Mutations in STIL, encoding a pericentriolar and centrosomal protein, cause primary microcephaly

Abstract

Primary microcephaly (MCPH) is an autosomal-recessive congenital disorder characterized by smaller-than-normal brain size and mental retardation. MCPH is genetically heterogeneous with six known loci: MCPH1-MCPH6. We report mapping of a novel locus, MCPH7, to chromosome 1p32.3-p33 between markers D1S2797 and D1S417, corresponding to a physical distance of 8.39 Mb. Heterogeneity analysis of 24 families previously excluded from linkage to the six known MCPH loci suggested linkage of five families (20.83%) to the MCPH7 locus. In addition, four families were excluded from linkage to the MCPH7 locus as well as all of the six previously known loci, whereas the remaining 15 families could not be conclusively excluded or included. The combined maximum two-point LOD score for the linked families was 5.96 at marker D1S386 at theta = 0.0. The combined multipoint LOD score was 6.97 between markers D1S2797 and D1S417. Previously, mutations in four genes, MCPH1, CDK5RAP2, ASPM, and CENPJ, that code for centrosomal proteins have been shown to cause this disorder. Three different homozygous mutations in STIL, which codes for a pericentriolar and centrosomal protein, were identified in patients from three of the five families linked to the MCPH7 locus; all are predicted to truncate the STIL protein. Further, another recently ascertained family was homozygous for the same mutation as one of the original families. There was no evidence for a common haplotype. These results suggest that the centrosome and its associated structures are important in the control of neurogenesis in the developing human brain.

Figure 1

Photographs of a Primary Microcephaly Patient Note reduced size of the head in this patient.

Figure 2

The Candidate Region of the MCPH7 Locus between Markers D1S2797 and D1S417 The known genes between markers D1S2797 and D1S2874 are shown. The arrows depict the direction of transcription of the genes. The genomic structure of STIL is also shown below. Three mutations identified during this study are marked in STIL.

Figure 3

DNA Sequence Analysis of STIL for Mutations in Individuals from Families IIS-17, IIS-28, IIS-3, and IIS-45 (A) Note a homozygous C→T change in affected individual V-1 from family IIS-17. Both parents III-1 and IV-1 are heterozygous for this change. Two other affected individuals, V-2 and V-3, are homozygous for this change. This change converts the CAA codon to a premature stop codon TAA. The CAA and TAA codons are underlined in chromatograms of the normal unrelated individual and the affected individual V-1. Arrows mark the position of the nucleotide change. (B) Note the deletion of the nucleotide residue G in affected individual III-1 from family IIS-28. The deleted nucleotide residue G is marked by an arrow in the chromatogram of the normal unrelated individual. The other affected individual, IV-2, is also homozygous for this deletion, and the rest of the individuals, II-1, II-2, III-2, III-3, and IV-1, are heterozygous for this deletion. The region around the deleted nucleotide residue G is underlined in chromatograms. (C) Note a homozygous G→A change affecting the donor splice site in the affected individual IV-1 from family IIS-3. Both parents (III-1 and III-2) and normal siblings (IV-2 and IV-3) are heterozygous for this change. Arrows mark the position of nucleotide residues involved in G→A change in chromatograms. (D) Note the deletion of the nucleotide residue G in affected individual IV-1 from family IIS-45. The deleted nucleotide residue G is marked by an arrow in the chromatogram of the normal unrelated individual. The other affected individual, IV-2, is also homozygous for this deletion, and both parents II-1 and III-1 are heterozygous for this deletion. The region around the deleted nucleotide residue G is underlined in chromatograms.