An FBN1 deep intronic mutation in a familial case of Marfan syndrome: an explanation for genetically unsolved cases?

Overcoming challenges associated with identifying FBN1 deep intronic variants through whole-genome sequencing

BACKGROUND

Marfan syndrome (MFS), caused by pathogenic variants of FBN1 (fibrillin-1), is a systemic connective tissue disorder with variable phenotypes and treatment responsiveness depending on the variant. However, a significant number of individuals with MFS remain genetically unexplained. In this study, we report novel pathogenic intronic variants in FBN1 in two unrelated families with MFS.

METHODS

We evaluated subjects with suspected MFS from two unrelated families using Sanger sequencing or multiplex ligation-dependent probe amplification of FBN1 and/or panel-based next-generation sequencing. As no pathogenic variants were identified, whole-genome sequencing was performed. Identified variants were analyzed by reverse transcription-PCR and targeted sequencing of FBN1 mRNA harvested from peripheral blood or skin fibroblasts obtained from affected probands.

RESULTS

We found causative deep intronic variants, c.6163+1484A>T and c.5788+36C>A, in FBN1. The splicing analysis revealed an insertion of in-frame or out-of-frame intronic sequences of the FBN1 transcript predicted to alter function of calcium-binding epidermal growth factor protein domain. Family members carrying c.6163+1484A>T had high systemic scores including prominent skeletal features and aortic dissection with lesser aortic dilatation. Family members carrying c.5788+36C>A had more severe aortic root dilatation without aortic dissection. Both families had ectopia lentis.

CONCLUSION

Variable penetrance of the phenotype and negative genetic testing in MFS families should raise the possibility of deep intronic FBN1 variants and the need for additional molecular studies. This study expands the mutation spectrum of FBN1 and points out the importance of intronic sequence analysis and the need for integrative functional studies in MFS diagnosis.

An FBN1 deep intronic variant is associated with pseudoexon formation and a variable Marfan phenotype in a five generation family

Exome sequencing of genes associated with heritable thoracic aortic disease (HTAD) failed to identify a pathogenic variant in a large family with Marfan syndrome (MFS). A genome-wide linkage analysis for thoracic aortic disease identified a peak at 15q21.1, and genome sequencing identified a novel deep intronic FBN1 variant that segregated with thoracic aortic disease in the family (LOD score 2.7) and was predicted to alter splicing. RT-PCR and bulk RNA sequencing of RNA harvested from fibroblasts explanted from the affected proband revealed an insertion of a pseudoexon between exons 13 and 14 of the FBN1 transcript, predicted to lead to nonsense mediated decay (NMD). Treating the fibroblasts with an NMD inhibitor, cycloheximide, greatly improved the detection of the pseudoexon-containing transcript. Family members with the FBN1 variant had later onset aortic events and fewer MFS systemic features than typical for individuals with haploinsufficiency of FBN1. Variable penetrance of the phenotype and negative genetic testing in MFS families should raise the possibility of deep intronic FBN1 variants and the need for additional molecular studies.

Combined cardiac surgery in a Marfan syndrome patient with severe scoliosis via lower hemisternotomy: a case report

Background

Scoliosis is one of the symptoms manifested by patients with Marfan syndrome (MFS). Deformity of the thoracic cavity due to severe scoliosis may cause difficulty during cardiac surgery in terms of the surgical approach and instrument manipulation; however, only a few reports have been available regarding the surgical case of MFS with severe scoliosis. Here, we report a case of combined aortic valve replacement and left atrial appendage closure in a patient with MFS who had severe scoliosis using lower hemisternotomy.

Case presentation

A 62-year-old female with MFS was referred to our hospital after being diagnosed with severe aortic regurgitation and paroxysmal atrial fibrillation with a history of cerebral thromboembolism. The aortic valve showed severe insufficiency due to cusp prolapse, whereas the aortic root was moderately dilated (42 mm). Echocardiography revealed severe regurgitation with reduced left ventricular ejection function (32%) and massive left ventricular diastolic dimension (88 mm). Moreover, combined aortic valve replacement and left atrial appendage closure was indicated. However, the patient had chest deformity due to severe scoliosis. Thus, conventional full sternotomy or thoracotomy was considered an inappropriate surgical approach. Lower hemisternotomy was selected on the basis of three-dimensional reconstruction imaging of the aorta, left atrial appendage, sternum, and rib. Sternal elevation and rib retraction with the costal arch folded back provided enough surgical field for the combined procedures to be safely conducted. The postoperative course was uneventful, except for predicted prolonged mechanical ventilation with the assistance of intraaortic balloon pumping. Thereafter, the patient has been free from any cardiac and cerebrovascular event.

