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

Abstract

Joubert syndrome (JS) is a rare autosomal recessive malformation syndrome, involving dysgenesis of the cerebellar vermis with accompanying brainstem malformations (comprising the molar tooth sign). JS is characterized by hypotonia, developmental delay, intermittent hyperpnea and apnea, and abnormal eye movements. A single locus for JS was previously identified on 9q34 in a consanguineous family of Arabian origin. However, linkage to this locus has subsequently been shown to be rare. We have ascertained 35 JS pedigrees for haplotype segregation analysis of genetic loci for genes with a putative role in cerebellar development. We examined the ZIC1 gene as a functional candidate for JS as Zic1(-/-) null mice have a phenotype reminiscent of JS. We undertook mutational analysis of ZIC1 by standard mutational analysis (dideoxy-fingerprinting (ddf)) of 47 JS probands, and fully sequenced the coding region in five of these probands. By these means, ZIC1 was excluded from playing a causal role in most cases of JS as no disease-associated mutations were identified. Further, linkage to the ZIC1 genetic locus (3q24) was excluded in 21 of 35 pedigrees by haplotype segregation analysis of closely spaced markers. The remaining 14 of 35 pedigrees were consistent with linkage. However, this number does not significantly depart from that expected by random chance (16.5) for this cohort. Therefore, this systematic approach has been validated as a means to prioritize functional candidate genes and enables us to confine mutational analysis to only those probands whose segregation is consistent with linkage to any given locus.

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

“…Given the lack of consanguinity and the small size of these families, the haplotype analysis likely indicates cosegregation of shared haplotypes and disease by chance. 16 We identified 15 different AHI1 mutations distributed in exons 6-19 in 13 families with JS (table 1; fig 1). None of the mutations had been described previously.…”

“…We examined the haplotypes for regions of homozygosity in consanguineous pedigrees and for compatibility with linkage by segregation analysis in multiplex pedigrees. 16 We sequenced the 28 exons encoding mRNA (GenBank mRNA accession number AJ4560824) and three alternatively spliced exons (GenBank accession numbers AJ459825, AK024085, and AI733147) of the AHI1 gene in DNA from a proband from each of 99 families, including those families not excluded by haplotype analysis for linkage to 6q23. Only a portion of the terminal 39 untranslated region was sequenced.…”

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

“…The developmental defects in cerebellar structures in JS led to the investigation of candidate genes that are important for early development of the cerebellum. However, no mutations in any genes known to contribute to cerebellar development have been identified (7)(8)(9). Instead, JBTS3 was found to associate with mutations in the Abelson helper integration site 1 gene (AHI1) (10,11).…”

Cerebellar vermis aplasia: Patient report and exclusion of the candidate genes EN2 and ZIC1

“…Heterozygous mice demonstrate hypoplasia of the cerebellar anterior vermis and significant abnormalities in locomotor tests reminiscent of the deficits seen in patients with ACV [Aruga et al, 1998; Ogura et al, 2001]. ZIC1 has been excluded as a causative gene of JS [Bennett et al, 2004]. However, we did not find mutations in EN2 and ZIC1 by direct sequencing analysis in our patients.…”

“…ZIC1 encodes a member of C 2 H 2 ‐type zinc finger proteins, which play an important role during CNS development. En2 and Zic1 have been recently excluded as a causative gene of Joubert syndrome (JS, MIM 213300), a rare neurological disorder presenting with agenesis or dysgenesis of the cerebellar vermis [Blair et al, 2002; Bennett et al, 2004].…”