Aline Verstraeten

Progress in unraveling the genetic etiology of Parkinson disease in a genomic era

Parkinson disease (PD) and Parkinson-plus syndromes are genetically heterogeneous neurological diseases. Initial studies into the genetic causes of PD relied on classical molecular genetic approaches in well-documented case families. More recently, these approaches have been combined with exome sequencing and together have identified 15 causal genes. Additionally, genome-wide association studies (GWASs) have discovered over 25 genetic risk factors. Elucidation of the genetic architecture of sporadic and familial parkinsonism, however, has lagged behind that of simple Mendelian conditions, suggesting the existence of features confounding genetic data interpretation. Here we discuss the successes and potential pitfalls of gene discovery in PD and related disorders in the post-genomic era. With an estimated 30% of trait variance currently unexplained, tackling current limitations will further expedite gene discovery and lead to increased application of these genetic insights in molecular diagnostics using gene panel and exome sequencing strategies.

DLB and PDD: a role for mutations in dementia and Parkinson disease genes?

Based on the substantial overlap in clinical and pathological characteristics of dementia with Lewy bodies (DLB) and Parkinson disease with dementia (PDD) with Alzheimer disease (AD) and Parkinson disease (PD) we hypothesized that these disorders might share underlying genetic factors. The contribution of both sequence and copy number variants (CNVs) in known AD and PD genes to the genetic etiology of DLB and PDD however is currently unclear. Therefore, we performed a gene-based mutation analysis of all major AD and PD genes in 99 DLB and 75 PDD patients, including familial and sporadic forms, from Flanders, Belgium. Also, copy number variants in APP, SNCA, and PARK2 were determined. In the AD genes we detected proven pathogenic missense mutations in PSEN1 and PSEN2, and 2 novel missense variants in PSEN2 and MAPT. In the PD genes we identified 1 SNCA duplication, the LRRK2 R1441C founder mutation and 4 novel heterozygous missense variants with unknown pathogenicity. Our results suggest a contribution of established AD and PD genes to the genetic etiology of DLB and PDD though to a limited extent. They do support the hypothesis of a genetic overlap between members of the Lewy body disease spectrum, but additional genes still have to exist.

Marfan Syndrome and Related Disorders: 25 Years of Gene Discovery

Marfan syndrome (MFS) is a rare, autosomal‐dominant, multisystem disorder, presenting with skeletal, ocular, skin, and cardiovascular symptoms. Significant clinical overlap with other systemic connective tissue diseases, including Loeys–Dietz syndrome (LDS), Shprintzen–Goldberg syndrome (SGS), and the MASS phenotype, has been documented. In MFS and LDS, the cardiovascular manifestations account for the major cause of patient morbidity and mortality, rendering them the main target for therapeutic intervention. Over the past decades, gene identification studies confidently linked the aforementioned syndromes, as well as nonsyndromic aneurysmal disease, to genetic defects in proteins related to the transforming growth factor (TGF)‐β pathway, greatly expanding our knowledge on the disease mechanisms and providing us with novel therapeutic targets. As a result, the focus of the developing pharmacological treatment strategies is shifting from hemodynamic stress management to TGF‐β antagonism. In this review, we discuss the insights that have been gained in the molecular biology of MFS and related disorders over the past 25 years.

Amyloid pathology influences aβ1-42 cerebrospinal fluid levels in dementia with lewy bodies.

A significant proportion of patients with dementia with Lewy bodies (DLB) show Alzheimers disease (AD) pathology like senile plaques and neurofibrillary tangles. Biomarkers in cerebrospinal fluid (CSF), such as amyloid-β1-42 (Aβ1-42), total tau (T-tau), and hyperphosphorylated tau (P-tau181P), are linked to the different pathological hallmarks of AD. We set up a study to investigate the influence of AD co-pathology on CSF biomarker concentrations and profiles in autopsy-confirmed DLB. DLB patients with senile plaques showed significantly lower CSF Aβ1-42 concentrations than DLB patients without senile plaques, but not compared to the AD patients. There were no significant differences in CSF T-tau or P-tau181P concentrations between DLB patients with and without neurofibrillary tangles. A correlation was found between the number of APOE ε4 alleles and Aβ1-42 CSF levels in DLB patients with senile plaques. Although the CSF biomarkers Aβ1-42, T-tau, and P-tau181P have an added diagnostic value for the differential dementia diagnosis, concomitant amyloid pathology in DLB limits the use of CSF Aβ1-42 for the differential diagnosis of AD versus DLB.

