A. De Paepe

LDL Receptor-Related Protein 5 (LRP5) Affects Bone Accrual and Eye Development

In humans, low peak bone mass is a significant risk factor for osteoporosis. We report that LRP5, encoding the low-density lipoprotein receptor-related protein 5, affects bone mass accrual during growth. Mutations in LRP5 cause the autosomal recessive disorder osteoporosis-pseudoglioma syndrome (OPPG). We find that OPPG carriers have reduced bone mass when compared to age- and gender-matched controls. We demonstrate LRP5 expression by osteoblasts in situ and show that LRP5 can transduce Wnt signaling in vitro via the canonical pathway. We further show that a mutant-secreted form of LRP5 can reduce bone thickness in mouse calvarial explant cultures. These data indicate that Wnt-mediated signaling via LRP5 affects bone accrual during growth and is important for the establishment of peak bone mass.

Emerging patterns of cryptic chromosomal imbalance in patients with idiopathic mental retardation and multiple congenital anomalies: a new series of 140 patients and review of published reports

Background: Chromosomal abnormalities are a major cause of mental retardation and multiple congenital anomalies (MCA/MR). Screening for these chromosomal imbalances has mainly been done by standard karyotyping. Previous array CGH studies on selected patients with chromosomal phenotypes and normal karyotypes suggested an incidence of 10–15% of previously unnoticed de novo chromosomal imbalances. Objective: To report array CGH screening of a series of 140 patients (the largest published so far) with idiopathic MCA/MR but normal karyotype. Results: Submicroscopic chromosomal imbalances were detected in 28 of the 140 patients (20%) and included 18 deletions, seven duplications, and three unbalanced translocations. Seventeen of 24 imbalances were confirmed de novo and 19 were assumed to be causal. Excluding subtelomeric imbalances, our study identified 11 clinically relevant interstitial submicroscopic imbalances (8%). Taking this and previously reported studies into consideration, array CGH screening with a resolution of at least 1 Mb has been undertaken on 432 patients with MCA/MR. Most imbalances are non-recurrent and spread across the genome. In at least 8.8% of these patients (38 of 432) de novo intrachromosomal alterations have been identified. Conclusions: Array CGH should be considered an essential aspect of the genetic analysis of patients with MCA/MR. In addition, in the present study three patients were mosaic for a structural chromosome rearrangement. One of these patients had monosomy 7 in as few as 8% of the cells, showing that array CGH allows detection of low grade mosaicisims.

Effect of Mutation Type and Location on Clinical Outcome in 1,013 Probands with Marfan Syndrome or Related Phenotypes and FBN1 Mutations: An International Study

Mutations in the fibrillin-1 (FBN1) gene cause Marfan syndrome (MFS) and have been associated with a wide range of overlapping phenotypes. Clinical care is complicated by variable age at onset and the wide range of severity of aortic features. The factors that modulate phenotypical severity, both among and within families, remain to be determined. The availability of international FBN1 mutation Universal Mutation Database (UMD-FBN1) has allowed us to perform the largest collaborative study ever reported, to investigate the correlation between the FBN1 genotype and the nature and severity of the clinical phenotype. A range of qualitative and quantitative clinical parameters (skeletal, cardiovascular, ophthalmologic, skin, pulmonary, and dural) was compared for different classes of mutation (types and locations) in 1,013 probands with a pathogenic FBN1 mutation. A higher probability of ectopia lentis was found for patients with a missense mutation substituting or producing a cysteine, when compared with other missense mutations. Patients with an FBN1 premature termination codon had a more severe skeletal and skin phenotype than did patients with an inframe mutation. Mutations in exons 24-32 were associated with a more severe and complete phenotype, including younger age at diagnosis of type I fibrillinopathy and higher probability of developing ectopia lentis, ascending aortic dilatation, aortic surgery, mitral valve abnormalities, scoliosis, and shorter survival; the majority of these results were replicated even when cases of neonatal MFS were excluded. These correlations, found between different mutation types and clinical manifestations, might be explained by different underlying genetic mechanisms (dominant negative versus haploinsufficiency) and by consideration of the two main physiological functions of fibrillin-1 (structural versus mediator of TGF beta signalling). Exon 24-32 mutations define a high-risk group for cardiac manifestations associated with severe prognosis at all ages.

