Joubert Syndrome in French Canadians and Identification of Mutations in CEP104

Abstract

Joubert syndrome (JBTS) is a primarily autosomal-recessive disorder characterized by a distinctive mid-hindbrain and cerebellar malformation, oculomotor apraxia, irregular breathing, developmental delay, and ataxia. JBTS is a genetically heterogeneous ciliopathy. We sought to characterize the genetic landscape associated with JBTS in the French Canadian (FC) population. We studied 43 FC JBTS subjects from 35 families by combining targeted and exome sequencing. We identified pathogenic (n = 32 families) or possibly pathogenic (n = 2 families) variants in genes previously associated with JBTS in all of these subjects, except for one. In the latter case, we found a homozygous splice-site mutation (c.735+2T>C) in CEP104. Interestingly, we identified two additional non-FC JBTS subjects with mutations in CEP104; one of these subjects harbors a maternally inherited nonsense mutation (c.496C>T [p.Arg166*]) and a de novo splice-site mutation (c.2572-2A>G), whereas the other bears a homozygous frameshift mutation (c.1328_1329insT [p.Tyr444fs*3]) in CEP104. Previous studies have shown that CEP104 moves from the mother centriole to the tip of the primary cilium during ciliogenesis. Knockdown of CEP104 in retinal pigment epithelial (RPE1) cells resulted in severe defects in ciliogenesis. These observations suggest that CEP104 acts early during cilia formation by regulating the conversion of the mother centriole into the cilia basal body. We conclude that disruption of CEP104 causes JBTS. Our study also reveals that the cause of JBTS has been elucidated in the great majority of our FC subjects (33/35 [94%] families), even though JBTS shows substantial locus and allelic heterogeneity in this population.

Figure 1

CEP104 Mutations in JBTS-Affected Families (A) Localization of the identified mutations in CEP104 (upper panel) and CEP104 (lower panel; 925 amino acids). The coiled-coiled (CC) domains (amino acid positions 209–289 and 677–725 according to UniProt: O60308) are highlighted in yellow. (B) Segregation of CEP104 mutations within families. (C) Brain MRI images of the JBTS individuals with CEP104 mutations from family 618 (a and a′), family A_I (b and b′), and GeneDx (c and c′). Axial T1-weighted (a) and T2-weighted (b and c) images show the molar tooth sign with deepened interpeduncular fossa and elongated, thickened, and abnormally orientated superior cerebellar peduncles. Sagittal T1-weighted images (a′, b′, and c’) show hypoplasia of the cerebellar vermis (arrow).

Figure 2

Locus and Allelic Heterogeneity of JBTS in FC Individuals The graph shows the number of different alleles associated with JBTS in FC individuals. The total number (n) of JBTS-affected families with mutations in the indicated genes is shown beneath the gene name. The number of families bearing the recurrent alleles is noted in parentheses following the allele name on the x axis. Deletion 1 refers to chr2: 110,826,262–110,978,224, and deletion 2 refers to chr2: 110,862,369–110,978,000 (hg19 genome assembly).