Astrid S. Plomp

Pseudoxanthoma elasticum: A clinical, histopathological, and molecular update

Pseudoxanthoma elasticum is an autosomally inherited disorder that is associated with the accumulation of mineralized and fragmented elastic fibers in the skin, Bruchs 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 Bruchs membrane. This review is an update on etiology of pseudoxanthoma elasticum, including its clinical and genetic features, pathogenesis, and biomolecular basis.

ABCC6–Mediated ATP Secretion by the Liver Is the Main Source of the Mineralization Inhibitor Inorganic Pyrophosphate in the Systemic Circulation—Brief Report

Objective— Mutations in ABCC6 underlie the ectopic mineralization disorder pseudoxanthoma elasticum (PXE) and some forms of generalized arterial calcification of infancy, both of which affect the cardiovascular system. Using cultured cells, we recently showed that ATP-binding cassette subfamily C member 6 (ABCC6) mediates the cellular release of ATP, which is extracellularly rapidly converted into AMP and the mineralization inhibitor inorganic pyrophosphate (PPi). The current study was performed to determine which tissues release ATP in an ABCC6-dependent manner in vivo, where released ATP is converted into AMP and PPi, and whether human PXE ptients have low plasma PPi concentrations. Approach and Results— Using cultured primary hepatocytes and in vivo liver perfusion experiments, we found that ABCC6 mediates the direct, sinusoidal, release of ATP from the liver. Outside hepatocytes, but still within the liver vasculature, released ATP is converted into AMP and PPi. The absence of functional ABCC6 in patients with PXE leads to strongly reduced plasma PPi concentrations. Conclusions— Hepatic ABCC6-mediated ATP release is the main source of circulating PPi, revealing an unanticipated role of the liver in systemic PPi homeostasis. Patients with PXE have a strongly reduced plasma PPi level, explaining their mineralization disorder. Our results indicate that systemic PPi is relatively stable and that PXE, generalized arterial calcification of infancy, and other ectopic mineralization disorders could be treated with PPi supplementation therapy.

Mutation Spectrum in RAB3GAP1, RAB3GAP2, and RAB18 and Genotype–Phenotype Correlations in Warburg Micro Syndrome and Martsolf Syndrome

Warburg Micro syndrome and Martsolf syndrome (MS) are heterogeneous autosomal‐recessive developmental disorders characterized by brain, eye, and endocrine abnormalities. Causative biallelic germline mutations have been identified in RAB3GAP1, RAB3GAP2, or RAB18, each of which encode proteins involved in membrane trafficking. This report provides an up to date overview of all known disease variants identified in 29 previously published families and 52 new families. One‐hundred and forty‐four Micro and nine Martsolf families were investigated, identifying mutations in RAB3GAP1 in 41% of cases, mutations in RAB3GAP2 in 7% of cases, and mutations in RAB18 in 5% of cases. These are listed in Leiden Open source Variation Databases, which was created by us for all three genes. Genotype–phenotype correlations for these genes have now established that the clinical phenotypes in Micro syndrome and MS represent a phenotypic continuum related to the nature and severity of the mutations present in the disease genes, with more deleterious mutations causing Micro syndrome and milder mutations causing MS. RAB18 has not yet been linked to the RAB3 pathways, but mutations in all three genes cause an indistinguishable phenotype, making it likely that there is some overlap. There is considerable genetic heterogeneity for these disorders and further gene identification will help delineate these pathways.

