Brain dystrophin, neurogenetics and mental retardation

Duchenne muscular dystrophy (DMD) and the allelic disorder Becker muscular dystrophy (BMD) are common X-linked recessive neuromuscular disorders that are associated with a spectrum of genetically based developmental cognitive and behavioral disabilities. Seven promoters scattered throughout the huge DMD/BMD gene locus normally code for distinct isoforms of the gene product , dystrophin, that exhibit nervous system developmental, regional and cell- type specificity. Dystrophin is a complex plasmalemmal-cytoskeletal linker protein that possesses multiple functional domains, autosomal and X-linked homologs and associated binding proteins that form multiunit signaling comp lexes whose composition is unique to each cellular and developmental contex t. Through additional interactions with a variety of proteins of the extrac ellular matrix, plasma membrane, cytoskeleton and distinct intracelluar com partments, brain dystrophin acquires the capability to participate in the m odulatory actions of a large number of cellular signaling pathways. During neural development, dystrophin is expressed within the neural tube and sele cted areas of the embryonic and postnatal neuraxis, and may regulate distin ct aspects of neurogenesis, neuronal migration and cellular differentiation . By contrast, in the mature brain, dystrophin is preferentially expressed by specific regional neuronal subpopulations within proximal somadendritic microdomains associated with synaptic terminal membranes. Increasing experi mental evidence suggests that in adult life, dystrophin normally modulates synaptic terminal integrity, distinct forms of synaptic plasticity and regi onal cellular signal integration. At a systems level, dystrophin may regula te essential components of an integrated sensorimotor attentional network. Dystrophin deficiency in DMD/BMD patients and in the mdx mouse model appear s to impair intracellular calcium homeostasis and to disrupt multiple prote in-protein interactions that normally promote information transfer and sign al integration from the extracellular environment to the nucleus within reg ulated microdomains. In DMD/BMD, the individual profiles of cognitive and b ehavioral deficits, mental retardation and other phenotypic variations appe ar to depend on complex profiles of transcriptional regulation associated w ith individual dystrophin mutations that result in the corresponding presen ce or absence of individual brain dystrophin isoforms that normally exhibit developmental, regional and cell-type-specific expression and functional r egulation. This composite experimental model will allow fine-level mapping of cognitive-neurogenetic associations that encompass the interrelationship s between molecular, cellular and systems levels of signal integration, and will further our understanding of complex gene-environmental interactions and the pathogenetic basis of developmental disorders associated with menta l retardation. (C) 2000 Elsevier Science B.V. All rights reserved.