Epileptogenic brain malformations: clinical presentation, malformative patterns and indications for genetic testing

Abstract

We review here those malformations of the cerebral cortex which are most often observed in epilepsy patients, for which a genetic basis has been elucidated or is suspected and give indications for genetic testing. There are three forms of lissencephaly (agyria-pachygyria) resulting from mutations of known genes, which can be distinguished because of their distinctive imaging features. They account for about 85% of all licence-phalies. Lissencephaly with posteriorly predominant gyral abnormality is caused by mutations of the LIS1 gene on chromosome 17. Anteriorly predominant lissencephaly in hemizygous males and subcortical band heterotopia (SBH) in heterozygous females are caused by mutations of the XLIS (or DCX) gene. Mutations of the coding region of XLIS were found in all reported pedigrees, and in most sporadic female patients with SBH. Missense mutations of both LIS1 and XLIS genes have been observed in some of the rare male patients with SBH. Autosomal recessive lissencephaly with cerebellar hypoplasia has been associated with mutations of the reelin gene. With few exceptions, children with lissencephaly have severe developmental delay and infantile spasms early in life. Patients with SBH have a mild to severe mental retardation with epilepsy of variable severity and type. X-linked bilateral periventricular nodular heterotopia (BPNH) consists of typical BPNH with focal epilepsy in females and prenatal lethality in males. About 88% of patients have focal epilepsy. Filamin A (FLNA) mutations have been reported in some families and in sporadic patients. Additional, possibly autosomal recessive gene(s) are likely to be involved in causing BPNH non-linked to FLN1. Tuberous sclerosis (TS) is a dominant disorder caused by mutations in at lest two genes, TSC1 and TSC2. 75% of cases are sporadic. Most patients with TS have epilepsy. Infantile spasms are a frequent early manifestation of TS. Schizencephaly (cleft brain) has a wide anatomo-clinical spectrum, including focal epilepsy in most patients. Familial occurrence is rare. Heterozygous mutations in the EMX2 gene have been reported in some patients. However, at present, there is no clear indication on the possible pattern of inheritance and on the practical usefulness that mutation detection in an individual with schizencephaly would carry in terms of genetic counselling. Amongst several syndromes featuring polymicrogyria, bilateral perisylvian polymicrogyria had familial occurrence on several occasions. Genetic heterogeneity is likely, including autosomal recessive, X-linked dominant, X-linked recessive inheritance and association to 22q11.2 deletions. FISH analysis for 22q11.2 is advisable in all patients with perisylvian polymicrogyria. Parents of an affected child with normal karyotype should be given up to a 25% recurrence risk.