STIL on my small brain: a new gene for microcephaly

Primary microcephaly (MCPH) is a genetically heterogeneous neurodevelopmental disorder characterized by marked reduction in cortical size that may result from failure of neurogenesis. The head circumference is typically greater than three standard deviations below the ageand sex-related population mean. Patients typically present with mild to severe mental retardation without other clinical manifestations. The mode of inheritance is autosomal-recessive and at least six known loci (MCPH1–MCPH6) have been mapped using traditional family-based linkage studies. To date, four genes encoding centrosomal proteins have been identified as causative for MCPH: MCPH1 on chromosome 8p23.1, CDK5RAP2 on 9q33.2, ASPM on 1q31.3, and CENPJ on 13q12.12. In the present study, Kumar et al. sought to identify novel MCPH loci by studying families previously excluded from linkage to any of the six known MCPH loci. They mapped a novel 8.39 Mb locus, MCPH7, on chromosome 1p32.3-p33 that contains 77 genes including STIL, which encodes a centrosomal protein. The identification of three homozygous mutations in patients with MCPH establishes STIL as a new gene for human brain development. Figure 1 briefly outlines the experimental design used to implicate mutations in STIL as a cause of MCPH. Kumar et al. genotyped 35 families of Indian descent using three markers from each of the six known MCPH loci. They excluded 24 families from linkage to these loci by calculating multipoint logarithm of the odds scores using the Mapmaker/Homoz program. Next, they genotyped three affected individuals from a single consanguineous family using 222 markers from a human genome-wide screening set, which identified homozygosity for a single marker (D1S2134) in all three individuals. Genotyping of an additional 14 markers from this region delineated a minimum candidate region of 8.39 Mb on 1p32.3p33, which represented a novel MCPH locus (MCPH7). The authors selected 4 of the 15 markers from the MCPH7 candidate region and genotyped all 24 unlinked families. In five families, evidence for a greater than 70% conditional probability of linkage to the MCPH7 locus was found. The authors also excluded four families from linkage to the MCPH7 locus, indicating the presence of at least one more novel MCPH locus. The authors were unable to conclusively include or exclude linkage to the MCPH7 locus in the remaining 15 families. The MCPH7 candidate region contains 77 genes of which 59 are expressed in the brain. Given that the four known microcephaly genes (MCPH1, CDK5RAP2, ASPM, and CENPJ ) are expressed primarily in the centrosome, the authors prioritized the list of 59 brain-expressing genes to determine which, if any, are expressed in the centrosome or involved in spindle pole body function. They identified at least one gene, STIL, with evidence of expression and/or function in the centrosome. In addition, the authors performed expression analyses in six different human fetal tissues and demonstrated that STIL is expressed in the brain at 16 weeks of gestation, a period of increased neuronal cell proliferation in the walls of the cerebral ventricles. Thus, the expression pattern of STIL is consistent with its hypothesized role in neuronal cell proliferation and the development of the cerebral cortex. To test the hypothesis that mutations in STIL underlie MCPH, Kumar et al. performed DNA sequence analyses of the entire STIL coding region in the MCPH7-linked families. In one family, a homozygous non-sense mutation (c.3715C → T; p.Gln1239X) in exon 18 that introduces a premature stop codon was identified in all three affected siblings. Both parents were heterozygous for this mutation. Furthermore, the mutation was not identified in 202 population-matched control chromosomes. In a second family, a homozygous 1 bp deletion (c.3655delG; p.Leu1218X) in exon 18 that is predicted to truncate the STIL protein was identified in both of the affected siblings