Genetic Testing for Cancer

US Pharm. 2012;37(9)(Oncology suppl):3-6.

ABSTRACT: Genetic testing is the analysis of a person’s DNA to determine whether the individual has a disease-causing gene alteration or is a carrier for a genetic disease. Specific genetic mutations have been linked to several types of cancer that may be identified with clinical tests. Inheritance of the mutated tumor suppressor gene BRCA1 or BRCA2 confers significant risk of developing breast, ovarian, and other cancers. Identifying certain genetic mutations (e.g., epidermal growth factor receptor) may help patients make informed treatment decisions.

Genetic testing is a type of medical test available for infants, children, and adults that examines chromosomes, genes, or proteins in order to identify mutations linked to a disease or disorder.^1,2 The test is performed on a person’s hair, saliva, tissue, blood, or amniotic fluid, but mainly blood, to determine changes in whole chromosomes. Genetic testing analyzes a person’s chromosomes (DNA), proteins, and certain metabolites to detect disease-related genotypes, phenotypes, or karyotypes for clinical purposes.^3-7 It is often used to determine changes that are associated with inherited medical disorders. Over 4,000 diseases have their roots in genes or stem from altered inherited genes.

The results of a genetic test may confirm or rule out a suspected genetic condition or help determine a person’s chance of developing or passing on a genetic disorder. Types of genetic tests often utilized include:

Predictive Testing⁷: Test results show which people have a higher chance of getting a disease before symptoms appear. One type of predictive test may screen for inherited genetic risk factors that make it more likely the person will develop certain cancers, such as colon or breast cancer.

Presymptomatic Testing⁸: Test results are utilized to predict or indicate which family members are at risk for a certain genetic condition (e.g., Huntington’s disease) already known to be present in their family, but do not have symptoms of that disease.

Genetic Test Results

Genetic tests look for large changes, such as a gene that has a section missing or added, or small changes, such as a missing, added, or altered chemical base within the DNA strand. Genetic tests may also detect genes with too many copies, individual genes that are too active, genes that are turned off, or genes that are lost entirely.⁹

A positive test result means that the laboratory found a change in a particular gene, chromosome, or protein of interest.¹ This result may be used for prenatal testing in order to confirm a diagnosis; indicate that a person is a carrier of a particular gene mutation; identify an increased risk of developing a disease in the future; or suggest the need for further testing.

A negative test result indicates that the laboratory did not find a dangerous copy or mutation of the gene, chromosome, or protein being considered.¹⁰ However, it is possible that the test missed a disease-causing genetic alteration because some tests cannot detect all of the possible genetic changes. This result can indicate that a person is not affected by a particular disorder, is not a carrier of a specific genetic mutation, or does not have an increased risk of developing a certain disease that can cause a particular disorder, and further testing may be required.

Genetic testing can determine whether a person has a disease-causing gene alteration or if someone is a carrier for a genetic disease.^1,9,10 A negative result can produce a tremendous sense of relief and may eliminate the need for frequent checkups and tests that are routine in families with a high risk of cancer.^11,12 Even a positive result can relieve uncertainty and allow a person to make informed decisions about the future. Other considerations of genetic testing are listed in TABLES 1 and 2.

Genetic Disorders

Specific genetic mutations that have been linked to several types of cancer may be identified with clinical tests. Genes can be altered, or mutated, in many ways.¹³ The most common gene change involves a single-base mismatch—a misspelling—placing the wrong base in the DNA. Genetic disorders in patients often involve the inheritance of a particular mutated disease-causing gene. The terms breast cancer susceptibility gene 1 (BRCA1) and 2 (BRCA2) refer to human genes that belong to a class known as tumor suppressors. In breast cancer, inheritance of a mutated BRCA1 or BRCA2 gene confers significant risk of developing the disease. Many genetic disorders are multifactorial inheritance disorders, meaning that they are caused by a combination of inherited mutations in multiple genes along with environmental factors.¹⁴

Gene mutation can greatly predispose a person to a number of diseases that run in the family and can significantly increase each family member’s risk of developing the disease.^13,14 A healthy body depends on the continuous interplay of thousands of proteins and chemicals acting in concert in just the right amounts and in just the right places; each properly functioning compound is the product of an intact healthy gene.¹² More than 4,000 diseases have their roots in genes or stem from altered genes inherited from one or both parents. It is estimated that inherited gene mutations account for 5% to 10% of breast cancers and 10% to 15% of ovarian cancers among white women in the United States.^15,16

Most cancers come from random gene mutations that develop in body cells during one’s lifetime—either as a mistake when cells are going through cell division or in response to injuries from environmental agents such as radiation or chemicals. Genes that control orderly replication of cells can become damaged, allowing cells to reproduce uncontrollably. Genetic disorders can be grouped as monogenetic, multifactorial, or chromosomal.

