Molecular basis for risk factors
Evidence for the molecular basis of colorectal cancer comes
from genetic analysis of tissues either from patients with a
family history of the disease or from patients with sporadic
adenomatous colorectal polyps or extensive ulcerative colitis.
The traditional view is that background rates of genetic
mutation, combined with several rounds of clonal expansion,
are necessary for a tumour to develop. It has recently
been argued, however, that inherent genetic instability not
only is necessary but may also be sufficient for cancer to
develop.
Sporadic colorectal adenomas
More than 70% of colorectal cancers develop from sporadic
adenomatous polyps, and postmortem studies have shown the
incidence of adenomas to be 30-40% in Western populations.
Polyps are asymptomatic in most cases and are often multiple.
Flat adenomas, which are more difficult to detect at endoscopy,
account for about 10% of all polyps and may have a higher rate
of malignant change or may predispose to a more aggressive
cancer phenotype.
Family history
Recognised familial syndromes account for about 5% of
colorectal cancers. The commonest hereditary syndromes are
familial adenomatous polyposis and heredity non-polyposis
colon cancer. Patients with these syndromes usually have a
family history of colorectal cancer presenting at an early age.
Attenuated familial adenomatous polyposis, juvenile polyposis
syndrome, and Peutz-Jeghers syndrome are rarer, mendelian
causes of colorectal cancer. In familial adenomatous polyposis
(a mendelian dominant disorder with almost complete
penetrance) there is a germline mutation in the tumour
suppressor gene for adenomatous polyposis coli (APC) on
chromosome 5.
Heredity non-polyposis colon cancer also shows dominant
inheritance, and cancers develop mainly in the proximal colon.
Patients with heredity non-polyposis colon cancer show
germline mutations in DNA mismatch repair enzymes (which
normally remove misincorporated single or multiple
nucleotide bases as a result of random errors during
recombination or replications). Mutations are particularly
demonstrable in DNA with multiple microsatellites
(“microsatellite instability”).
In addition to the well recognised syndromes described
above, clusters of colorectal cancer occur in families much more
often than would be expected by chance. Postulated reasons for
this increased risk include “mild” APC and mismatch repair
gene mutations, as well as polymorphisms of genes involved in
nutrient or carcinogen metabolism.
Risk from ulcerative colitis
Several studies have indicated that patients with ulcerative colitis
have a 2-8.2 relative risk of colorectal cancer compared with the
normal population, accounting for about 2% of colorectal
cancers. One of the factors influencing an individual’s risk is
duration of colitis—the cumulative incidence of colorectal
cancer is 5% at 15 years and 8-13% at 25 years. The extent of
disease is also important: patients with involvement of right and
transverse colon are more likely to develop colorectal cancer
(the relative risk in these patients is 15 compared with the
normal population). Coexisting primary sclerosing cholangitis
independently increases the relative risk of ulcerative colitis
associated neoplasia (UCAN) by 3-15%. In addition, high grade
dysplasia in random rectosigmoid biopsies is associated with an
unsuspected cancer at colectomy in 33% of patients
Molecular basis of adenoma
carcinoma sequence and UCAN
Cancers arising in colitis versus those in adenomas
Important clinical and biological differences exist between the
adenoma carcinoma sequence and ulcerative colitis associated
neoplasia. Firstly, cancer in ulcerative colitis probably evolves
from microscopic dysplasia with or without a mass lesion rather
than from adenomas. Secondly, the time interval from the
presence of adenoma to progression to carcinoma probably
exceeds the interval separating ulcerative colitis associated
dysplasia from ulcerative colitis associated neoplasia. Thirdly,
patients with a family history of colorectal cancer (but not
ulcerative colitis associated neoplasia) and who also have
ulcerative colitis are at further increased risk, suggesting additive
factors.
