Diverse phenotypic consequences of mutations affecting the C-terminus of FLNA.

UNLABELLED

Filamin A, the filamentous protein encoded by the X-linked gene FLNA, cross-links cytoskeletal actin into three-dimensional networks, facilitating its role as a signalling scaffold and a mechanosensor of extrinsic shear forces. Central to these functions is the ability of FLNA to form V-shaped homodimers through its C-terminal located filamin repeat 24. Additionally, many proteins that interact with FLNA have a binding site that includes the C-terminus of the protein. Here, a cohort of patients with mutations affecting this region of the protein is studied, with particular emphasis on the phenotype of male hemizygotes. Seven unrelated families are reported, with five exhibiting a typical female presentation of periventricular heterotopia (PH), a neuronal migration disorder typically caused by loss-of-function mutations in FLNA. One male presents with widespread PH consistent with previous male phenotypes attributable to hypomorphic mutations in FLNA. In stark contrast, two brothers are described with a mild PH presentation, due to a missense mutation (p.Gly2593Glu) inserting a large negatively charged amino acid into the hydrophobic dimerisation interface of FLNA. Co-immunoprecipitation, in vitro cross-linking studies and gel filtration chromatography all demonstrated that homodimerisation of isolated FLNA repeat 24 is abolished by this p.Gly2593Glu substitution but that extended FLNA(Gly2593Glu) repeat 16-24 constructs exhibit dimerisation. These observations imply that other interactions apart from those mediated by the canonical repeat 24 dimerisation interface contribute to FLNA homodimerisation and that mutations affecting this region of the protein can have broad phenotypic effects.

KEY MESSAGES

• Mutations in the X-linked gene FLNA cause a spectrum of syndromes. • Genotype-phenotype correlations are emerging but still remain unclear. • C-term mutations can confer male lethality, survival or connective tissue defects. • Mutations leading to the latter affect filamin dimerisation. • This deficit is compensated for by remotely acting domains elsewhere in FLNA.