Monogenetic Disorders¹⁷: These types of genetic disorders (e.g., sickle cell disease, cystic fibrosis, polycystic kidney disease, autism, and Tay-Sachs disease) are caused by a mutation of a single gene. The mutation may be present in one or both chromosomes inherited from parents. Monogenetic disorders are classified as “dominant” and “recessive” diseases. Dominant genetic diseases are caused by the presence of the disease gene on just one of the two inherited parental chromosomes. In dominant diseases, the chance of a child inheriting the disease is 50%. Recessive genetic diseases result from inheriting two defective recessive genes, one from each parent. The chance of a child inheriting a recessive disease is 25%.

Multifactorial Inheritance Disorders^13,17: Heart disease, diabetes, and most cancers are caused by a combination of small inherited variations in genes, often acting with environmental factors. Behaviors (e.g., alcoholism), obesity, mental illness, and Alzheimer’s disease are also multifactorial, involving multiple genes.

Chromosome Disorders^13,17: Certain disorders (e.g., Down syndrome, chronic myeloid leukemia, Prader-Willi syndrome) are caused by structural changes within chromosomes; by an excess or deficiency of genes located on chromosomes, e.g., an extra copy of a chromosome or the absence or nonexpression of a group of genes on a chromosome; or by chromosomal translocation in which portions of two different chromosomes are exchanged.

BRCA1 and BRCA2 Genes

As previously mentioned, the BRCA1 and BRCA2 genes belong to a class of genes known as tumor suppressors. Evidence of their existence was first documented in 1990.¹⁸ In normal cells, BRCA1 and BRCA2 help ensure the stability of the cell’s genetic material (DNA) and help prevent uncontrolled cell growth.¹⁵ Mutation of these genes has been linked to the development of hereditary breast and ovarian cancer. A woman with a BRCA1 or BRCA2 mutation is at a higher risk for developing breast, ovarian, and other cancers than a woman without an alteration. Although much less common, men with BRCA2 gene mutations have higher rates of breast cancer, pancreatic cancer, and prostate cancer than men without an altered gene.^15,19,20

BRCA1 Gene: The BRCA1 gene is located on the long (q) arm of chromosome 17.²¹ The protein encoded by the BRCA1 gene combines with other tumor suppressors, DNA damage sensors, and signal transducers to form a large multisubunit protein complex know as the BRCA1-associated genome surveillance complex (BASC).²² The BRCA1 protein repairs double-strand breaks in one or both strands of DNA. BRCA1 is expressed in the cells of breast and other tissue, where it helps repair damaged DNA or destroy cells if the DNA cannot be repaired. If the BRCA1 gene itself is damaged, it may not be repaired properly, and this increases the risk of cancer.

Specific variations of the BRCA1 gene lead to an increased risk for breast cancer as part of a hereditary breast-ovarian cancer syndrome. Women with an abnormal BRCA1 or BRCA2 gene have up to a 60% risk of developing breast cancer by age 90 years; increased risk of developing ovarian cancer is about 55% risk for women with BRCA1 mutations and about 25% for women with BRCA2 mutations.²³ In addition to breast cancer, mutations in the BRCA1 gene also increase the risk of ovarian and fallopian tube cancers in women and prostate cancers in men; they also greatly increase risks for a subset of leukemias and lymphomas.²⁴

Some women who inherit a defective BRCA1 or BRCA2 gene have risks for breast and ovarian cancer that are so high and seem so selective that many mutation carriers choose to have prophylactic surgery.^25,26 An innate genomic deficit in a tumor suppressor gene is thought to impair normal responses and exacerbate the susceptibility to disease in organ targets. For example, women with the BRCA1 breast cancer susceptibility gene have an 80% chance of developing breast cancer by the age of 65 years. Family members who test negative for the BRCA1 mutation are not exempt from breast cancer risk; over time, they can acquire breast cancer–associated genetic changes at the same rate as the general population.

