Subcortical band heterotopia (SBH) and classical lissencephaly (LIS) result from deficient neuronal migration which causes mental retardation and epilepsy. A single LIS/SBH locus on Xq22.3-q24 was mapped by linkage analysis and physical mapping of the breakpoint in an X;2 translocation. A recently identified gene, doublecortin (DCX), is expressed in fetal brain and mutated in LIS/SBH patients. We have identified four novel missense mutations in the gene, one familial mutation with LIS in a male and SBH in the carrier females, one de novo mutation in an SBH female, and two mutations in sporadic SBH female patients. The DCX gene is found to be expressed exclusively at a very high level in the adult frontal lobe. We have also cloned the X-linked mouse doublecortin (Dcx) gene. It encodes isoforms of a highly hydrophilic 40 kDa protein, homologous to its human counterpart and containing several potential phosphorylation sites. Both human and mouse DCX proteins are homologous to a CNS protein containing a Ca2+/calmodulin kinase domain, suggesting that the DCX protein may belong to a novel class of intracellular proteins involved in neuronal migration through Ca2+-dependent signaling.
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We are grateful to the family members and individuals who participated in this study. We thank M. Elizabeth Ross, W. Blackburn and R.E. Stevenson for discussions during this study; D. Schlessinger and C.E. Schwartz for valuable suggestions and critical reading of the manuscript; R. Nagaraja for sharing mapping information from the Xq24 region and P. Lalley for somatic cell hybrid DNA-containing mouse X chromosome. We also thank B. Hane, L. Jones, S.-C. Yates, S. Minnerath and S. McMillan for technical assistance. This study was supported by a grant from the National Institutes of Health (R01-NS35515; A.K.S. and W.B.D.).