Conclusions

Lower hemisternotomy can be useful for combined cardiac surgery in MFS with severe scoliosis.

Functional Analysis of an Intronic FBN1 Pathogenic Gene Variant in a Family With Marfan Syndrome

Marfan syndrome (MFS) is an autosomal dominant connective tissue disorder that canonically affects the ocular, skeletal, and cardiovascular system, in which aortic tear and rupture is the leading cause of death for MFS patients. Genetically, MFS is primarily associated with fibrillin-1 (FBN1) pathogenic variants. However, the disease-causing variant in approximately 10% of patients cannot be identified, partly due to some cryptic mutations that may be missed using routine exonic sequencing, such as non-coding intronic variants that affects the RNA splicing process. We present a 32-year female with typical MFS systemic presentation that reached to a clinical diagnosis according to the revised Ghent nosology. We performed whole-exome sequencing (WES) but the report failed to identify known causal variants when analyzing the exonic sequence. However, further investigation on the exon/intron boundaries of the WES report revealed a candidate intronic variant of the fibrillin 1 (FBN1) gene (c.248-3 C>G) that predicted to affect the RNA splicing process. We conducted minigene splicing analyses and demonstrated that the c.248-3 C>G variant abolished the canonical splicing site of intron 3, leading to activation of two cryptic splicing sites and causing insertion (c.248-1_248-2insAG and c.248-1_248-282ins). Our study not only characterizes an intronic variant to the mutational spectrum of the FBN1 gene in MFS and its aberrant effect on splicing, but highlights the importance to not neglect the exon/intron boundaries when reporting and assessing WES results. We point out the need of conducting functional analysis to verify the pathogenicity of intronic mutation, and the opportunity to re-consider the standard diagnostic approaches in cases of clinically diagnosed MFS with normal or variant of unknown significance genetic results.

Expanding the clinical spectrum of COL2A1 related disorders by a mass like phenotype

MASS phenotype is a connective tissue disorder clinically overlapping with Marfan syndrome and caused by pathogenic variants in FBN1. We report four patients from three families presenting with a MASS-like phenotype consisting of tall stature, arachnodactyly, spinal deformations, dural ectasia, pectus and/or feet deformations, osteoarthritis, and/or high arched palate. Gene panel sequencing was negative for FBN1 variants. However, it revealed likely pathogenic missense variants in three individuals [c.3936G > T p.(Lys1312Asn), c.193G > A p.(Asp65Asn)] and a missense variant of unknown significance in the fourth patient [c.4013G > A p.(Ser1338Asn)] in propeptide coding regions of COL2A1. Pathogenic COL2A1 variants are associated with type II collagenopathies comprising a remarkable clinical variablility. Main features include skeletal dysplasia, ocular anomalies, and auditory defects. A MASS-like phenotype has not been associated with COL2A1 variants before. Thus, the identification of likely pathogenic COL2A1 variants in our patients expands the phenotypic spectrum of type II collagenopathies and suggests that a MASS-like phenotype can be assigned to various hereditary disorders of connective tissue. We compare the phenotypes of our patients with related disorders of connective tissue and discuss possible pathomechanisms and genotype–phenotype correlations for the identified COL2A1 variants. Our data recommend COL2A1 sequencing in FBN1-negative patients suggestive for MASS/Marfan-like phenotype (without aortopathy).

