What's New in ACS Surgery
What Is Microsatellite Instability and Why Should We Care?


from ACS Surgery: Principles & Practice
Posted 11/19/2004
Dieter Hahnloser, MD, Stephen N. Thibodeau, PhD



Microsatellites are short segments of repetitive DNA bases that are
scattered throughout the genome; they are found predominantly in
noncoding DNA. Microsatellite instability (MSI) can be defined as a
change of any length resulting from either insertion or deletion of
repeating units in a microsatellite within a tumor, as compared with
normal tissue. MSI results from failure of the cell to repair errors
made during DNA replication. A group of intranuclear proteins,
collectively known as the mismatch repair (MMR) system, is
responsible for removing these errors. Failure allows errors to
accumulate and MSI to develop; this facilitates malignant
transformation.

MSI and Colon Cancer
Deficiency in MMR occurs in approximately 15% to 20% of all
colorectal cancers. In hereditary cases (e.g., hereditary
nonpolyposis colorectal cancer [HNPCC]), the cause of MMR deficiency
is mutational inactivation of a mismatch repair gene, usually MLH1 or
MSH2. In sporadic cases, however, a noninherited methylation of the
promoter sequence of MLH1 leads to the deficiency of mismatch repair.
The tumor phenotype associated with either hereditary or acquired
loss of DNA mismatch repair competency is called MSI-H (or high-
frequency microsatellite instability).

Are MSI-H Colon Cancers Different?
Cancers demonstrating MSI-H appear to develop via an alternative
genetic pathway, because aneuploidy and loss of heterozygosity are
rarely encountered, and tumor suppressor genes, such as APC and p53,
or the oncogene K-ras are seldom mutated. MSI-H tumors more often
arise in the right colon and tend to have a female predilection.
Also, distinctive histopathologic features are found, such as poor
differentiation, mucinous or signet-ring cell pattern, extensive
lymphocytic infiltration, and Crohn-like lymphoid reaction around the
tumor. MSI-H tumors have been associated with primary cancers of
larger size but more favorable stage distribution.

Most interestingly, and despite the fact that unfavorable histologic
features are more common in MSI-H cancers than in non-MSI-H cancers,
several studies have shown MSI-H to be an independent indicator of
good prognosis. Additionally, in a recent study, patients with MSI-H
stage II or stage III colon cancers who received no adjuvant
chemotherapy had a better overall survival rate than patients who
received adjuvant treatment (83% and 69%, respectively).[1] By
contrast, adjuvant chemotherapy improved survival in patients with
microsatellite-stable tumors (76% survival in patients who received
chemotherapy versus 68% in those who did not). This finding suggests
that MSI-H may also predict the effectiveness of treatment and that
MSI testing should be conducted routinely to direct rational adjuvant
chemotherapy. However, the authors of this study carefully urged
against altering clinical practice until these data are confirmed in
future prospective, randomized trials.

MSI and HNPCC
HNPCC is an autosomal dominant syndrome characterized by early onset
(average age at onset is less than 45 years) and the development of
neoplastic lesions in a variety of tissues (e.g., colorectal,
endometrial, gastric, renal, and ovarian tissues, as well as skin).
Of those families having the clinical diagnosis of HNPCC, only
approximately 70% will have evidence of defective MMR. Additionally,
of those families that do have defective MMR, an abnormality may be
present in one of at least four different MMR genes (hMLH1, hMSH2,
hMSH6, and PMS2). The clinical syndrome associated with hereditary
defects in DNA MMR is now specifically referred to as Lynch syndrome.
With the elucidation of the molecular etiology of Lynch syndrome,
characteristic tumor changes can now be used as defining features:
the presence of MSI-H and the absence of MMR protein expression by
immunohistochemistry (IHC) are both hallmarks of defective MMR.
Although over 90% of Lynch syndrome-associated cancers are MSI-H,
they account for only 2% to 3% of all colorectal cancers; 15% to 20%
of sporadically occurring colorectal cancers also demonstrate MSI-H.
Thus, the finding of MSI-H should alert caregivers to the possibility
of Lynch syndrome, but MSI-H does not necessarily equate with the
diagnosis of Lynch syndrome. The Bethesda guidelines for testing
colorectal tumors for MSI, which were developed in 1996 and revised
in 2002, are useful for identifying patients at risk for having Lynch
syndrome[2] (see Table 1).

Tests for MSI
The current gold standard for assessing tumor DNA mismatch repair
competency is molecular MSI testing. This is a labor-intensive test
that involves extracting DNA from both tumor and normal tissue
excised at surgery. This is clinically troublesome, because surgeons
would like to know preoperatively if a patient is likely to have
HNPCC/Lynch syndrome. This information might change the extent of the
colectomy: in HNPCC patients, the incidence of metachronous
colorectal cancers at 10 years is 40%; therefore, subtotal colectomy
or total proctocolectomy is recommended for these patients. In
addition, this information could lead to consideration of
simultaneous hysterectomy or oophorectomy.

Because in the vast majority of MSI-H tumors, MLH1, MSH2, or MSH6 is
either mutated or methylated and therefore their protein products are
absent in tumor tissue, IHC can be performed for those proteins. IHC
can be conducted on tiny tumor fragments such as those typically
obtained from needle biopsy or colonoscopic biopsy (such fragments
yield insufficient DNA for MSI testing). In addition, IHC is less
expensive than MSI testing, and the results are available within a
day or two. Furthermore, if one of the proteins is absent, mutational
analyses can be directed to the corresponding gene. IHC has a 100%
specificity and a 92% sensitivity for screening for DNA mismatch
repair defect. The final decision to conduct MSI testing, IHC
testing, or both must be made on a case-by-case basis; the decision
is based on local expertise in these techniques, cost analyses, and
whether the test is being used for screening or as a diagnostic test.

Why Should We Care?
It is important for clinicians to recognize a hereditary cancer
syndrome within families and individuals, because identification of
such a disease will have an impact on cancer screening, surveillance,
and management of the affected individual and of family members who
are at risk. The identification of a deleterious germline mutation
constitutes a genetically defined diagnosis of hereditary colorectal
cancer and is important for the treatment of the disease and
subsequent surveillance; in addition, it allows predictive testing to
be performed on unaffected relatives at a relatively low cost and
with essentially 100% accuracy. Identifying patients with Lynch
syndrome can now be accomplished by testing tumors for MSI, by
assessing for the absence of MMR protein expression with IHC, or
both. In addition to providing diagnostic information, MSI and IHC
analyses may have important prognostic and therapeutic implications.






Dieter Hahnloser, MD, University Hospital Zurich, Switzerland;
Stephen N. Thibodeau, PhD, Mayo Clinic College of Medicine



ACS Surgery 2004. © 2004 WebMD Inc.
All rights reserved.