Joubert syndrome: A haplotype segregation strategy and exclusion of the zinc finger protein of cerebellum 1 (ZIC1) gene

Huntingtin-associated protein 1 interacts with Ahi1 to regulate cerebellar and brainstem development in mice

Joubert syndrome is an autosomal recessive disorder characterized by congenital malformation of the cerebellum and brainstem, with abnormal decussation in the brain. Mutations in the Abelson helper integration site 1 gene, which encodes the protein AHI1, have been shown to cause Joubert syndrome. In this study, we found that mouse Ahi1 formed a stable complex with huntingtin-associated protein 1 (Hap1), which is critical for neonatal development and involved in intracellular trafficking. Hap1-knockout mice showed significantly reduced Ahi1 levels, defective cerebellar development, and abnormal axonal decussation. Suppression of Ahi1 also decreased the level of Hap1; and truncated Ahi1, which corresponds to the mutations in Joubert syndrome, inhibited neurite outgrowth in neuronal culture. Reducing Hap1 expression suppressed the level and internalization of TrkB, a neurotrophic factor receptor that mediates neurogenesis and neuronal differentiation, which led to decreased TrkB signaling. These findings provide insight into the pathogenesis of Joubert syndrome and demonstrate the critical role of the Ahi1-Hap1 complex in early brain development.

Genetic basis of Joubert syndrome and related disorders of cerebellar development

Over three decades have passed since Marie Joubert described the original proband for Joubert syndrome, a rare neurological disorder featuring absence of the cerebellar vermis (i.e. midline). Efforts at deciphering the molecular basis for this disease have been complicated by the clinical and genetic heterogeneity as well as extensive phenotypic overlap with other syndromes. However, progress has been made in recent years with the mapping of three genetic loci and the identification of mutations in two genes, AHI1 and NPHP1. These genes encode proteins with some shared functional domains, but their role in brain development is unclear. Clues may come from studies of related syndromes, including Bardet-Biedl syndrome and nephronophthisis, for which all of the encoded proteins localize to primary cilia. The data suggest a tantalizing connection between intraflagellar transport in cilia and brain development.

AHI1 mutations cause both retinal dystrophy and renal cystic disease in Joubert syndrome

BACKGROUND

Joubert syndrome (JS) is an autosomal recessive disorder characterised by hypotonia, ataxia, mental retardation, altered respiratory pattern, abnormal eye movements, and a brain malformation known as the molar tooth sign (MTS) on cranial MRI. Four genetic loci have been mapped, with two genes identified (AHI1 and NPHP1).

METHODS

We screened a cohort of 117 JS subjects for AHI1 mutations by a combination of haplotype analysis and sequencing of the gene, and for the homozygous NPHP1 deletion by sequencing and marker analysis.

RESULTS

We identified a total of 15 novel AHI1 mutations in 13 families, including nonsense, missense, splice site, and insertion mutations, with some clustering in the WD40 domains. Eight families were consanguineous, but no single founder mutation was apparent. In addition to the MTS, retinal dystrophy was present in 11 of 12 informative families; however, no subjects exhibited variable features of JS such as polydactyly, encephalocele, colobomas, or liver fibrosis. In contrast to previous reports, we identified two families with affected siblings who developed renal disease consistent with nephronophthisis (NPH) in their 20s. In addition, two individuals with classic NPH were found to have homozygous NPHP1 deletions.

CONCLUSIONS

Overall, 11% of subjects had AHI1 mutations, while approximately 2% had the NPHP1 deletion, representing a total of less than 15% in a large JS cohort. Some preliminary genotype-phenotype correlations are possible, notably the association of renal impairment, specifically NPH, in those with NPHP1 deletions. Subjects with AHI1 mutations may be at risk of developing both retinal dystrophy and progressive kidney disease.

Cerebellar vermis aplasia: Patient report and exclusion of the candidate genesEN2 andZIC1

Cerebellar vermis aplasia (ACV, OMIM 117360) is a rare malformation of the cerebellum, with only few familial patients reported so far. Main clinical features of this rare disorder include floppiness and delayed milestones in early infancy, preceding mild cerebellar ataxia, non-progressive clinical course, normal or slightly delayed intelligence, and occasional nystagmus. Neuroimaging reveals selective involvement of the cerebellum, which is prominent in the vermis. Because of the large preponderance of female patients, X-linked dominant transmission was suggested by [Fenichel and Phillips (1989); Arch Neurol 46:582-583], and subsequent reports only concern female patients. Only one family with male-to-male transmission presenting with a generalized atrophy of the cerebellum rather than a more localized vermis aplasia has been reported so far. We report on a family in which father and son are affected by a mild form of ACV, thus confirming an autosomal mode of inheritance of the disease. Our patients showed a progressive improvement of their motor abilities, neurological examination of the father being actually normal except for a mild mental retardation. We also evaluated the potential role of two candidate genes, EN2 and ZIC1, responsible for abnormal cerebellar development in murine knock-out models. However, molecular analysis failed to reveal any causative mutation in the coding sequence of the two genes in our patients. The understanding of the genetic basis of autosomal dominant ACV would allow a better classification of isolate cerebellar malformations and might permit to understand cell differentiation and migration in the developing central nervous system.