Analysis of Pathogenic Pseudoexons Reveals Novel Mechanisms Driving Cryptic Splicing

Understanding pre-mRNA splicing is crucial to accurately diagnosing and treating genetic diseases. However, mutations that alter splicing can exert highly diverse effects. Of all the known types of splicing mutations, perhaps the rarest and most difficult to predict are those that activate pseudoexons, sometimes also called cryptic exons. Unlike other splicing mutations that either destroy or redirect existing splice events, pseudoexon mutations appear to create entirely new exons within introns. Since exon definition in vertebrates requires coordinated arrangements of numerous RNA motifs, one might expect that pseudoexons would only arise when rearrangements of intronic DNA create novel exons by chance. Surprisingly, although such mutations do occur, a far more common cause of pseudoexons is deep-intronic single nucleotide variants, raising the question of why these latent exon-like tracts near the mutation sites have not already been purged from the genome by the evolutionary advantage of more efficient splicing. Possible answers may lie in deep intronic splicing processes such as recursive splicing or poison exon splicing. Because these processes utilize intronic motifs that benignly engage with the spliceosome, the regions involved may be more susceptible to exonization than other intronic regions would be. We speculated that a comprehensive study of reported pseudoexons might detect alignments with known deep intronic splice sites and could also permit the characterisation of novel pseudoexon categories. In this report, we present and analyse a catalogue of over 400 published pseudoexon splice events. In addition to confirming prior observations of the most common pseudoexon mutation types, the size of this catalogue also enabled us to suggest new categories for some of the rarer types of pseudoexon mutation. By comparing our catalogue against published datasets of non-canonical splice events, we also found that 15.7% of pseudoexons exhibit some splicing activity at one or both of their splice sites in non-mutant cells. Importantly, this included seven examples of experimentally confirmed recursive splice sites, confirming for the first time a long-suspected link between these two splicing phenomena. These findings have the potential to improve the fidelity of genetic diagnostics and reveal new targets for splice-modulating therapies.

Sudden death due to a novel nonsense mutation in Marfan syndrome

BACKGROUND

Marfan syndrome is a hereditary connective tissue disease accompanied by autosomal dominant inheritance; that mainly arises from a mutation in the fibrillin-1 gene (FBN1). Aortic dissection and rupture are the common and lethal complications of MFS and may cause sudden unexpected death.

METHOD

A man aged 34 was admitted to the hospital due to persistent pain in his abdomen 12 h post-drinking and suddenly died 10 h later. A forensic autopsy was performed to identify the underlying mechanism of death. Due to the high suspected of MFS, Sanger sequencing was performed, and a novel mutation was detected in the deceased. To clarify the underlying mechanism of this mutation, real-time quantitative polymerase chain reaction was conducted and Western blot analysis was performed in vitro.

RESULTS

A novel PTC mutation c.933C > A in FBN1 was found. Through family history inspection and Sanger sequencing, other MFS patients in the present family were confirmed. The pathologic changes in the aorta in the present case showed media cystic degeneration, disordered arrangement of elastic fibers and a significant reduction in fibrillin 1 compared with the control. The mutation led to significant reduction inFBN1 mRNA and fibrillin-1 in cells in vitro, and overexpression of phospho-Smad2 was observed.

CONCLUSION

We confirmed a novel pathogenic PTC mutation in the FBN1gene through Sanger sequencing, and the pathological changes and underlying mechanisms were also identified. The present work not only extends the pathogenic mutation spectrum of MFS, but also stresses the role of forensic autopsy, genetic analysis and functional validation of novel mutations in cases of sudden death associated with congenital diseases.

Marfan syndrome and the eye clinic: from diagnosis to management

Marfan syndrome (MFS) is an autosomal dominantly inherited disorder affecting the cardiovascular, ocular and musculoskeletal systems. Frequently, clinical suspicion and subsequent diagnosis begins in the ophthalmology clinic. Importantly, the ophthalmologist has a responsibility to cater not only to the eye, but also to be involved in a holistic approach for these patients. In this review, we discuss how MFS may present to an eye clinic, including clinical features, ocular morbidity, genetic diagnosis and management. Although this condition is ideally managed by a multidisciplinary team, our focus will be on MFS and the eye, including other conditions which may present with similar phenotypes. The ophthalmologist's role as the potential first contact for a patient with suspected MFS is crucial in making the proper investigations and referral, with the knowledge that not all ectopia lentis cases are MFS and vice versa. Management of ocular conditions in MFS may range from simple observation to surgical intervention; current options will be discussed.

