UCSF

Proper establishment of cortical structures during early brain development is vital to normal brain function. A key component of normal lamination of the cerebral cortex is the migration of neurons after they are born from precursor cells to their specific destination in one of the six cortical layers. Neuronal migration involves dynamic changes to microtubules and other cytoskeletal components at the tip of an extending axon or dendrite with the aid of microtubule-associated proteins (MAPs). Doublecortin (DCX) is a MAP that is highly and specifically expressed in immature, migrating neurons. Dysfunction of X-linked DCX causes lissencephaly in males, a malformation characterized by a lack of gyri in the cortex, and subcortical band heterotropia (SBH) or a "double cortex" in females. The role DCX plays in neuronal migration is not well understood and studies in mouse models have only investigated the effects of a complete knockout (KO) of Dcx, which resulted in no cortical lamination phenotype in male or female mice, in contrast to the conspicuous phenotypes observed in humans. However, documented disease-causing human DCX mutations involve a missense mutation in one of DCX’s microtubule binding domains, which has been shown to not remove DCX function entirely.Despite characterization of human-specific mutations and mouse knockout models, there remains an unmet need for elucidating the cellular and molecular mechanisms of patient-specific mutations in their native genetic context. I hypothesize that the limited phenotypes in Dcx KO mice are due to compensation by other proteins in Dcx’s absence, and not due to intrinsic species differences. In particular, I predict that the mutant phenotype observed in the cortex is due to altered binding to microtubules during neuronal migration, producing a dominant negative effect. To begin to elucidate the role of Doublecortin in regulation of neuronal migration, I have introduced a human disease-causing DCX mutation (T203R) into the endogenous mouse Dcx locus for in vivo experiments. These studies involving a disease-causing, patient-derived mutation of the endogenous Doublecortin locus in mouse contexts have yielded important insights into the biology of Doublecortin and cortical development, addressing unresolved mechanisms of patient-specific DCX mutations at the molecular and cellular level.