Heterozygous mutations in SIX3 and SHH are associated with schizencephaly and further expand the clinical spectrum of holoprosencephaly.

Schizencephaly (SCH) is a clinically and etiologically heterogeneous cerebral malformation presenting as unilateral or bilateral hemispheric cleft with direct connection between the inner and outer liquor spaces. The SCH cleft is usually lined by gray matter, which appears polymicrogyric implying an associated impairment of neuronal migration. The majority of SCH patients are sporadic, but familial SCH has been described. An initial report of heterozygous mutations in the homeobox gene EMX2 could not be confirmed in 52 patients investigated in this study in agreement with two independent SCH patient cohorts published previously. SCH frequently occurs with additional cerebral malformations like hypoplasia or aplasia of the septum pellucidum or optic nerve, suggesting the involvement of genes important for the establishment of midline forebrain structures. We therefore considered holoprosencephaly (HPE)-associated genes as potential SCH candidates and report for the first time heterozygous mutations in SIX3 and SHH in a total of three unrelated patients and one fetus with SCH; one of them without obvious associated malformations of midline forebrain structures. Three of these mutations have previously been reported in independent patients with HPE. SIX3 acts directly upstream of SHH, and the SHH pathway is a key regulator of ventral forebrain patterning. Our data indicate that in a subset of patients SCH may develop as one aspect of a more complex malformation of the ventral forebrain, directly result from mutations in the SHH pathway and hence be considered as yet another feature of the broad phenotypic spectrum of holoprosencephaly.