Neuropathologic findings in patients with primary autosomal recessive microcephaly

Autosomal recessive primary microcephaly (MCPH, syn. Microcephalia vera) is a rare, genetically heterogeneous disease, in which patients exhibit microcephaly by the 32nd week of gestation. Genetic causes of MCPH subtypes 1–6 include mutations in genes encoding microcephalin (MCPH1), cyclin dependent kinase 5 regulatory associated protein 2 CDK5RAP2 (MCPH3), abnormal spindle-like, microcephaly associated ASPM (MCPH5), centromeric protein J CENPJ (MCPH6) as well as linkage to the two loci 19q13.1–13.2 (MCPH2) and 15q15-q21 (MCPH4). Severe microcephaly is characterized by a significantly reduced occipito-frontal head circumference (OCF) of more than three standard deviations below the mean for age, sex, and ethnicity. Microcephaly correlates closely with reduced brain volume and mental retardation, but patients with MCPH typically do not have further neurological findings. Imaging studies show brains of normal architecture but of reduced size. The latter is particularly evident in the cerebral cortex, which shows a simplified cerebral cortex structure, and there is also a slightly reduced white matter volume. Individual patients with MCPH provide evidence of periventricular neuronal heterotopias suggesting neuronal migration defects. It is not known whether the brain pathology in patients with MCPH is the result of defective cell proliferation/differentiation, cell migration and/or increased cell death. Neuropathological studies, which are rare and were performed at a time when a genetic diagnosis could not be performed, are thus highly warranted. Here, we summarize current knowledge on histological findings in MCPH patients. Infants with Microcephalia vera exhibited a significantly reduced brain volume with an almost preserved convolution pattern and no myelination disturbances. Cytoarchitectonic anomalies, seen predominantly in the first two cortical layers, included the appearance of compact cell clusters in the second layer. This can be interpreted as being indicative of a proliferation/differentiation and/or migration defect.