Mutations in B9D1 and MKS1 cause mild Joubert syndrome: expanding the genetic overlap with the lethal ciliopathy Meckel syndrome

Joubert syndromeis a clinically and genetically heterogeneous ciliopathycharacterized by a typical cerebellar and brainstem malformation (the “molar tooth sign”), and variable multiorgan involvement. To date, 24 genes have been found mutated in Joubert syndrome, of which 13 also cause Meckel syndrome, a lethal ciliopathywith kidney, liver and skeletal involvement. Here we describe four patients with mild Joubert phenotypes who carry pathogenic mutations in either MKS1 or B9D1, two genes previously implicated only in Meckel syndrome.