A novel Q378X mutation exists in the transmembrane transporter protein ABCC6 and its pseudogene: implications for mutation analysis in pseudoxanthoma elasticum

Abstract

Pseudoxanthoma elasticum (PXE) is an inherited disorder of the elastic tissue with characteristic progressive calcification of elastic fibers in skin, eye, and the cardiovascular system. Recently mutations in the ABCC6 gene, encoding a transmembrane transporter protein, were identified as cause of the disease. Surprisingly, sequence and RFLP analysis for exon 9 with primers corresponding to flanking intronic sequence in diseased and haplotype negative members from all of our families and in a control population revealed either a homozygous or heterozygous state for the Q378X (1132C-->T) nonsense mutation in all individuals. With the publication of the genomic structure of the PXE locus we had identified the starting point of a large genomic segmental duplication within the locus in the cytogenetic interval defined by the Cy19 and Cy185 somatic cell hybrid breakpoints on chromosome 16p13.1. By means of somatic cell hybrid mapping we located this starting point telomeric to exon 10 of ABCC6. The duplication, however, does not include exon 10, but exons 1-9. These findings suggest that one or several copies of an ABCC6 pseudogene (psiABCC6) lie within this large segmental duplication. At least one copy contains exons 1-9 and maps to the chromosomal interval defined by the Cy163 and Cy11 breakpoints. Either this copy and/or an additional copy of psiABCC6 within Cy19-Cy183 carries the Q378X mutation that masks the correct identification of this nonsense mutation as being causative in pseudoxanthoma elasticum. Long-range PCR of exon 9 starting from sequence outside the genomic replication circumvents interference from the psiABCC6 DNA sequences and demonstrates that the Q378X mutation in the ABCC6 gene is associated with PXE in some families. These findings lead us to propose that gene conversion mechanisms from psiABCC6 to ABCC6 play a functional role in mutations causing PXE.

Mutational analysis of theABCC6 gene and the proximalABCC6 gene promoter in German patients with pseudoxanthoma elasticum (PXE)

“…Among the 42 different ABCC6 mutations detected in our PXE cohort, 32 had been described elsewhere (Bergen et al, 2000;Germain et al, 2000;Ringpfeil et al, 2000;Struk et al, 2000;Cai et al, 2001;Le Saux et al, 2001;Pulkkinen et al, 2001;Hu et al, 2003;Chassaing et al, 2004;Gheduzzi et al, 2004;Götting et al, 2004;Hendig et al, 2005;Schulz et al, 2005a;Schulz et al, 2005b), and 10 were novel DNA variations, namely c.37-1G>A, p.W38S, p.L252F, p.V415A, p.R487Q, p.N497K, c.1574_1575insG, p.S1049A, p.L1063R and c.3505_3506+2delAGGT. The mutations occurred in 10 PXE patients from 9 unrelated families and were not present in normal healthy controls (Tables 1 and 2).…”

Novel deletions causing pseudoxanthoma elasticum underscore the genomic instability of the ABCC6 region

“…Indeed, Cai et al have shown that the ABCC6 pseudogenes contribute to mutations in the parent gene. 22,23 Until now, a total of 16 different large deletions (exons, whole gene deletions) have been described. 7,[9][10][11][12][13][14] Because of the wide variety of repeat elements in the ABCC6 region, we anticipated that several of the unidentified mutant alleles in our PXE cohort would consist of deletions and insertions.…”

ABCC6/MRP6 mutations: further insight into the molecular pathology of pseudoxanthoma elasticum

“…18 Interpretation of the results from mutational analysis of ABCC6/MRP6 is complicated by the presence of two ABCC6/MRP6 pseudogenes in the genome and the multiple presence of larger and smaller deletions spanning (part of) the gene. 10,[19][20][21][22] In addition, still unknown molecular or environmental factors that could influence the variable clinical expression of the disease in patients complicate the correct assessment of genotype-phenotype relations. In this study, we present and discuss the results of ABCC6/ MRP6 mutation analysis in 59 PXE patients and families from The Netherlands.…”