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

Pseudoxanthoma elasticum (PXE) is a hereditary disease characterized by progressive dystrophic mineralization of the elastic fibres. PXE patients frequently present with skin lesions and visual acuity loss. Recently, we and others showed that PXE is caused by mutations in the ABCC6/MRP6 gene. However, the molecular pathology of PXE is complicated by yet unknown factors causing the variable clinical expression of the disease. In addition, the presence of ABCC6/MRP6 pseudogenes and multiple ABCC6/MRP6-associated deletions complicate interpretation of molecular genetic studies. In this study, we present the mutation spectrum of ABCC6/MRP6 in 59 PXE patients from the Netherlands. We detected 17 different mutations in 65 alleles. The majority of mutations occurred in the NBF1 (nucleotide binding fold) domain, in the eighth cytoplasmatic loop between the 15th and 16th transmembrane regions, and in NBF2 of the predicted ABCC6/MRP6 protein. The R1141X mutation was by far the most common mutation identified in 19 (32.2%) patients. The second most frequent mutation, an intragenic deletion from exon 23 to exon 29 in ABCC6/MRP6, was detected in 11 (18.6%) of the patients. Our data include 11 novel ABCC6/MRP6 mutations, as well as additional segregation data relevant to the molecular pathology of PXE in a limited number of patients and families. The consequences of our data for the molecular pathology of PXE are discussed.

Proposal for updating the pseudoxanthoma elasticum classification system and a review of the clinical findings

Pseudoxanthoma elasticum (PXE) is a systemic disorder affecting elastic tissues most markedly in skin, retina, and blood vessels. It is caused by mutations in the ABCC6 gene and is transmitted in an autosomal recessive fashion. In 1994 a new classification system for PXE was published as the result of a consensus conference. Since then the ABCC6 gene has been discovered. We propose that there is a need for a classification system incorporating all relevant systemic symptoms and signs, based on standardized clinical, histological, and molecular biological examination techniques. We re‐evaluated the histopathologic PXE signs and propose a classification system with unambiguous criteria leading to a consistent diagnosis of definitive, probable, or possible PXE world‐wide. We put this proposed classification forward to encourage further debate on the diagnosis of this multi‐organ disorder.

Pfeiffer Syndrome Type 2: Further delineation and review of the literature

We present 5 unrelated patients, 3 boys and 2 girls, with Pfeiffer syndrome (PS) type 2. They all had cloverleaf skull, severe proptosis, ankylosis of the elbows, broad thumbs and/or broad halluces and variable accompanying anomalies. We review the literature on all subtypes of PS. Most patients with PS type 2 died shortly after birth. Causes of death include pulmonary problems, brain abnormalities, prematurity and post-operative complications. DNA studies were performed in 3 of the 5 patients. Two of them showed a 1036T --> C mutation in the fibroblast growth factor receptor 2 (FGFR2) gene, that was earlier reported in PS and in Crouzon syndrome. Probably most, if not all, PS type 2 cases are caused by a de novo mutation in the FGFR2 gene or in another, yet unidentified gene. To date all type 2 cases have been non-familial. A low recurrence risk for parents can be advised.

Identification of 34 novel and 56 known FOXL2 mutations in patients with blepharophimosis syndrome

Blepharophimosis syndrome (BPES) is caused by loss‐of‐function mutations in the single‐exon forkhead transcription factor gene FOXL2 and by genomic rearrangements of the FOXL2 locus. Here, we focus on 92 new intragenic FOXL2 mutations, 34 of which are novel. Specifically, we found 10 nonsense mutations (11%), 13 missense mutations (14%), 40 deletions or insertions leading to a frameshift (43%), and 29 in‐frame changes (32%), of which 28 (30%) lead to a polyalanine expansion. This study confirms the existence of two previously described mutational hotspots. Moreover, we gained novel insights in genotype‐phenotype correlations, emphasizing the need to interpret genotype‐phenotype correlations individually and always in the context of further clinical observations.

Complete COL1A1 allele deletions in osteogenesis imperfecta

Purpose: To identify a molecular genetic cause in patients with a clinical diagnosis of osteogenesis imperfecta (OI) type I/IV.Methods: The authors performed multiplex ligation-dependent probe amplification analysis of the COL1A1 gene in a group of 106 index patients.Results: In four families with mild osteogenesis imperfecta and no other phenotypic abnormalities, a deletion of the complete COL1A1 gene on one allele was detected, a molecular finding that to our knowledge has not been described before, apart from a larger chromosomal deletion detected by fluorescent in situ hybridization encompassing the COL1A1 gene in a patient with mild osteogenesis imperfecta and other phenotypic abnormalities. Microarray analysis in three of the four families showed that it did not concern a founder mutation.Conclusion: The clinical picture of complete COL1A1 allele deletions is a comparatively mild type of osteogenesis imperfecta. As such, multiplex ligation-dependent probe amplification analysis of the COL1A1 gene is a useful additional approach to defining the mutation in cases of suspected osteogenesis imperfecta type I with no detectable mutation.

