Homozygosity mapping in families with Joubert syndrome identifies a locus on chromosome 9q34.3 and evidence for genetic heterogeneity

Abstract

Joubert syndrome is a rare developmental defect of the cerebellar vermis, with autosomal recessive inheritance. The phenotype is highly variable and may include episodic hyperpnea, abnormal eye movements, hypotonia, ataxia, developmental delay, and mental retardation. Even within sibships the phenotype may vary, making it difficult to establish the exact clinical diagnostic boundaries of Joubert syndrome. To genetically localize the gene region, we have performed a whole-genome scan in two consanguineous families of Arabian/Iranian origins, with multiple affected probands. In one family, we detected linkage to the telomeric region of chromosome 9q, close to the marker D9S158, with a multipoint LOD score of Z=+3.7. The second family did not show linkage to this region, giving a first indication of genetic heterogeneity underlying Joubert syndrome. These findings were supported by subsequent analysis of two smaller families-one compatible with linkage to 9q; the other, unlinked. We conclude that Joubert syndrome is clinically and genetically heterogeneous and that one locus maps to chromosome 9q.

Figure 1

Pedigrees and most likely haplotypes for markers on chromosome 9q34 for families A–D. Symbols of affected individuals are blackened.

Figure 2

Assignment of markers D9S164, D9S1818, D9S114, D9S1826, D9158, and D9S1838 to the long arm of chromosome 9. Marker distances are taken from Dib et al. (1996).

Figure 3

Multipoint analysis performed with Linkmap (from the FASTLINK package version 4.0P) for family A, with the markers D9S1826, D9S158, and D9S1838, resulting in a maximal LOD score of Z=+3.7 at marker D9S158.