Identification of microcephalin, a protein implicated in determining the size of the human brain

Abstract

Primary microcephaly (MIM 251200) is an autosomal recessive neurodevelopmental condition in which there is a global reduction in cerebral cortex volume, to a size comparable with that of early hominids. We previously mapped the MCPH1 locus, for primary microcephaly, to chromosome 8p23, and here we report that a gene within this interval, encoding a BRCA1 C-terminal domain-containing protein, is mutated in MCPH1 families sharing an ancestral 8p23 haplotype. This gene, microcephalin, is expressed in the developing cerebral cortex of the fetal brain. Further study of this and related genes may provide important new insights into neocortical development and evolution.

Figure 1

Schematic of the 2.1-mb MCPH1 critical region. Microsatellite markers are in boldface type. An asterisk (*) denotes flanking markers for the ancestral haplotype. Sequenced BAC clones are shown as horizontal lines, and end-sequenced BAC clones as horizontal lines with blackened circles at the ends. The prefix “AC” denotes a novel microsatellite marker. Marker content determined by PCR (or electronically by BLAST) is indicated by vertical lines. NT = STS not tested against clone. The two genes in the critical region are shown, with their directions of transcription indicated by arrows. Vertical lines indicate the positions of individual exons.

Figure 2

Genotyping results from one affected individual and parents from families 1 and 2, showing a region of identical alleles shared between both families (boxed), indicative of a founder effect.

Figure 3

Microcephalin structure and mutation. A, Gene organization. Scale bar represents 100 bp of exon sequence. B, Predicted domain structure of the wild-type (WT) and mutant protein. BRCT domains are indicated by blue boxes. The position of the S25X mutation is indicated by an arrow. C, Nonsense mutation in exon 2 in affected individual from family 1.

Figure 4

Expression pattern of microcephalin; RT-PCR analysis of human fetal tissues. Upper panel, a 570-bp microcephalin fragment amplified by use of primers from exons 1 and 6, to include the entire first BRCT domain. Lower panel, GAPDH control.

Figure 5

ClustalX alignment of human and mouse microcephalin proteins. Identical amino acids shaded in black, conservative substitutions in gray. BRCT domains (as predicted by SMART) are boxed.

Figure 6

Expression of microcephalin mRNA in the fetal mouse brain. A, B, and C Expression of microcephalin mRNA in 7-μm E13.5 fetal mouse sagittal brain sections. Antisense microcephalin riboprobe (A), toluidine blue counterstained section (B), sense microcephalin riboprobe (C). lv = lateral ventricle; s = corpus striatum; d = diencephalon; mv = mesencephalic vesicle.