Studies into the Underlying Aetiology of Periventricular Heterotopia

Abstract

Understanding the genetic and molecular components involved in cortical development will enhance the understanding of the pathogenesis of malformations affecting the brain. This project encompasses three strategies to progress the understanding of human cortical development, extend knowledge of the genetic aetiology of the neuronal migration disorder periventricular heterotopia (PH), and provide connections to molecular pathways. The first strategy explored the dimerisation capabilities of a mutant FLNA protein identified in a surviving male with PH. This mutation, located in the dimerisation interface, was shown to disrupt homodimerisation of FLNA, an interaction crucial for actin cross-linking. Biochemical studies showed that this mutant FLNA can form homodimers utilizing a second dimerisation site. In addition, FLNA and FLNB were shown to be capable of forming heterodimers although the low level at which this interaction occured calls into doubt the physiological significance of this interaction. The second strategy investigated whether copy number changes at the FLNA locus are a mutational mechanism underlying PH. Five PH patients, including one male, were identified with copy number changes of FLNA that include full length and intragenic deletions and intragenic duplications. The male with an intragenic duplication presented with severe intestinal dysmotility, classified as X-linked chronic idiopathic intestinal pseudo-obstruction. Subsequent protein expression studies showed that the mutation was associated with reduced FLNA expression. The final strategy aimed to identify new PH loci by detecting genomic rearrangements using genome wide arrays. An initial study to prove the validity of this approach identified the monogenetic cause of osteopathia striata with cranial sclerosis, a skeletal dysplasia. Six PH patients were identified with genomic deletions which could be divided into two categories; deletions overlapping with common microdeletion syndromes (2) and novel copy number changes (3). The majority of patients with microdeletion syndromes do not present with PH suggesting that the presence of this neurological presentation can be a secondary consequence of these genomic alterations with variable penetrance. The analysis of four patients with three novel genomic events proved complex and demonstrated that the clinical interpretation of detected copy number variants in relation to PH is challenging. This study advanced the knowledge regarding PH by extending the biochemical understanding of the cellular functions of FLNA and detecting intragenic copy number changes as a new mutational mechanism at this locus. The search for novel PH genes uncovered no novel genetic contributors but did suggest that this approach may have validity in localising new loci for PH and improving the understanding of genes and signalling pathways underlying human brain development.