The molecular genetics of Marfan syndrome and related disorders.

Marfan syndrome (MFS), a relatively common autosomal dominant hereditary disorder of connective tissue with prominent manifestations in the skeletal, ocular, and cardiovascular systems, is caused by mutations in the gene for fibrillin-1 (FBN1). The leading cause of premature death in untreated individuals with MFS is acute aortic dissection, which often follows a period of progressive dilatation of the ascending aorta. Recent research on the molecular physiology of fibrillin and the pathophysiology of MFS and related disorders has changed our understanding of this disorder by demonstrating changes in growth factor signalling and in matrix-cell interactions. The purpose of this review is to provide a comprehensive overview of recent advances in the molecular biology of fibrillin and fibrillin-rich microfibrils. Mutations in FBN1 and other genes found in MFS and related disorders will be discussed, and novel concepts concerning the complex and multiple mechanisms of the pathogenesis of MFS will be explained.

The Ehlers–Danlos syndrome, a disorder with many faces

The Ehlers–Danlos syndromes (EDSs) comprise a heterogeneous group of diseases, characterized by fragility of the soft connective tissues and widespread manifestations in skin, ligaments, joints, blood vessels and internal organs. The clinical spectrum varies from mild skin and joint hyperlaxity to severe physical disability and life‐threatening vascular complications. The current Villefranche classification recognizes six subtypes, most of which are linked to mutations in genes encoding fibrillar collagens or enzymes involved in post‐translational modification of these proteins. Mutations in type V and type III collagen cause classic or vascular EDS respectively, while mutations involving the processing of type I collagen are involved in the kyphoscoliosis, arthrochalasis and dermatosparaxis type of EDS. Establishing the correct EDS subtype has important implications for genetic counseling and management and is supported by specific biochemical and molecular investigations. Over the last years, several new EDS variants have been characterized which call for a refinement of the Villefranche classification. Moreover, the study of these diseases has brought new insights into the molecular pathogenesis of EDS by implicating genetic defects in the biosynthesis of other extracellular matrix (ECM) molecules, such as proteoglycans and tenascin‐X, or genetic defects in molecules involved in intracellular trafficking, secretion and assembly of ECM proteins.

