HEREDITARY NONPOLYPOSIS COLORECTAL CANCER: GENETICS AND DIAGNOSIS
THE ALPHABET SOUP OF GENETICS AND DIAGNOSIS EXPLAINED
With a myriad of vexing abbreviations and obscure terminology, the genetics controlling the formation of colonic polyps and malignancies may be difficult to appreciate. Paradoxically, the physician, positioned at the beginning of the diagnostic effort, is often the person most hampered by a lack of basic understanding of this genetic alphabet soup. All that is required to overcome this obstacle is a refresher in basic genetics and a high index of clinical suspicion. As the science underlying the development of inherited colorectal cancer has become better understood, the clinician has become better equipped to stand at the forefront of the diagnostic and treatment effort.
Part I of this series will examine the roles of the pathologist and geneticist in diagnosing Hereditary Nonpolyposis Colorectal Cancer. Part II will discuss screening and treatment and the roles of the epidemiologist, the diagnosticians and the surgeon. In Parts III and IV, Familial Adenomatous Polyposis will be discussed.
SPORADIC, FAMILIAL AND INHERITED DISEASE
Of all colon cancers, eighty to ninety percent occur sporadically, with no known etiology. Ten to fifteen percent of patients have familial colorectal cancer, meaning that there are two or more colorectal malignancies found in a given family and that a specific causative gene has not been identified. Five percent of patients have an inherited or hereditary form of colon cancer and a causative genetic abnormality has been found to be associated with the malignancy.
Generally, inherited colorectal cancers are divided into two groups. The first group is composed of those malignancies arising in a background of epithelial polyposis or hamartomatous colonic polyps. Familial adenomatous polyposis is the most common syndrome in this group. The second group is represented by malignancies arising in grossly normal appearing mucosa with few or no visible underlying polyps. Hereditary nonpolyposis colorectal cancer is the most common syndrome in this group.
HEREDITARY NONPOLYPOSIS COLORECTAL CANCER
Hereditary nonpolyposis colorectal cancer (HNPCC) or Lynch syndrome accounts for four percent of all colorectal cancers. Originally characterized in 1895 as a familial clustering of colorectal and other types of cancer, and then re-described in 1971 by Henry Lynch, HNPCC is now defined molecularly as an inherited, cancer-predisposing syndrome secondary to a deleterious germline mutation in one of a group of DNA mismatch repair (MMR) genes.(1,2) MMR genes correct sequence errors in DNA that result from faulty replication. The MMR genes are part of a post-replication DNA repair system. This genetic machinery is as complex as the genetic replication machinery itself, underscoring the importance of the MMR system.
An estimated one hundred fifty thousand Americans may be carriers of the HNPCC mutation and have a ninety percent lifetime risk of developing some form of cancer. Up to eighty percent of carriers will develop colorectal cancer by age seventy years. Up to seventy percent of female carriers are at risk of developing endometrial cancer by age seventy. With germline mutation analysis, the aberrant genetic fingerprint is often detectable prior to the development of cancer. Unfortunately, our present day capabilities have not advanced to the point of being able to use this information to repair the faulty gene and prevent the development of disease in affected individuals. However, we do possess the ability to reduce cancer risk in selected patients by removing susceptible organs before malignant transformation occurs.(3)
UNDERSTANDING THE DISEASE: THE PATHOLOGIST
HNPCC, or Lynch syndrome, is divided into Lynch I and Lynch II. Muir-Torre Syndrome and Colorectal Cancer Type X are also members of the HNPCC group. Muir-Torre is not commonly seen in the clinical setting. These diseases are often discovered by the pathologist postoperatively upon finding certain pathological features during the examination of the resected surgical specimen.
In Lynch I, colorectal cancer is the most commonly occurring malignancy. The lifetime risk of developing a colorectal cancer in an individual with Lynch I syndrome is eighty percent. The average age at diagnosis is forty four years old, compared to age sixty four in the sporadic form of colon cancer. Multiple generations are usually affected. Most of the neoplasms are poorly differentiated and located proximal to the splenic flexure. Synchronous lesions occur in seven percent of cases compared with one percent in sporadic cases. Metachronous lesions are found in forty five percent of patients with HNPCC compared with five percent of those with sporadic colon cancer, signaling a possible mismatch repair defect. Colorectal cancer occurring in the absence of a visible polyp or polyposis is the final phenotypic expression of the MMR mutation. It is important to note that a polyp, whether visible or occult, is the precursor lesion of the colorectal cancer.
