Two main forms of hereditary colorectal cancer can be distinguished:
hereditary nonpolyposis colorectal cancer (HNPCC) and familial
adenomatous polyposis (FAP). Both conditions have an autosomal
dominant pattern of inheritance. Hereditary nonpolyposis colorectal
cancer accounts for approximately 1%-5% of all colorectal cancer
cases and FAP accounts for 1% of such cancers . With some exceptions,
FAP can be attributed to a germ-line defect in the adenomatous
polyposis coli (APC) gene on chromosome 5q . This gene was
localized in 1987 and isolated in 1991. As a result, presymptomatic
DNA-based diagnosis of FAP has become possible.
Patients with FAP exhibit a characteristic clinical picture of
multiple adenomatous polyps in the large bowel. In contrast, patients
with HNPCC do not present with pathognomonic signs. Therefore,
clinical diagnosis of HNPCC has depended on family studies. Until
recently the basic genetic defect of HNPCC was unknown. However,
in 1993 a gene for HNPCC was localized on chromosome 2p through
linkage analysis of two large HNPCC kindreds .
An important additional finding was provided by studies of tumors
from HNPCC patients. Most of these tumors have a characteristic
pattern of widespread genetic alterations, the so-called microsatellite
instability or replication error positive phenotype . This
finding directed further studies on the nature of the HNPCC-associated
gene on chromosome 2. This gene could then be identified as one
normally involved in DNA mismatch repair [5,6]. Thus, HNPCC appears
to be due basically to a disturbance of DNA mismatch repair, which
leads to genetic instability in somatic cells. Within a year,
three additional HNPCC-associated DNA mismatch repair genes were
cloned [7,8]. Apparently, HNPCC is a genetically heterogeneous
condition, with different genes being involved in different families.
Identification of the gene defects underlying HNPCC introduces
the prospect of presymptomatic diagnosis for at-risk family members.
Although DNA testing is potentially beneficial for these individuals,
it also is associated with various psychosocial consequences.
In this article, genetic counseling of HNPCC families will be
addressed, with emphasis on presymptomatic DNA-based diagnosis.
For the purpose of illustration, clinical information on and mutation
assays in two of our HNPCC pedigrees will be presented.
As a group, HNPCC patients share certain features: early onset
of disease, predominantly proximal tumor localization, a high
incidence of multiple primary colorectal cancers, and possible
manifestation of other tumor types, notably, endometrial cancer
(Table 1). However, as mentioned above, pathognomonic clinical
characteristics have not been identified. Therefore, HNPCC can
be recognized only by its autosomal dominant inheritance pattern.
Penetrance (the percentage of gene carriers who exhibit disease)
is approximately 90%; skipped generations seldom occur .
In 1990, at the second meeting of the International Collaborative
Group on HNPCC in Amsterdam, minimal criteria for the identification
of HNPCC kindreds to be included in collaborative studies were
proposed. These criteria are generally known as the Amsterdam
criteria (Table 2) . Since extracolonic tumors are not included,
the diagnosis of HNPCC may be missed if one strictly adheres to
these criteria for diagnostic purposes .
Clinically, HNPCC cannot be identified if the family history is
negative. A patient with HNPCC due to a new mutation can be identified
only by means of direct mutation studies.
In summary, the clinical diagnosis of HNPCC is based on observation
of early-onset colorectal cancer and other tumors, notably, endometrial
cancer, in successive generations. In large families with many
affected individuals, the diagnosis may be straightforward. In
small families with only a few affected individuals, the clinical
diagnosis of HNPCC must often remain tentative.
The DNA in every cell is continuously exposed to injury by various
intrinsic and extrinsic factors. Usually, the consequences of
damage to the DNA are not severe since the molecule is subject
to various control systems that prevent the occurrence of mutations.
These systems recognize insults, and subsequent pathways may lead
to either repair of the errors or programmed cell death.
DNA repair systems can be separated into two main groups: nucleotide
excision repair and mismatch repair. DNA mismatch repair is the
repair of base-pair anomalies that occur during DNA replication
. The four DNA mismatch repair genes implicated in HNPCC are
human homologs of bacterial and yeast DNA mismatch repair genes.
The characteristics of these four genes are summarized in Table
3. In most HNPCC families, the condition appears to be due to
a germ-line mutation in either the hMSH2 or hMLH1 gene .
Approaches to DNA-Based Diagnosis
There are two main approaches to presymptomatic DNA-based diagnosis:
linkage analysis and direct mutation analysis. In these studies,
the genomic DNA is usually isolated from blood samples.
In linkage analysis, haplotypes (chromosome regions) that harbor
the gene of interest are examined. This method is based on a comparison
of haplotypes from affected and unaffected family members. Since
different genes are involved in HNPCC, linkage analysis, if feasible,
is performed as the first step in identifying the gene locus involved
in the family under investigation. Often, linkage studies are
of limited value due to small family size or the limited availability
of blood samples.
Direct mutation analysis focuses on the individual patient and
is not dependent on family studies. Various DNA studies used to
investigate HNPCC families are summarized in Table 4.
Some mutations, in particular, missense mutations that lead to
an amino-acid substitution in the protein product, may not be
causally related to the disease. Mutations that result in protein
truncation (as a result of base-pair substitutions, deletions,
or insertions leading to the generation of stop codons) are expected
to be pathogenic. Therefore, new methods for the in vitro detection
of protein truncation will probably become increasingly important
for diagnostic purposes .
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