Global investigation and meta-analysis of the C9orf72 (G4C2)n repeat in Parkinson disease

Objectives: The objective of this study is to clarify the role of (G4C2)n expansions in the etiology of Parkinson disease (PD) in the worldwide multicenter Genetic Epidemiology of Parkinsons Disease (GEO-PD) cohort. Methods: C9orf72 (G4C2)n repeats were assessed in a GEO-PD cohort of 7,494 patients diagnosed with PD and 5,886 neurologically healthy control individuals ascertained in Europe, Asia, North America, and Australia. Results: A pathogenic (G4C2)n>60 expansion was detected in only 4 patients with PD (4/7,232; 0.055%), all with a positive family history of neurodegenerative dementia, amyotrophic lateral sclerosis, or atypical parkinsonism, while no carriers were detected with typical sporadic or familial PD. Meta-analysis revealed a small increase in risk of PD with an increasing number of (G4C2)n repeats; however, we could not detect a robust association between the C9orf72 (G4C2)n repeat and PD, and the population attributable risk was low. Conclusions: Together, these findings indicate that expansions in C9orf72 do not have a major role in the pathogenesis of PD. Testing for C9orf72 repeat expansions should only be considered in patients with PD who have overt symptoms of frontotemporal lobar degeneration/amyotrophic lateral sclerosis or apparent family history of neurodegenerative dementia or motor neuron disease.

Loss-of-function mutations in the X-linked biglycan gene cause a severe syndromic form of thoracic aortic aneurysms and dissections

Purpose:Thoracic aortic aneurysm and dissection (TAAD) is typically inherited in an autosomal dominant manner, but rare X-linked families have been described. So far, the only known X-linked gene is FLNA, which is associated with the periventricular nodular heterotopia type of Ehlers-Danlos syndrome. However, mutations in this gene explain only a small number of X-linked TAAD families.Methods:We performed targeted resequencing of 368 candidate genes in a cohort of 11 molecularly unexplained Marfan probands. Subsequently, Sanger sequencing of BGN in 360 male and 155 female molecularly unexplained TAAD probands was performed.Results:We found five individuals with loss-of-function mutations in BGN encoding the small leucine-rich proteoglycan biglycan. The clinical phenotype is characterized by early-onset aortic aneurysm and dissection. Other recurrent findings include hypertelorism, pectus deformity, joint hypermobility, contractures, and mild skeletal dysplasia. Fluorescent staining revealed an increase in TGF-β signaling, evidenced by an increase in nuclear pSMAD2 in the aortic wall. Our results are in line with those of prior reports demonstrating that Bgn-deficient male BALB/cA mice die from aortic rupture.Conclusion:In conclusion, BGN gene defects in humans cause an X-linked syndromic form of severe TAAD that is associated with preservation of elastic fibers and increased TGF-β signaling.Genet Med 19 4, 386–395.

Contribution of VPS35 genetic variability to LBD in the Flanders-Belgian population

VPS35 was recently identified as a novel autosomal dominant gene for Parkinson disease. In this study, we aimed to determine the contribution of simple and complex VPS35 variations to the genetic etiology of the spectrum of Lewy body disorders (LBD) in a Flanders-Belgian patient cohort (n = 677). We identified 3 novel missense variations in addition to 1 silent and 1 intronic variation predicted to activate a cryptic splice site, but no copy number variations. Despite the absence of these rare variations in the control group (n = 800), we could not attain convincing evidence for pathogenicity by segregation analysis or in silico predictions. Hence, our data do not support a major role for VPS35 variations in the genetic etiology of Lewy body disorders in the Flanders-Belgian population.

