Molecular and cellular mechanisms of corpus callosum anomalies – CILAXCAL

Corpus callosum is the main midline brain structure connecting the homologous cortical areas of both hemispheres. Corpus callosum malformations (CCM) are the most frequent brain malformation in human with an incidence of 1 /4,000 newborn. CCM are now diagnosed antenatally and often associated with chromosomal anomalies and mendelian syndromes. Even though apparently isolated, children with CCM have an uncertain neuro-developmental outcome with possible moderate learning disabilities or epilepsy and severe cognitive disorder. Therefore, counseling remains challenging in the absence of the known aetiology that might drive the prognosis. Recurrence is observed in almost 5 % of cases, and data suggest both recessive and dominant de novo inheritance. The major aim of this study is to unravel the genetic causes and find prognostic factors for CCM in human by combining human genetics, 3D neuroimaging, neuropathological and functional analyses in cellular and animal models.

CCM can result from disruption in any one of the multiple steps of callosal development including patterning and formation of the midline, specification of callosal neurons and growth and targeting of callosal axons in the contralateral hemisphere. Genetic studies in mouse and in syndromic CCM have provided valuable insight into the underlying mechanisms of CCM. In particular, both intrinsic and extrinsic guidance cues as well as cell adhesion processes during neural outgrowth appear crucial for the CC to develop correctly. Interestingly, our recent studies and identification of the ciliary gene KIF7 as the causal gene for acrocallosal syndrome together with the frequent association of CCM to ciliopathies, has raised the question of the involvement of primary cilia defect in corpus callosum development. Our second aim is thus to understand the physiopathological mechanisms underlying CCM in human ciliopathies. To this end, we will take advantages of mouse models of ciliopathies presenting with CCM. In particular, we will precisely dissect the cellular and molecular origins of neuronal wiring defects associated with Kif7 invalidation, but also in Tmem67/Mks3 knock out mouse, a model that recapitulates the brain phenotypic variability of another ciliopathy associating CCM, the Meckel/Joubert syndrome.

Facing the even larger genetic heterogeneity of syndromic CCM that can be observed in more than 300 entities, we propose to undertake first a non hypothesis-driven approach by whole exome sequencing of 50 trios selected from two complementary and well characterized cohorts consisting of 200 cases with CC agenesis or hypolasia : fetuses following termination of pregnancies that underwent a fetal and neuropathological examination (team 1), and chilren and adults with CCM and mental retardation included in a National “PHRC” program (team 3). A second step will consist to screen the rest of the cohorts for the genes found by this approach along with a targeted next generation sequencing of genes involved in two functional modules frequently associated to CCM in human and mouse: primary cilium and axon guidance/neuronal adhesion.

The combined and complementary expertise of the teams in congenital malformations / ciliopathies (teams 1 & 2) and neurology / axon guidance defects (team 3) will allow to functionally characterize new genes identified by this study, unravel the physiopathology of corpus callosum anomalies in ciliopathies, and shed light on midline molecular pathways underlying the formation of the CC in human.

Identifying the genetic cause in individuals with CCM will allow establishing correlations with the underlying molecular defect or brain morphology that will include Diffusion Tensor Imaging (DTI) in combination with 3D-tractography. A better knowledge of the genetic causes and prognostic factors for CCM is the challenge of the next decade for the clinical management of fetuses and children.