Mutations in Fibrillin-1 Cause Congenital Scleroderma: Stiff Skin Syndrome

Stiff skin syndrome, an autosomal dominant congenital form of scleroderma, is caused by mutations in the domain of fibrillin-1 that mediates integrin binding. Variation in Rare Disease Linked to Common Skin Disorder In the epic words of British physician and researcher Dr. William Harvey, “Nature is nowhere accustomed more openly to display her secret mysteries than in cases where she shows traces of her workings apart from the beaten path; nor is there any better way to advance the proper practice of medicine than to give our minds to the discovery of the usual law of nature by the careful investigation of cases of rarer forms of disease. For it has been found in almost all things, that what they contain of useful or of applicable nature, is hardly perceived unless we are deprived of them, or they become deranged in some way.” Such is the case for rare genetic diseases, which have provided the framework to understand some of the most devastating common diseases from just the simple permutation of a gene. Scleroderma, which literally means a pathological hardening of the skin, manifests as a complex phenotype. To better understand the etiology of scleroderma, Loeys and colleagues investigate four families with more than 10 affected individuals with a rare congenital form called stiff skin syndrome—which together account for more than 25% of the total cases currently documented in the literature—and identify the key mutations in a gene that encodes the connective tissue protein fibrillin-1. Fibrosis is not only seen in the context of systemic connective tissue disorders, but rather is often the major signature of the inflammatory burden in many common disorders, providing incentive to understand the factors critical in the initiation and maintenance of profibrotic programs. Mutations causing stiff skin syndrome cluster within a single domain of fibrillin-1 that mediates integin binding. Low amounts of an activated protein kinase that is triggered by ligand-integrin interactions provided further evidence for altered cell matrix interactions. This associates with evidence for activation of the TGFβ signaling cascade, a mechanism to instruct cells to deposit collagens and other matrix elements both during normal wound healing and in various fibrotic states. These data are consistent with a model in which integrins provide a means for cells to sample the matrix and to adjust their synthetic repertoire accordingly. Loss of this feedback would culminate in fibrosis. The results garnered from these patients with this rare disease prompted the authors to examine skin biopsies from five patients with common scleroderma, which surprisingly revealed that each scleroderma patient showed all of the abnormalities seen in stiff skin syndrome. These findings reinforce the power of studying rare genetic disorders to inform the underlying causes of common diseases, but also unveil a potential avenue for therapeutic intervention in common disorders that include fibrosis. The predisposition for scleroderma, defined as fibrosis and hardening of the skin, is poorly understood. We report that stiff skin syndrome (SSS), an autosomal dominant congenital form of scleroderma, is caused by mutations in the sole Arg-Gly-Asp sequence–encoding domain of fibrillin-1 that mediates integrin binding. Ordered polymers of fibrillin-1 (termed microfibrils) initiate elastic fiber assembly and bind to and regulate the activation of the profibrotic cytokine transforming growth factor–β (TGFβ). Altered cell-matrix interactions in SSS accompany excessive microfibrillar deposition, impaired elastogenesis, and increased TGFβ concentration and signaling in the dermis. The observation of similar findings in systemic sclerosis, a more common acquired form of scleroderma, suggests broad pathogenic relevance.

Exclusion of mutations in the gene for type III collagen (COL3A1) as a common cause of intracranial aneurysms or cervical artery dissections: Results from sequence analysis of the coding sequences of type III collagen from 55 unrelated patients

We performed detailed DNA sequencing analysis on type III collagen cDNA from 58 patients with either intracranial artery aneurysms or cervical artery dissections. The 58 patients were of seven different nationalities; among the patients were three pairs of relatives, so that 55 were unrelated, and of these, 29 had at least one blood relative with either an intracranial artery aneurysm or a cervical artery dissection. The age of the patients at the time of diagnosis ranged from 15 to 68 years (mean ± SD = 40.3 ± 11.0). The study group consisted of 25 males and 33 females. The analysis covered 3,232 nucleotides of significant (nonredundant) sequences per allele; therefore, we analyzed as many as 355,520 nucleotides. Mutations in the coding sequences for the triple-helical domain of type III collagen were excluded in 40 individuals with intracranial aneurysms and 18 individuals with cervical artery dissections. Direct sequencing of polymerase chain reaction products allowed mutations to be excluded with a high degree of confidence. Mutations that markedly decreased expression from one allele were also excluded in 42 of the 58 individuals, since the presence of both bases at one or more polymorphic sites in the 42 patients showed that two alleles were transcribed. The results indicated that mutations in the gene for type III procollagen (COL3A1) are not a common cause of either intracranial artery aneurysms or cervical artery dissections.

Deletions involving long-range conserved nongenic sequences upstream and downstream of FOXL2 as a novel disease-causing mechanism in blepharophimosis syndrome.