Optimising the mutation screening strategy in Marfan syndrome and identifying genotypes with more severe aortic involvement

Background

Marfan syndrome (MFS) is a systemic connective tissue disorder with life-threatening manifestations affecting the ascending aorta. MFS is caused by dominant negative (DN) and haploinsufficient (HI) mutations of the FBN1 gene. Our aim was to identify mutations of MFS patients with high detection rate and to investigate the use of a gene panel for patients with Marfanoid habitus. We also aimed to examine correlations between genotype and cardiovascular manifestations to predict “malignant” mutations.

Methods

136 individuals were enrolled. In the first phase, next-generation sequencing (NGS) and Sanger sequencing were performed for 57 patients to screen the FBN1 gene, followed by multiplex ligation-dependent probe amplification (MLPA) in negative cases. For repeated negative results, NGS gene panel involving 9 genes was used. In the second phase, 79 patients were tested primarily with the same gene panel, negative samples were tested by MLPA.

Results

84 pathogenic mutations were detected, out of which 78 affected FBN1, 6 non-FBN1 mutations (2 TGFB2, 1 TGFBR2, 2 TGFBR1, 1 SMAD3) are associated with Loeys-Dietz syndrome (LDS). LDS patients had lower systemic score and they were younger, but their aortic involvement did not differ. MLPA detected 4 multi-exon deletions of FBN1 gene, which could not be identified by our first-step screening method. Aortic involvement (aortic dissection and/or dilation) did not differ significantly among HI and DN mutations (p = 0.061). Combined group of HI and DN mutations eliminating a disulphide-bonding cysteine (DN Cys) had significantly higher aortic involvement rate than DN mutations not eliminating a disulphide-bonding cysteine (DN non-Cys) (p < 0.001). Patients with DN Cys required significantly more aortic surgeries than HI and DN non-Cys mutations (p = 0.042 and p = 0.015, respectively).

Conclusions

Due to the relevant number of mutations affecting genes other than FBN1, preferred approach for testing individuals with Marfanoid habitus is using a gene panel rather than single-gene analysis, followed by MLPA for negative samples. DN Cys and HI mutations should be considered as risk factors for aortic involvement. Genetic testing for patients with Marfanoid features and a systemic score under 7 is recommended, as LDS patients may have lower scores, but they may have severe cardiovascular manifestations.

Full-Mouth Rehabilitation With Implant-Prosthesis in Marfan Syndrome Patient: Clinical Report and Literature Review

The purpose of this case report is to present implant-retained maxillary and mandibular complete overdentures in a patient with Marfan syndrome. The patient initially presented with generalized periodontitis (stage IV, grade C). Due to the progressive nature of periodontal disease, the patient elected to have implant-retained maxillary and mandibular complete dentures. Bilateral maxillary sinus augmentation was performed 6 months before full-mouth extraction, alveoloplasty, and immediate implant placement. Maxillary and mandibular immediate overdentures were delivered. After 4 months of healing, the final overdenture was fabricated. The patient was seen regularly throughout the healing process for peri-implant maintenance. Soft-tissue grafts were completed to increase the thickness of the mucosa around the implants. The patient has been followed for 2 years and is functioning well without major complications. For patients with Marfan syndrome, implant-retained prostheses are a viable treatment option in the presence of a failing dentition.

The genetics and biomechanics of thoracic aortic diseases

Thoracic aortic aneurysms and aortic dissections (TAAD) are highly fatal emergencies within cardiothoracic surgery. With increasing age, thoracic aneurysms become more prevalent and pose an even greater threat when they develop into aortic dissections. Both diseases are multifactorial and are influenced by a multitude of physiological and biomechanical processes. Structural stability of aorta can be disrupted by genes, such as those for extracellular matrix and contractile protein, as well as telomere dysfunction, which leads to senescence of smooth muscle and endothelial cells. Biomechanical changes such as increased luminal pressure imposed by hypertension are also very prevalent and lead to structural instability. Furthermore, ageing is associated with a pro-inflammatory state that exacerbates degeneration of vessel wall, facilitating the development of both aortic aneurysms and aortic dissection. This literature review provides an overview of the aetiology and pathophysiology of both thoracic aneurysms and aortic dissections. With an improved understanding, new therapeutic targets may eventually be identified to facilitate treatment and prevention of these diseases.

