DCX lissencephaly / subcortical band heterotopia
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A 7-month-old boy presents with severe developmental delay and intractable seizures. Brain MRI shows a smooth cerebral surface with markedly reduced gyri (lissencephaly). His older sister carries the same DCX variant; she has a milder phenotype of childhood-onset epilepsy and mild intellectual disability, and her MRI shows bilateral symmetric bands of gray matter sitting beneath the cortical ribbon ("double cortex").
- X-linked, DCX (doublecortin, Xq22.3-q23). DCX is a microtubule-associated protein required for radial migration of neurons during cortical development.
- Hemizygous males: classic X-linked lissencephaly: diffuse agyria/pachygyria, severe intellectual disability, intractable epilepsy, often early death.
- Heterozygous females: subcortical band heterotopia (SBH, "double cortex") because random X-inactivation produces a mosaic of cells with and without functional DCX. Cells with active wild-type DCX migrate normally to form the cortex; cells with active mutant DCX stall in transit and form a band of gray matter beneath it.
- The sex difference is the textbook example of how X-inactivation can produce two strikingly different phenotypes from a single mutation.
- Males: lissencephaly (smooth brain) → severe ID, refractory epilepsy, microcephaly, hypotonia → spasticity
- Females: SBH → epilepsy (often adolescent onset), variable intellectual disability ranging from normal to moderate
- Phenotype severity in females correlates with band thickness on MRI (thicker band → more severe)
- MRI is diagnostic: agyria/pachygyria in males, SBH in females
- DCX sequencing; if negative in lissencephaly, sequence LIS1 (PAFAH1B1) and consider deletion testing for Miller-Dieker syndrome (17p13.3)
- Antiseizure medication; epilepsy in the male phenotype is typically refractory and may require multiple agents or epilepsy surgery evaluation
- Supportive care for severe neurodevelopmental disability in males
- Genetic counseling around recurrence (see Pearls)
- One mutation, two phenotypes, driven entirely by X-inactivation. This is a classic teaching case for sex-dependent expression of an X-linked allele.
- For an affected female (SBH), each pregnancy carries ~50% risk of transmission: sons → severe lissencephaly, daughters → SBH.
- Distinguish DCX lissencephaly (X-linked, gradient is anterior > posterior) from Miller-Dieker syndrome / LIS1 (17p13.3 deletion, gradient is posterior > anterior). The reverse anteroposterior gradient is a classic MRI distinction.
- Contrast SBH with periventricular nodular heterotopia: SBH bands sit beneath the cortex (mid-migration arrest); PNH nodules sit against the ventricle wall (failure to leave the ventricular zone at all).
"DCX = Double CorteX": The female phenotype is a literal "double cortex": a band of heterotopic gray matter parallel to the true cortex.