Misregulation of mitotic chromosome segregation in a new type of autosomal recessive primary microcephaly

Primary autosomal recessive microcephaly (MCPH) is a congenital disorder characterized by a pronounced reduction of brain size and mental retardation. We present here a consanguineous Turkish family clinically diagnosed with MCPH and without linkage to any of the known loci (MCPH1-MCPH7). Autozygosity mapping identified a homozygous region of 15.8 Mb on chromosome 10q11.23-21.3, most likely representing a new locus for MCPH. Although we were unable to identify the underlying genetic defect after extensive molecular screening, we could delineate a possible molecular function in chromosome segregation by the characterization of mitosis in the patients' cells. Analyses of chromosome nondisjunction in T-lymphocytes and fibroblasts revealed a significantly elevated rate of nondisjunction in the patients' cells as compared with controls. Mitotic progression was further explored by immunofluorescence analyses of several chromosome and spindle associated proteins. We detected a remarkable alteration in the anaphase distribution of Aurora B and INCENP, which are key regulators of chromosome segregation. In particular, a fraction of both proteins remained abnormally loaded on chromosomes during anaphase in MCPH patients' cells while in cells of normal control subjects both proteins are completely transferred to the spindle midzone. We did not observe any other alterations regarding cell cycle progression, chromosome structure or response to DNA damage. Our observations point toward a molecular role of the underlying gene product in the regulation of anaphase/telophase progression possibly through interaction with chromosomal passenger proteins. In addition, our findings represent further evidence for the proposed role of MCPH genes in the regulation of mitotic progression.