Understanding human DNA variants affecting pre-mRNA splicing in the NGS era

Pre-mRNA splicing, an essential step in eukaryotic gene expression, relies on recognition of short sequences on the primary transcript intron ends and takes place along transcription by RNA polymerase II. Exonic and intronic auxiliary elements may modify the strength of exon definition and intron recognition. Splicing DNA variants (SV) have been associated with human genetic diseases at canonical intron sites, as well as exonic substitutions putatively classified as nonsense, missense or synonymous variants. Their effects on mRNA may be modulated by cryptic splice sites associated to the SV allele, comprehending exon skipping or shortening, and partial or complete intron retention. As splicing mRNA outputs result from combinatorial effects of both intrinsic and extrinsic factors, in vitro functional assays supported by computational analyses are recommended to assist SV pathogenicity assessment for human Mendelian inheritance diseases. The increasing use of next-generating sequencing (NGS) targeting full genomic gene sequence has raised awareness of the relevance of deep intronic SV in genetic diseases and inclusion of pseudo-exons into mRNA. Finally, we take advantage of recent advances in sequencing and computational technologies to analyze alternative splicing in cancer. We explore the Catalog of Somatic Mutations in Cancer (COSMIC) to describe the proportion of splice-site mutations in cis and trans regulatory elements. Genomic data from large cohorts of different cancer types are increasingly available, in addition to repositories of normal and somatic genetic variations. These are likely to bring new insights to understanding the genetic control of alternative splicing by mapping splicing quantitative trait loci in tumors.

A Novel Heterozygous Intronic Mutation in the FBN1 Gene Contributes to FBN1 RNA Missplicing Events in the Marfan Syndrome

Marfan syndrome (MFS) is an autosomal dominantly inherited connective tissue disorder, mostly caused by mutations in the fibrillin-1 (FBN1) gene. We, by using targeted next-generation sequence analysis, identified a novel intronic FBN1 mutation (the c.2678-15C>A variant) in a MFS patient with aortic dilatation. The computational predictions showed that the heterozygous c.2678-15C>A intronic variant might influence the splicing process by differentially affecting canonical versus cryptic splice site utilization within intron 22 of the FBN1 gene. RT-PCR and Western blot analyses, using FBN1 minigenes transfected into HeLa and COS-7 cells, revealed that the c.2678-15C>A variant disrupts normal splicing of intron 22 leading to aberrant 13-nt intron 22 inclusion, frameshift, and premature termination codon. Collectively, the results strongly suggest that the c.2678-15C>A variant could lead to haploinsufficiency of the FBN1 functional protein and structural connective tissue fragility in MFS complicated by aorta dilation, a finding that further expands on the genetic basis of aortic pathology.

Gross deletions in FBN1 results in variable phenotypes of Marfan syndrome

BACKGROUND

A mutation in FBN1 is primarily attributed to Marfan syndrome (MFS). So far, >1800 unique FBN1 mutations have been identified, with the vast majority being single-nucleotide substitutions, small deletions, and insertions. The rearrangement of large fragments of FBN1 accounts for only 1.7% of all variants. The aim of this study was to investigate the characteristics of large genomic rearrangements in FBN1 among MFS patients and to evaluate the correlations between genotype and phenotype.

METHODS

Systematic sequencing of the disease-related genes FBN1, TGFBR1, and TGFBR2, was carried out previously for 26 unrelated patients with MFS. No small mutations were found. Subsequently, multiplex ligation-dependent probe amplification was performed for the detection of copy number variations in these patients. The breakpoints were determined by gap PCR and sequencing. Transcription level analysis was conducted in patients whose RNA sample was available.

RESULTS

Four gross deletions were identified in FBN1. Three deletions (exons 6, 48-53, and 49-50) were predicted to be in-frame deletions; the remaining deletion (exons 1-36) was expected to induce the loss of one copy of the FBN1 gene. The breakpoints of these four deletions were cloned, and revealed deletion sizes of 16,551, 10,346, 4563, and 187,047bp, respectively. Patients with in-frame deletions of exons 48-53 and 49-50 showed severe clinical phenotypes; Patient with an exon 6 deletion showed mild potential MFS phenotypes. And the patient had classic MFS with a deletion of exons 1-36.

