Joubert syndrome in the Netherlands. Clinical and genetic aspects

Abstract

The ciliopathies, a collective of diseases caused by dysfunction of the primary cilia, have proven to be one of the most exciting groups of new diseases of the last ten years. Joubert syndrome (JBS), a rare neurodevelopmental disorder described for the first time in 1969, has turned out to be one of the quintessential ciliopathies. JBS shows the wide range of clinical features and genetic heterogeneity typical of the ciliary diseases. Our knowledge on the syndrome has expanded considerably over the last twenty years (for a general overview, I also refer to some of the excellent reviews 1-6). However, many questions on the phenotypical variability and the complex genetic background of JBS have not been answered yet. The literature on JBS started with the report of Marie Joubert et al. in 1969 7. She described four siblings with hyperventilation, abnormal eye movements and psychomotor retardation. Boltshauser described the first European cases 8; this is why JBS is also known as Joubert-Boltshauser syndrome. In the initial reports hallmarks of the syndrome were considered to be mental (and motor) retardation, hypotonia, and, specifically, periodic hyperpnea, abnormal eye movements, and vermis aplasia. Several authors recognized the overlap of clinical features with other syndromes (e.g. with orofacialdigital syndrome type 1 9, or CHARGE syndrome 10). In the years following the initial descriptions, many reports have been published adding additional symptoms to the phenotype. For instance, retinal colobomas as part of the Joubert phenotype were reported by Lindhout et al. (1980) 11, and Aicardi et al. (1983) 12, and polydactyly by Egger et al. (1982) 13. The first reports on retinal dysplasia/Lebers congenital amaurosis were in the early eighties of the last century 12,14,15, and in the nineties cystic kidney disease and hepatic fibrosis were recognized as features of JBS 16. Over the years, lumpers and splitters have defined the shifting boundaries between JBS and syndromes with overlapping symptoms, e.g. COACH syndrome (cerebellar vermis hypoplasia, oligophrenia, congenital ataxia, coloboma, and hepatic fibrosis 17), and ARIMA syndrome (cerebellar hypoplasia, retinal disease and cystic kidney disease) 18. The term “Joubert syndrome and related disorders (JSRD)” was coined to cope with this ongoing debate. Little has been published on JBS in the Netherlands, since the description of the first case in 1980 1 At the start of this study, we aimed to obtain an overview of the Dutch population of JBS patients, to estimate the Dutch birth prevalence, and to gain insight into the clinical course and genetic background of Dutch JBS cases. This thesis describes the results of our efforts to solve these questions, and related topics that we encountered on the way. Chapters 2 and 3 are focused on clinical aspects of JBS. In Chapter 2 we investigated hearing loss in the Dutch JBS patient cohort. Many studies have addressed the visual features of JBS, but none have investigated hearing. Hearing loss has been reported in some ciliopathies, e.g. Alstrom disease and BBS. Given the fact that many JBS patients have reduced vision and impaired speech development, adequate hearing is extremely important. In Chapter 3 we discuss the diagnostic pitfalls in JBS, especially the correct interpretation of MRI results when evaluating the molar tooth sign. Two patients are described with an initial diagnosis of JBS, who turned out to have chromosomal rearrangements not related to JBS. In Chapter 4 we report a family with a JBS-like phenotype with a pedigree in line with X-linked inheritance. We were especially interested in this family because of the excess of male patients in the Dutch JBS population and our expectation to find involvement of X-linked genes. Chapters 5 to 7 describe the results of molecular studies in the Dutch JBS cohort. In Chapter 5, we investigated the AHI1 gene, the NPHP1 gene, and the CYCLIN D1 gene. At the start of this study, Sanger sequencing was the only tool available. At the time AHI1 and NPHP1 had just been identified as JBS causing genes. Since large families suitable for linkage analysis or consanguineous families suitable for homozygosity mapping are relatively rare in the Dutch population, we chose a candidate gene approach based on a combination of JBS loci and animal models, and also investigated CYCLIN D1 as a possible new JBS gene (OMIM # 168461). In Chapter 6 and 7 we utilized the new possibilities of next generation sequencing. We developed a targeted NGS array containing 22 JBS genes and named it the Joubertome. The molecular cause of JBS remains elusive in about half of the cases, and therefor we added around 600 candidate genes selected from the literature and the ciliary proteome database [http://v3.ciliaproteome.org]. The results of the Joubertome analysis and the finding of MKS1 as a novel gene for JBS are reported in Chapter 6 and 7.