Lynch II describes the association of colorectal cancer with extracolonic malignancies. In women with Lynch II, there is a fifty to seventy percent lifetime risk of developing endometrial cancer, with the average age of diagnosis being forty six(4). Other malignancies associated with Lynch II are: ovarian cancer (three to thirteen percent), gastric cancer (two to thirteen percent), transitional cell carcinoma of the ureter and renal pelvis (one to twelve percent), small bowel cancer occurring most commonly in the duodenum and jejunum (four to seven percent), central nervous system tumors, most often glioblastomas (one to four percent), and hepatobiliary cancer (two percent)(5)
Muir-Torre Syndrome is a rare syndrome consisting of multiple benign and malignant neoplasms. It may be a variant of Lynch II. Sebaceous gland adenomas are the most common marker of the disease and are often found on the head. Keratoacanthomas, which may begin as a red nodule and progress to a shiny nodule with telangiectasia and a central crust, are located on the face and dorsum of the hands. Visceral carcinomas are frequent, occurring in one half of patients with Muir-Torre. The colonic neoplasms are the most frequent malignancies found in Muir-Torre syndrome and are usually located proximal to the splenic flexure. Genitourinary tumors are the second most common malignancy(6).
Familial Colorectal Cancer Type X is a disease in which those involved meet the clinical criteria of the nonpolyposis syndrome but do not have the MMR defect found in the nonpolyposis syndromes. More specifically, patients with Familial Colorectal Cancer Type X do not have a demonstrable mismatch repair mutation in one of the known MMR genes. The risk of developing colorectal cancer or extracolonic neoplasia is lower than in those patients with HNPCC, and the age of diagnosis of colon cancer is higher than in patients with Lynch Syndrome. Colonic malignancies are not predominantly right sided as in patients with Lynch Syndrome. Unlike patients with the Lynch II syndrome, these individuals do not seem to develop malignancies in other organ systems(7).
UNDERSTANDING THE GENETICS: THE GENETICIST
HNPCC is transmitted through germ cells in an autosomal dominant fashion and is highly penetrant. Germline cells are those cells passed down through generations. The commonly involved genes are MSH2, found in sixty percent of HNPCC mutations and MSH6, found in ten percent of HNPCC mutations. Both are located on chromosome two. MLH1, located on chromosome three is responsible for thirty percent of mutations. Numerous other genes account for rare cases of HNPCC. These genes normally produce proteins responsible for removing and repairing specific nucleotide sequences in DNA which may have become corrupt as a result of faulty replication. One copy of the mutant HNPCC gene is found in all cells and in all tissues of carriers. A second, normal copy of the gene from the unaffected parent is also present in all cells. Any event causing a mutation and inactivation of this second normal gene in colorectal epithelium or other susceptible epithelium causes a transcription silencing of an important part of the MMR genetic machinery. The mutation is considered to be a “second hit” as both genes coding for the production of mismatch repair proteins are now non-functional. Without mismatch repair, there is a rapid accumulation of somatic mutations and a neoplastic cascade leading to tumor development, which is the ultimate expression of the HNPCC phenotype.
The defect in mismatch repair genes also leads to mutations in “bystander” genes, known as microsatellites. Microsatellites are short, non-coding, tandemly repeated DNA sequences of one to six nucleotide bases located primarily on the telomere or centromere, but also located next to the coding region of MMR genes. These sequences are unique to each individual. They can be affected by a mutation termed microsatellite instability (MSI). MSI results from the erroneous insertion, deletion or mis-incorporation of bases during DNA replication or recombination, with failure of the mismatch repair system to correct these errors(8). In HNPCC, mutant microsatellites begin to accumulate and can be detected in the tumor tissue of ninety five percent of affected patients using fluorescent multiplex polymerase chain reaction-capillary electrophoresis. The tumor microsatellite nucleotide repeats are compared with the repeats found in normal tissue adjacent to the tumor tissue. The tumor is considered to be microsatellite unstable if the tumor repeats are different from the normal tissue repeats. MSI testing can be performed on fresh tissue or fixed paraffin blocks. In 1993, the genetics underlying mismatch repair were elucidated, allowing for MSI testing of tumor tissue in an attempt to diagnose HNPCC. (9,10,11,12) Depending on the number of abnormal nucleotide repeats found in the tumor tissue, results are reported as MSI-H (high), MSI-L (low) or MSI-S (stable).