BRCA2 Gene: The BRCA2 gene is located on the long (q) arm of chromosome 13.² Although the structures of the BRCA1 and BRCA2 genes are very different, some of their functions are interrelated. The proteins made by both genes are essential for repairing damaged DNA. The BRCA2 protein binds to and regulates the protein produced by the RAD51 gene to repair breaks in DNA caused primarily by natural and medical radiation and other environmental exposure.²⁷

Numerous mutations in the BRCA2 gene have been identified, many of which cause an increased risk of cancer. BRCA2 mutations are usually insertions or deletions of a small number of DNA-based pairs in the gene.²³ As a result of these mutations, the protein product of the BRCA2 gene is abnormal and does not function properly, and defective BRCA2 proteins are unable to repair mutations that occur in other genes. As a result, mutations build up and can cause cells to divide in an uncontrolled way and form a tumor.

People who have two mutated copies of the BRCA2 gene have one type of Fanconi anemia. This condition is caused by extremely reduced levels of the BRCA2 protein in cells, which allows the accumulation of damaged DNA. Patients with Fanconi anemia are also prone to several types of leukemia; to solid tumors, particularly of the head, neck, skin, and reproductive organs; and to bone marrow suppression leading to anemia.²⁸ In addition to breast cancer in men and women, mutations in BRCA2 also lead to an increased risk of ovarian, fallopian tube, prostate, and pancreatic cancers, as well as malignant melanoma.

All germline BRCA1 mutations identified to date have been inherited, suggesting the possibility of a large “founder” effect in which a certain mutation is common to a well-defined population group and can theoretically be traced back to a common ancestor.²² A striking example of a founder mutation occurs in Iceland, where a single BRCA2 (999del5) mutation accounts for virtually all breast/ovarian cancer families.^21,29 Data from a recent study suggest that a BRCA2 mutation may also be associated with less favorable clinical features that result in higher risk for both recurrence and death.³⁰

Epidermal Growth Factor Receptor Tests

Classification for breast cancer is often based on an assessment of tumor biology, i.e., the status or presence of hormonal estrogen receptors (ER) and progesterone receptors (PgR or PR), as well as human epidermal growth factor receptors (EGFR, also known as ErbB2 or HER2Neu) overexpression.^31-35 About 15% to 20% of breast cancer test results are negative for estrogen receptors (ER-), progesterone receptors (PR-), and HER2 (HER-); these patients are diagnosed as having triple-negative breast cancer (TNBC).³⁶ When the status of all three is negative, it means that cancer growth is not supported by the hormones estrogen and progesterone, nor by the presence of too many HER2 receptors,^36,37 and that other factors are supporting cell proliferation. About 20% to 30% of breast cancers have too many HER2 receptors.^36,37 In normal, healthy breast cells, HER2 receptors receive signals that stimulate cell growth. With too many HER2 receptors, however, breast cancer cells multiply too quickly. Researchers have also determined that 92% of TNBCs express the mucin1 (MUC1) oncoprotein target.³⁷ This protein serves a protective function by binding to pathogens and also functions in a cell-signaling capacity. Overexpression, aberrant intracellular localization, and changes in glycosylation of this protein have been associated with carcinomas.³⁵

Test for the G84E Mutation

Similar to the role of BRCA1 and BRCA2 screenings for prostate cancer risk, a new test for the G84E mutation of the HOXB13 gene is being studied.³⁸ The HOXB13 gene plays a key role in the development and functioning of the prostate gland, and its G84E mutation is associated with prostate cancer susceptibility. Researchers found a 14-fold increase in prostate cancer risk in men with the mutation who had early onset of the disease (55 years or younger) or a family history of cancer. This new genetic test could potentially identify younger patients who may benefit from earlier or additional screenings.³⁸

Summary

When it comes to identifying or predicting the risk for some genetic disorders, genome sequencing is not a crystal ball. But it certainly provides the clinician and patient with additional information that may be extremely valuable in making informed choices in life and in treating a genetic disorder, especially certain cancers. However, genetic testing should not be used as a substitute for conventional risk-management strategies, including routine medical checkups and healthy life-style habits. It is important to remember that cancer is not just a genetic disorder but is caused by both genetic and epigenetic factors, which are equally important with respect to the pathophysiology of the disease.

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