CONCLUSIONS

We characterized four large genomic rearrangements in FBN1. FBN1 haploinsufficiency correlated with a classic MFS phenotype, while in-frame deletions between exons 24-53 of FBN1 tended to cause severe clinical phenotypes.

Hereditary Influence in Thoracic Aortic Aneurysm and Dissection

Thoracic aortic aneurysm is a potentially life-threatening condition in that it places patients at risk for aortic dissection or rupture. However, our modern understanding of the pathogenesis of thoracic aortic aneurysm is quite limited. A genetic predisposition to thoracic aortic aneurysm has been established, and gene discovery in affected families has identified several major categories of gene alterations. The first involves mutations in genes encoding various components of the transforming growth factor beta (TGF-β) signaling cascade (FBN1, TGFBR1, TGFBR2, TGFB2, TGFB3, SMAD2, SMAD3 and SKI), and these conditions are known collectively as the TGF-β vasculopathies. The second set of genes encode components of the smooth muscle contractile apparatus (ACTA2, MYH11, MYLK, and PRKG1), a group called the smooth muscle contraction vasculopathies. Mechanistic hypotheses based on these discoveries have shaped rational therapies, some of which are under clinical evaluation. This review discusses published data on genes involved in thoracic aortic aneurysm and attempts to explain divergent hypotheses of aneurysm origin.

Successful Application of Whole Genome Sequencing in a Medical Genetics Clinic

A pilot program was initiated using whole genome sequencing (WGS) to diagnose suspected genetic disorders in the Genetics Clinic at Children's Hospital of Wisconsin. Twenty-two patients underwent WGS between 2010 and 2013. Initially, we obtained a 14% (3/22) diagnosis rate over 2 years; with subsequent reanalysis, this increased to 36% (8/22). Disease causing variants were identified in SKIV2L, CECR1, DGKE, PYCR2, RYR1, PDGFRB, EFTUD2, and BCS1L. In 75% (6/8) of diagnosed cases, the diagnosis affected treatment and/or medical surveillance. Additionally, one case demonstrated a homozygous A18V variant in VLDLR that appears to be associated with a previously undescribed phenotype.

FBN1: The disease-causing gene for Marfan syndrome and other genetic disorders

FBN1 encodes the gene for fibrillin-1, a structural macromolecule that polymerizes into microfibrils. Fibrillin microfibrils are morphologically distinctive fibrils, present in all connective tissues and assembled into tissue-specific architectural frameworks. FBN1 is the causative gene for Marfan syndrome, an inherited disorder of connective tissue whose major features include tall stature and arachnodactyly, ectopia lentis, and thoracic aortic aneurysm and dissection. More than one thousand individual mutations in FBN1 are associated with Marfan syndrome, making genotype-phenotype correlations difficult. Moreover, mutations in specific regions of FBN1 can result in the opposite features of short stature and brachydactyly characteristic of Weill-Marchesani syndrome and other acromelic dysplasias. How can mutations in one molecule result in disparate clinical syndromes? Current concepts of the fibrillinopathies require an appreciation of tissue-specific fibrillin microfibril microenvironments and the collaborative relationship between the structures of fibrillin microfibril networks and biological functions such as regulation of growth factor signaling.

Genetic testing in Marfan syndrome

Genetic testing is aiding rapid diagnosis of Marfan syndrome as a basis for management of eye, heart and skeletal disease. The affected patient's mutation can be used as a basis for prenatal or postnatal diagnosis of offspring. Preimplantation genetic diagnosis, the technique of choice, can ensure an unaffected pregnancy.

Prevalence, incidence, and age at diagnosis in Marfan Syndrome

BACKGROUND

Marfan syndrome is a genetic disorder with considerable morbidity and mortality. Presently, clinicians use the 2010 revised Ghent nosology, which includes optional genetic sequencing of the FBN1 gene, to diagnose patients. So far, only a few studies based on older diagnostic criteria have reported a wide range of prevalence and incidence. Our aim was to study prevalence, incidence, and age at diagnosis in patients with Marfan syndrome.

METHOD

Using unique Danish patient-registries, we identified all possible Marfan syndrome patients recorded by the Danish healthcare system (1977-2014). Following, we confirmed or rejected the diagnosis according to the 2010 revised Ghent nosology.

