Defective chromosome segregation, microtubule bundling and nuclear bridging in inner centromere protein gene (Incenp)-disrupted mice

INCENP is a chromosomal passenger protein which relocates from the centrome re to theI spindle midzone during the metaphase-anaphase transition, ultima tely being discarded in the cell midbody at the completion of cytokinesis. Using homologous recombination, we have generated Incenp gene-targeted hete rozygous mice that are phenotypically indistinguishable from their wild-typ e littermates. Intercrossing the heterozygotes results in no live-born homo zygous Incenp-disrupted progeny, indicating an early lethality. Day 3.5 aff ected pre-implantation embryos contain large, morphologically abnormal cell s that fail to fully develop a blastocoel cavity or thrive in utero and in culture. Chromatin and tubulin immunocytochemical stainings of these and da y 2.5 affected embryos reveal a high mitotic index, no discernible metaphas e or anaphase stages, complete absence of midbodies, micronuclei formation, morphologically irregular macronuclei with large chromosome complements, m ultipolar mitotic configurations, binucleated cells, internuclear bridges a nd abnormal spindle bundling. The phenotype is consistent with a defect in the modulation of microtubule dynamics, severely affecting chromosome segre gation and resulting in poorly resolved chromatin masses, aberrant karyokin esis and internuclear bridge formation. These latter occurrences could pose a physical barrier blocking cytokinesis.