The expression of a gene requires not only a normal coding sequence but also intact regulatory regions, which can be located at large distances from the target genes, as demonstrated for an increasing number of developmental genes. In previous mutation studies of the role of FOXL2 in blepharophimosis syndrome (BPES), we identified intragenic mutations in 70% of our patients. Three translocation breakpoints upstream of FOXL2 in patients with BPES suggested a position effect. Here, we identified novel microdeletions outside of FOXL2 in cases of sporadic and familial BPES. Specifically, four rearrangements, with an overlap of 126 kb, are located 230 kb upstream of FOXL2, telomeric to the reported translocation breakpoints. Moreover, the shortest region of deletion overlap (SRO) contains several conserved nongenic sequences (CNGs) harboring putative transcription-factor binding sites and representing potential long-range cis-regulatory elements. Interestingly, the human region orthologous to the 12-kb sequence deleted in the polled intersex syndrome in goat, which is an animal model for BPES, is contained in this SRO, providing evidence of human-goat conservation of FOXL2 expression and of the mutational mechanism. Surprisingly, in a fifth family with BPES, one rearrangement was found downstream of FOXL2. In addition, we report nine novel rearrangements encompassing FOXL2 that range from partial gene deletions to submicroscopic deletions. Overall, genomic rearrangements encompassing or outside of FOXL2 account for 16% of all molecular defects found in our families with BPES. In summary, this is the first report of extragenic deletions in BPES, providing further evidence of potential long-range cis-regulatory elements regulating FOXL2 expression. It contributes to the enlarging group of developmental diseases caused by defective distant regulation of gene expression. Finally, we demonstrate that CNGs are candidate regions for genomic rearrangements in developmental genes.

A new recurrent inversion, inv(7)(p15q34), leads to transcriptional activation of HOXA10 and HOXA11 in a subset of T-cell acute lymphoblastic leukemias

Chromosomal translocations with breakpoints in T-cell receptor (TCR) genes are recurrent in T-cell malignancies. These translocations involve the TCRαδ gene (14q11), the TCRβ gene (7q34) and to a lesser extent the TCRγ gene at chromosomal band 7p14 and juxtapose T-cell oncogenes next to TCR regulatory sequences leading to deregulated expression of those oncogenes. Here, we describe a new recurrent chromosomal inversion of chromosome 7, inv(7)(p15q34), in a subset of patients with T-cell acute lymphoblastic leukemia characterized by CD2 negative and CD4 positive, CD8 negative blasts. This rearrangement juxtaposes the distal part of the HOXA gene cluster on 7p15 to the TCRβ locus on 7q34. Real time quantitative PCR analysis for all HOXA genes revealed high levels of HOXA10 and HOXA11 expression in all inv(7) positive cases. This is the first report of a recurrent chromosome rearrangement targeting the HOXA gene cluster in T-cell malignancies resulting in deregulated HOXA gene expression (particularly HOXA10 and HOXA11) and is in keeping with a previous report suggesting HOXA deregulation in MLL-rearranged T- and B cell lymphoblastic leukemia as the key factor in leukaemic transformation. Finally, our observation also supports the previous suggested role of HOXA10 and HOXA11 in normal thymocyte development.

Mutation analysis of P73 and TP53 in Merkel cell carcinoma.

The p73 gene has been mapped to 1p36.33, a region which is frequently deleted in a wide variety of neoplasms including tumours of neuroectodermal origin. The p73 protein shows structural and functional homology to p53. For these reasons, p73 was considered as a positional and functional candidate tumour suppressor gene. Thus far, mutation analysis has provided no evidence for involvement of p73 in oligodendrogliomas, lung carcinoma, oesophageal carcinoma, prostatic carcinoma and hepatocellular carcinoma. In neuroblastoma, two mutations have been observed in a series of 140 tumours. In view of the occurrence of 1p deletions in Merkel cell carcinoma (MCC) and the location of p73 we decided to search for mutations in the p73 gene in five MCC cell lines and ten MCC tumours to test potential tumour suppressor function for this gene in MCC. In view of the possible complementary functions of p73 and TP53 we also examined the status of the TP53 gene. Sequence analysis of the entire coding region of the p73 gene revealed previously reported polymorphisms in four MCCs. In one MCC tumour, a mis-sense mutation located in the NH2-terminal transactivation region of the p73 gene was found. These results show that p73, analogous to neuroblastoma, is infrequently mutated in MCC. This is also the first report in which the role of TP53 in MCC has been investigated by sequencing the entire coding region of TP53. TP53 mis-sense mutations and one non-sense mutation were detected in three of 15 examined MCCs, suggesting that TP53 mutations may play a role in the pathogenesis or progression of a subset of MCCs. Moreover, typical UVB induced C to T mutations were found in one MCC cell line thus providing further evidence for sun-exposure in the aetiology of this rare skin cancer.