RESULTS

We identified a total of 1628 persons with possible Marfan syndrome. We confirmed the diagnosis in 412, whereof 46 were deceased, yielding a maximum prevalence of 6.5/100,000 at the end of 2014. The annual median incidence was 0.19/100,000 (range: 0.0-0.7) which increased significantly with an incidence rate ratio of 1.03 (95% CI: 1.02-1.04, p < 0.001). We found a median age at diagnose of 19.0 years (range: 0.0-74). The age at diagnosis increased during the study period, uninfluenced by the changes in diagnostic criteria. We found no gender differences.

CONCLUSION

The increasing prevalence of Marfan syndrome during the study period is possibly due to build-up of a registry. Since early diagnosis is essential in preventing aortic events, diagnosing Marfan syndrome remains a task for both pediatricians and physicians caring for adults.

Buried in the Middle but Guilty: Intronic Mutations in the TCIRG1 Gene Cause Human Autosomal Recessive Osteopetrosis

Autosomal recessive osteopetrosis (ARO) is a rare genetic bone disease with genotypic and phenotypic heterogeneity, sometimes translating into delayed diagnosis and treatment. In particular, cases of intermediate severity often constitute a diagnostic challenge and represent good candidates for exome sequencing. Here, we describe the tortuous path to identification of the molecular defect in two siblings, in which osteopetrosis diagnosed in early childhood followed a milder course, allowing them to reach the adult age in relatively good conditions with no specific therapy. No clearly pathogenic mutation was identified either with standard amplification and resequencing protocols or with exome sequencing analysis. While evaluating the possible impact of a 3'UTR variant on the TCIRG1 expression, we found a novel single nucleotide change buried in the middle of intron 15 of the TCIRG1 gene, about 150 nucleotides away from the closest canonical splice site. By sequencing a number of independent cDNA clones covering exons 14 to 17, we demonstrated that this mutation reduced splicing efficiency but did not completely abrogate the production of the normal transcript. Prompted by this finding, we sequenced the same genomic region in 33 patients from our unresolved ARO cohort and found three additional novel single nucleotide changes in a similar location and with a predicted disruptive effect on splicing, further confirmed in one of them at the transcript level. Overall, we identified an intronic region in TCIRG1 that seems to be particularly prone to splicing mutations, allowing the production of a small amount of protein sufficient to reduce the severity of the phenotype usually associated with TCIRG1 defects. On this basis, we would recommend including TCIRG1 not only in the molecular work-up of severe infantile osteopetrosis but also in intermediate cases and carefully evaluating the possible effects of intronic changes.

Molecular pathogenesis of Marfan syndrome

Marfan syndrome (MFS) is a genetic disorder that affects multiple organs. Mortality imposed by aortic aneurysm and dissections represent the most serious clinical manifestation of MFS. Progressive pathological aortic root enlargement as the result of degeneration of microfibril architecture and consequential loss of extracellular matrix integrity due to fibrillin-1 (FBN1) mutations are commonly diagnosed clinical manifestations of MFS. However, overlapping clinical manifestations with other aneurysmal disorders present a significant challenge in early and accurate diagnosis of MFS. While FBN1 mutations, abnormal transforming growth factor-β signaling and dysregulated matrix metalloproteinases have been implicated in MFS, clinically accepted risk-stratifying biomarkers have yet to be reliably identified. In this review, we summarize current consensus and recent insights in the understanding of MFS pathogenesis. Finally, we introduce the application of induced pluripotent stem cells (iPSCs) as cellular models for MFS and its potential as a novel platform into providing better appreciation of mechanisms underlying MFS diverse manifestations in the cardiovascular system.

VariantDB: a flexible annotation and filtering portal for next generation sequencing data

Interpretation of the multitude of variants obtained from next generation sequencing (NGS) is labor intensive and complex. Web-based interfaces such as Galaxy streamline the generation of variant lists but lack flexibility in the downstream annotation and filtering that are necessary to identify causative variants in medical genomics. To this end, we built VariantDB, a web-based interactive annotation and filtering platform that automatically annotates variants with allele frequencies, functional impact, pathogenicity predictions and pathway information. VariantDB allows filtering by all annotations, under dominant, recessive or de novo inheritance models and is freely available at http://www.biomina.be/app/variantdb/.