Large deletions of the KCNV2 gene are common in patients with cone dystrophy with supernormal rod response

Cone dystrophy with supernormal rod response (CDSRR) is considered to be a very rare autosomal recessive retinal disorder. CDSRR is associated with mutations in KCNV2, a gene that encodes a modulatory subunit (Kv8.2) of a voltage‐gated potassium channel. In this study, we found that KCNV2 mutations are present in a substantial fraction (2.2–4.3%) of a sample of 367 independent patients with a variety of initial clinical diagnoses of cone malfunction, indicating that CDSRR is underdiagnosed and more common than previously thought. In total, we identified 20 different KCNV2 mutations; 15 of them are novel. A new finding of this study is the substantial proportion of large deletions at the KCNV2 locus that accounts for 15.5% of the mutant alleles in our sample. We determined the breakpoints and size of all five different deletions, which ranged between 10.9 and 236.8 kb. Two deletions encompass the entire KCNV2 gene and one also includes the adjacent VLDLR gene. Furthermore, we investigated N‐terminal amino acid substitution mutations for its effect on interaction with Kv2.1 using yeast two‐hybrid technology. We found that these mutations dramatically reduce or abolish this interaction suggesting a lack of assembly of heteromeric Kv channels as one underlying pathomechanism of CDSRR. 32:1398–1406, 2011. ©2011 Wiley Periodicals, Inc.

Phenotype and genotype in 103 patients with tricho-rhino-phalangeal syndrome.

Tricho-rhino-phalangeal syndrome (TRPS) is characterized by craniofacial and skeletal abnormalities, and subdivided in TRPS I, caused by mutations in TRPS1, and TRPS II, caused by a contiguous gene deletion affecting (amongst others) TRPS1 and EXT1. We performed a collaborative international study to delineate phenotype, natural history, variability, and genotype-phenotype correlations in more detail. We gathered information on 103 cytogenetically or molecularly confirmed affected individuals. TRPS I was present in 85 individuals (22 missense mutations, 62 other mutations), TRPS II in 14, and in 5 it remained uncertain whether TRPS1 was partially or completely deleted. Main features defining the facial phenotype include fine and sparse hair, thick and broad eyebrows, especially the medial portion, a broad nasal ridge and tip, underdeveloped nasal alae, and a broad columella. The facial manifestations in patients with TRPS I and TRPS II do not show a significant difference. In the limbs the main findings are short hands and feet, hypermobility, and a tendency for isolated metacarpals and metatarsals to be shortened. Nails of fingers and toes are typically thin and dystrophic. The radiological hallmark are the cone-shaped epiphyses and in TRPS II multiple exostoses. Osteopenia is common in both, as is reduced linear growth, both prenatally and postnatally. Variability for all findings, also within a single family, can be marked. Morbidity mostly concerns joint problems, manifesting in increased or decreased mobility, pain and in a minority an increased fracture rate. The hips can be markedly affected at a (very) young age. Intellectual disability is uncommon in TRPS I and, if present, usually mild. In TRPS II intellectual disability is present in most but not all, and again typically mild to moderate in severity. Missense mutations are located exclusively in exon 6 and 7 of TRPS1. Other mutations are located anywhere in exons 4-7. Whole gene deletions are common but have variable breakpoints. Most of the phenotype in patients with TRPS II is explained by the deletion of TRPS1 and EXT1, but haploinsufficiency of RAD21 is also likely to contribute. Genotype-phenotype studies showed that mutations located in exon 6 may have somewhat more pronounced facial characteristics and more marked shortening of hands and feet compared to mutations located elsewhere in TRPS1, but numbers are too small to allow firm conclusions.