Cancer cells frequently induce aberrant centrosomes, which have been implicated 
in cancer initiation and progression. Human colorectal cancer cells, HCT116, 
contain aberrant centrioles composed of disorganized cylindrical microtubules 
and displaced appendages. These cells also express unique centrosome-related 
structures associated with a subset of centrosomal components, including 
gamma-tubulin, centrin and PCM1. During hydroxyurea treatment, these abnormal 
structures become more abundant and undergo a change in shape from small dots to 
elongated fibers. Although gamma-tubulin seems to exist as a ring complex, the 
abnormal structures do not support microtubule nucleation. Several lines of 
evidence suggest that the fibers correspond to a disorganized form of centriolar 
microtubules. Plk4, a mammalian homolog of ZYG-1 essential for initiation of 
centriole biogenesis, is not associated with the gamma-tubulin-specific abnormal 
centrosomes. The amount of Plk4 at each centrosome was less in cells with 
abnormal centrosomes than cells without gamma-tubulin-specific abnormal 
centrosomes. In addition, the formation of abnormal structures was abolished by 
expression of exogenous Plk4, but not SAS6 and Cep135/Bld10p, which are 
downstream regulators required for the organization of nine-triplet 
microtubules. These results suggest that HCT116 cells fail to organize the 
ninefold symmetry of centrioles due to insufficient Plk4.