An alternative detection technique consists of using antibodies to normal MMR gene proteins, combined with imunohistochemistry (IHC) fluorescent staining. IHC testing can be performed on fresh tissue or fixed paraffin blocks. Lack of staining is usually considered to be a positive test result, indicating loss of the normal protein product. This is due to the existence of a mutant, non-functioning mismatch repair gene. In 1996, monoclonal antibodies to mismatch repair gene proteins were discovered, allowing for this additional technique in the search for MMR mutations.(13,14)
Both MSI testing and IHC staining evaluate the phenotypic results of the HNPCC MMR gene mutation and are considered to be surrogate markers for HNPCC. (15,16,17) Cases of false positives results and rare false negative result exist and these must be considered as the clinician begins the evaluation. Higher detection sensitivities of up to ninety eight percent have been reported when using MSI testing in combination with IHC, as compared to using either test by itself. Tumor testing has a high sensitivity, and a lack of tumor microsatellite instability (MSI-S or MSI-L) or normal IHC staining effectively rules out the possibility of having classic Lynch I or Lynch II. No genetic mutation in a known MMR gene will be found on further testing. However, as these patients meet the clinical criteria for having an inherited colorectal cancer, they have been termed Familial Colorectal Cancer Type X, or “the other half of HNPCC”.(18,19)
Additionally, positive results (MSI-H) do not guarantee that a germline mutation will be found. An important and not uncommon example of a false positive test result which, if left undiscovered could lead to expensive and time consuming testing, is caused by hypermethylation and subsequent transcription silencing of MLH1. This is thought to be the etiology of fifteen percent of sporadic colorectal cancers. This is an epigenetic (non-mutational) change and means that although the underlying DNA morphology and sequence is normal, gene functioning is affected by a superimposed error, in this case, methylation of MLH1. MSI testing will be positive for microsatellite instability, but will not distinguish sporadic from inherited disease. The βRAF gene manufactures a βRAF protein which is involved in transmitting signals related to cell growth. A βRAF gene mutation is present in the majority of sporadic tumors with hypermethylation, but is not found in cases of HNPCC germline mutations. The combination of MSI testing, MLH1 hypermethylation testing and βRAF mutation analysis can help distinguish sporadic colorectal cancer from HNPCC and help to avoid otherwise unnecessary genetic testing and the resulting patient anxiety.
Germline analysis, with the discovery of a genetic mutation in one of the MMR genes is an important step in diagnosing HNPCC. It is performed on samples of whole blood. However, germline analysis is not widely available for all patients. Detection of the deleterious germline mutation has become the ultimate diagnostic criterion for HNPCC. The mutant gene is identified, as is the exact nucleotide mutation. This valuable information can be used in screening and in genetic counseling of family members. Patients identified as having a mutation in MSH2 or MLH1 comprise ninety percent of HNPCC patients. Therefore, germline testing directed toward these genes will yield the most obvious benefit. Finding a germline mutation in patients with MSI-H tumors or in tumors with absent IHC staining represents the ultimate diagnostic confirmation of HNPCC. However, up to fifty percent of clinically defined individuals with HNPCC do not display a mutation in one of the known MMR genes and are considered to have Familial Colorectal Cancer Type X.
To summarize the most common test results, in patients meeting the clinical criteria for having HNPCC (criteria to be discussed in part II), who have a colorectal cancer, which is MSI-S or MSI-L or shows normal IHC staining, classic Lynch Syndrome is effectively ruled out and the patient is considered to have Familial Colorectal Cancer Type X. If the tumor is found to be MSI-H, it may be a Lynch I or Lynch II tumor. Germline testing will confirm or rule out the diagnosis of HNPCC. If a germline mutation is not found in an MSI-H tumor, it may represent a case of sporadic colorectal cancer secondary to hypermethylation of MLH1. Further testing for this possibility may be performed by testing for a combination of a βRAF mutation and MLH1 hypermethylation using a test kit which can specifically evaluate this possibility.
Finally, there are many families with several members who have colorectal cancer but who do not demonstrate an underlying genetic basis for the disease. All available genetic test results are normal and non-informative. Clearly not every mutant gene involved in the production of a colon malignancy has been identified. Until such time as the genetic basis for colorectal cancer is completely delineated, clinicians will have to rely upon clinical guidelines to begin the screening process.
Part II will examine screening strategies and treatment recommendations for HNPCC.
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