Brain malformation in syndromic craniosynostoses, a primary disorder of white matter: a review

Syndromic craniosynostoses (Saethre-Chotzen, Pfeiffer 1, 2, 3, Apert, Crouzon, mainly) are particular in this that a single gene defect (mostly fibroblast growth factor receptor [FGFR] 2) generates different clinical phenotypes that characterize these syndromes. Significant brain abnormalities have been reported in all syndromes. However, whether these abnormalities are secondary to the bone disease or primary (e.g. callosal agenesis) is still controversial. Recent evidence suggests that the white matter defect might be a primary disorder.

From the review of the literature and the analysis of our cases, it appears that three categories of brain abnormalities can be found. (1) The global distortion of the brain is likely mechanical and in keeping with the deformity of the skull. (2) The chronic tonsillar herniation (Chiari I deformity) is likely mechanical also and a consequence of the small size of the posterior fossa, especially after an early closure (before 24 m) of the lambdoid suture. (3) On the contrary, the constellation of abnormalities that selectively involve the white matter (non-progressive, non-destructive ventriculomegaly, callosal agenesis or thinning, agenesis of septum pellucidum, paucity of the antero-mesial temporal white matter, pyramidal hypoplasia) is much more likely to constitute a primary disorder.

Recent neurobiological evidence supports this point of view. L1 cell adhesion molecule (L1CAM) gene plays a major role in the development of the white matter and its mutation in humans (callosal agenesis, retardation, adducted thumbs, spasticity, and hydrocephalus syndrome, Bickers-Adams syndrome) or in mice causes similar defects of the corpus callosum, septum pellucidum, centrum semi-ovale, and cortico-spinal tracts. To operate, L1CAM need interactions with FGFRs, whose defects are causal to the syndromic craniosynostoses. It seems logical to assumes that the FGFR defects generate both the skull abnormalities and, by lack of interaction with L1CAM, the primary defect of the white matter. The mental deficiency that is common in these patients therefore is likely to be part of the disease (through the L1CAM-FGFR interaction) rather than a consequence of the skull size or of the associated hydrocephalus.