Chromosomal instability
Aneuploidy indicates gross losses or gains in chromosomal
DNA and is often seen in many human primary tumours and
premalignant conditions. It has been shown that aneuploid
“fields” tend to populate the epithelium of patients with
ulcerative colitis even in histologically benign colitis. These
changes may occur initially in some cases by loss of one allele at
a chromosomal locus (loss of heterozygosity) and may imply the
presence of a tumour suppressor gene at that site. Loss of both
alleles at a given locus (homozygous deletion) is an even
stronger indicator of the existence of a tumour suppressor
gene. Loss of heterozygosity occurs clonally in both the
adenoma carcinoma sequence and ulcerative colitis associated
neoplasia. Many of these loci are already associated with one or
more known candidate tumour suppressor genes. These include
3p21 ( catenin gene), 5q21 (APC gene), 9p (p16 and p15
genes), 13q (retinoblastoma gene), 17p (p53 gene), 17q (BRCA1
gene), 18q (DCC and SMAD4 genes), and less frequently 16q
(E cadherin gene).
The p53 gene locus is the commonest site demonstrating
loss of heterozygosity. p53 is a DNA binding protein
transcriptional activator and can arrest the cell cycle in response
to DNA damage—hence its title “guardian of the genome.” The
effect of normal (wild-type) p53 is antagonised by mutation or
by action of the antiapoptotic gene Bcl-2, which is significantly
less frequently overexpressed in ulcerative colitis associated
neoplasia than in the adenoma carcinoma sequence. Most
mutations in p53 cause the protein to become hyperstable and
lead to its accumulation in the nucleus.
A second tumour suppressor gene necessary for
development of sporadic colorectal cancer is APC, which is
inactivated in > 80% of early colorectal cancers. Consequently
this gene has been termed the “gatekeeper” for adenoma
development as adenoma formation requires perturbation of
the APC gene’s function or that of related proteins such as
catenins. An important function of the APC gene is to prevent
the accumulation of molecules associated with cancer, such as
catenins. Accumulation of catenins can lead to the transcription
of the oncogene c-myc, giving a proliferative advantage to the
cell. APC mutations occur later and are somewhat less common
in ulcerative colitis associated neoplasia (4-27%) than in
sporadic colorectal adenomas and carcinomas. Catenins also
bind E cadherin, which functions as a tumour suppressor gene
in the gastrointestinal tract. It is currently thought that mutated
catenins may not bind to APC and thus accumulate.
Microsatellite instability
A further important category of alteration studied in the
adenoma carcinoma sequence and ulcerative colitis associated
neoplasia is microsatellite instability. This comprises length
alterations of oligonucleotide repeat sequences that occur
somatically in human tumours. This mechanism is also
responsible for the germline defects found in heredity
non-polyposis colon cancer. The incidence of microsatellite
instability has been noted to be about 15% for adenomas and
25% for colorectal cancers overall. Microsatellite instability also
occurs in patients with ulcerative colitis and is fairly common in
premalignant (dysplastic) and malignant lesions (21% and 19%
respectively). Indeed it has also been reported in “histologically
normal” ulcerative colitis mucosa. It can therefore be considered
to be an early event in the adenoma carcinoma sequence and in
ulcerative colitis associated neoplasia.
Prognosis
The prognosis of colorectal cancer is determined by both
pathological and molecular characteristics of the tumour.
Pathology
Pathology has an essential role in the staging of colorectal
cancer. There has been a gradual move from using Dukes’s
classification to using the TNM classification system as this is
thought to lead to a more accurate, independent description of
the primary tumour and its spread. More advanced disease
naturally leads to reduced disease-free interval and survival.
Independent factors affecting survival include incomplete
resection margins, grade of tumour, and number of lymph
nodes involved (particularly apical node metastasis—main node
draining a lymphatic segment).
Molecular biology
Reports on correlations between tumour genotype and
prognosis are currently incomplete. However, analysis of
survival data from patients with sporadic colorectal cancer and
from those with colorectal cancer associated with familial
adenomatous polyposis and hereditary non-polyposis colon
cancer has not shown any reproducible significant differences
between these groups. In premalignancy, however, the onset of
p53 mutations in histologically normal mucosa in ulcerative
colitis suggests that detection of such mutations may be a useful
strategy in determining mucosal areas with a high risk of
dysplastic transformation.
