemistry
Citation Excerpt :
A key finding in our present experiments was that three PXE-causing mutant forms of ABCC6 expressed in Sf9 cell membranes had markedly reduced transport activities, indicating that loss-of-function mutations are responsible for PXE. To date, 43 mutations in the humanABCC6 gene, associated with pseudoxanthoma elasticum, have been identified (1-4, 20, 41, 42). This current distribution pattern of PXE mutations revealed a cluster in the region of the gene encoding the C-terminal region of the protein (20): 11 of the 23 missense mutations identified were found within the C-proximal ABC domain, whereas only two missense mutations were found in the N-proximal ABC domain.
Mutations in the ABCC6 (MRP6) gene cause pseudoxanthoma elasticum (PXE), a rare heritable disorder resulting in the calcification of elastic fibers. In the present study a cDNA encoding a full-length normal variant of ABCC6 was amplified from a human kidney cDNA library, and the protein was expressed in Sf9 insect cells. In isolated membranes ATP binding as well as ATP-dependent active transport by ABCC6 was demonstrated. We found that glutathione conjugates, including leukotriene C₄ and N-ethylmaleimideS-glutathione (NEM-GS), were actively transported by human ABCC6. Organic anions (probenecid, benzbromarone, indomethacin), known to interfere with glutathione conjugate transport of human ABCC1 and ABCC2, inhibited the ABCC6-mediated NEM-GS transport in a specific manner, indicating that ABCC6 has a unique substrate specificity. We have also expressed three missense mutant forms of ABCC6, which have recently been shown to cause PXE. MgATP binding was normal in these proteins; ATP-dependent NEM-GS or leukotriene C₄ transport, however, was abolished. Our data indicate that human ABCC6 is a primary active transporter for organic anions. In the three ABCC6 mutant forms examined, the loss of transport activity suggests that these mutations result in a PXE phenotype through a direct influence on the transport activity of this ABC transporter.
Phenotypic features and genetic findings in a cohort of italian pseudoxanthoma elasticum patients and update of the ophthalmologic evaluation score
2021, Journal of Clinical Medicine

View all citing articles on Scopus

¹: To whom correspondence should be addressed at Laboratoire de Génétique. Hôpital Broussais, 96, rue Didot, 75014 Paris, France. Fax: + 33 1 45 41 02 34. E-mail: [email protected].

View full text

Regular ArticleHomozygosity for the R1268Q Mutation in MRP6, the Pseudoxanthoma Elasticum Gene, Is Not Disease-Causing

Abstract

Genomics

Biochim. Biophys. Acta

Am. J. Hum. Genet.

Clin. Dermatol.

Br. J. Dermatol.

Hum. Mol. Genet.

Genome Res.

J. Mol. Med.

Regular Article
Homozygosity for the R1268Q Mutation in MRP6, the Pseudoxanthoma Elasticum Gene, Is Not Disease-Causing