TGF-β signalopathies as a paradigm for translational medicine

This review focusses on impact of a better knowledge of pathogenic mechanisms of Marfan and related disorders on their treatment strategies. It was long believed that a structural impairment formed the basis of Marfan syndrome as deficiency in the structural extracellular matrix component, fibrillin-1 is the cause of Marfan syndrome. However, the study of Marfan mouse models has revealed the strong involvement of the transforming growth factor-β signalling pathway in the pathogenesis of Marfan. Similarly, this pathway was demonstrated to be key in the pathogenesis of Loeys-Dietz and Shprintzen-Goldberg syndrome. The elucidation of the underlying pathogenic mechanisms has led to new treatment strategies, targeting the overactive TGF-β pathway. Various clinical trials are currently investigating the potential new treatment options. A meta-analysis will contribute to a better understanding of the various trial results.

Aetiology and management of hereditary aortopathy

Aortic aneurysms are a major health problem because they account for 1–2% of all deaths in the Western population. Although abdominal aortic aneurysms (AAAs) are more prevalent than thoracic aortic aneurysms (TAAs), TAAs have been more exhaustively studied over the past 2 decades because they have a higher heritability and affect younger individuals. Gene identification in both syndromic and nonsyndromic TAA is proceeding at a rapid pace and has already pinpointed >20 genes associated with familial TAA risk. Whereas these genes explain <30% of all cases of familial TAA, their functional characterization has substantially improved our knowledge of the underlying pathological mechanisms. As such, perturbed extracellular matrix homeostasis, transforming growth factor-β signalling, and vascular smooth muscle cell contractility have been proposed as important processes in TAA pathogenesis. These new insights enable novel treatment options that are currently being investigated in large clinical trials. Moreover, together with the advent of next-generation sequencing approaches, these genetic findings are promoting a shift in the management of patients with TAA by enabling gene-tailored interventions. In this Review, we comprehensively describe the molecular landscape of familial TAA, and we discuss whether familial TAA, from a biological point of view, can serve as a paradigm for the genetically more complex forms of the condition, such as sporadic TAA or AAA.

Candidate Gene Resequencing in a Large Bicuspid Aortic Valve-Associated Thoracic Aortic Aneurysm Cohort: SMAD6 as an Important Contributor

Bicuspid aortic valve (BAV) is the most common congenital heart defect. Although many BAV patients remain asymptomatic, at least 20% develop thoracic aortic aneurysm (TAA). Historically, BAV-related TAA was considered as a hemodynamic consequence of the valve defect. Multiple lines of evidence currently suggest that genetic determinants contribute to the pathogenesis of both BAV and TAA in affected individuals. Despite high heritability, only very few genes have been linked to BAV or BAV/TAA, such as NOTCH1, SMAD6, and MAT2A. Moreover, they only explain a minority of patients. Other candidate genes have been suggested based on the presence of BAV in knockout mouse models (e.g., GATA5, NOS3) or in syndromic (e.g., TGFBR1/2, TGFB2/3) or non-syndromic (e.g., ACTA2) TAA forms. We hypothesized that rare genetic variants in these genes may be enriched in patients presenting with both BAV and TAA. We performed targeted resequencing of 22 candidate genes using Haloplex target enrichment in a strictly defined BAV/TAA cohort (n = 441; BAV in addition to an aortic root or ascendens diameter ≥ 4.0 cm in adults, or a Z-score ≥ 3 in children) and in a collection of healthy controls with normal echocardiographic evaluation (n = 183). After additional burden analysis against the Exome Aggregation Consortium database, the strongest candidate susceptibility gene was SMAD6 (p = 0.002), with 2.5% (n = 11) of BAV/TAA patients harboring causal variants, including two nonsense, one in-frame deletion and two frameshift mutations. All six missense mutations were located in the functionally important MH1 and MH2 domains. In conclusion, we report a significant contribution of SMAD6 mutations to the etiology of the BAV/TAA phenotype.