The Cell: Basic Structure and Function
More severe, however, is the impact of failing checkpoints
that control the centrosomes themselves. Cancer cells that
arise through the CIN pathway are characterized by an uneven
number and uneven distribution of centrosomes during mitosis.
This results in a total disorder of the normally bipolar spindle
apparatus and leads to complex multipolar spindle structures
that during mitosis result in disruption of the chromatids and
a complex uneven distribution of the chromosomal material to
the emerging daughter cells (Fig. 1.10) .
The major checkpoints that control these processes appear
to act at the transition of the G1 phase of the cell cycle to the
S phase and are controlled by cyclins and cyclin-dependent
kinase complexes. Interestingly, these processes are targeted by
two important viral oncoproteins encoded by high-risk human
papillomaviruses (HPV), the HPV E6 and E7 proteins, that
induce and maintain transformation of cervical cells as we will
learn later in this chapter. This results in failing control of the
mitotic processes and in particular distribution of chromosomes
during mitosis and severe numerical and structural alterations
of the chromosomes of the emerging daughter cells. The affected
cells usually undergo apoptosis, induced primarily by p53 gene
or related genes. Thus most emerging cancer cells develop molec-
ular mechanisms for evading this cellular suicide mechanism.
In the case of papillomavirus-associated cancers it is the HPV
E6 protein that binds to and inactivates p53. In other cancers
not induced by oncogenic HPVs, p53 functions are usually abro-
gated by an inactivating mutation or deletion of the p53 gene
itself or related genes within the same functional pathway.
Cells that achieve to evade the suicide control may survive the
genomic catastrophe and form the initial cells of an outgrowing
cancer. The emerging disproportionate distribution of chromo-
somal material in these transformed cell clones induce various
important morphological alterations of the affected cells' nuclei
that are the cornerstones of cytopathology (Fig. 1.11). Aneuploid
nuclei, coarser texture of the chromatin, changes in the size and
shape of the nuclei, hyper- and hypochromasia, and altered
shape and number of nucleoli are all immanent consequences
of the desegregation of chromosomes during mitosis of cells that
have lost control over the strictly bipolar mitotic figures, result-
ing in chaotic multipolar mitotic spindle complexes (Fig. 1.10).
Alternatively, cancer cells may arise through the MSI path-
way.9,10 Cancers that emerge through this pathway are char-
acterized by substantially different biological properties. They
usually do not display numerical or structural changes of the
chromosomes or the mitotic figures.11
Cells of these tumors
usually divide normally. Consequently, these tumor cells do not
display aneuploidy, aberrant mitotic figures or gross alterations
of their chromosomes. They usually remain diploid without
major morphological alteration of their nuclei. However, errors
emerge through a more subtle, superficially less brutal mecha-
nism that in its clinical consequences may end as disastrously
as the chromosomal instable cancers. In these cases the cancer
cells acquire increasing mutations of the DNA sequence itself.
After each round of DNA replication in the S-phase of the cell
cycle usually hundreds and thousands of mutations, which
need to be checked and repaired before the cell cycle proceeds
to avoid very high accumulation, occur in the replicated genetic
code due to misannealing and mispairing. Hence, all cells in
nature develop a sophisticated proofreading mechanism medi-
ated by the
DNA-MMR complex,
a multiprotein complex that
proofreads and fixes the mutations. If distinct components of
this repair complex are lost, the proofreading capacity decreases
and mutations particularly in thermodynamically sensitive
DNA sequences occur. These lead to the rapid accumulation of
mutations particularly in repetitive DNA sequences that con-
sist of longer stretches of mononucleotides (mononucleotide
repeats). Since these sequences are also commonly referred to
as microsatellites, this latter mechanism of carcinogenesis is
referred to as microsatellite instability. MSI is observed in up
to 15% of colorectal cancers, a subset of endometrial cancers,
and a number of urinary tract cancers. But it is also found in
a number of endometrial and bladder cancers, leukemias and
lymphomas, and skin cancers. It is the hallmark of an inher-
ited cancer predisposition syndrome referred to as hereditary
non-polyposis colon cancer syndrome. HNPCC syndrome is
characterized by inherited mutations of defect copies of genes
Fig. 1.11 Comparative genomic
hybridization (CGH) and spectral karyotype
hybridization (SKY).
The average CGH ratio
profiles for the diploid cell line DLD-1 and the
aneuploid cell line HT29 are presented in (A)
and (B). Note the remarkably stable genome of
DLD-1 with only three copy number variations
(chromosomes 2, 6, and 11). HT29 shows a
highly aberrant ratio profile, with copy number
alterations occurring on 13 chromosomes.
The gains of 7, 8q, 13, and 20q are common
aberrations in colorectal carcinomas. SKY of
metaphase chromosomes prepared from these
cell lines is shown in (C) and (D). No numerical
aberrations were identified in the diploid cell
line (C), whereas trisomies were common in
the aneuploid cell line HT29 (D). All aberrations
detected by SKY were also seen by CGH analysis.
This indicates that no reciprocal, balanced
chromosomal translocations have occurred. Data
and images were taken from Ghadimi et al.11
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