What Are Genes?

Each individual has unique characteristics that define who they are. These characteristics can be defined as a gene. Genes are acquired from the individual’s parents transferred to the offspring. There are approximately about 25,000 genes and 23 pairs of chromosomes compiled in a standard human body [1]. Genes are strands of DNA (deoxyribonucleic acid) inside each cell that give instruction to each cell on its function and when to grow and divide. Each gene is made up of a unique DNA sequence that contains a code, or set of instructions, to make a certain protein, each of which has a specific job or function in the body.Image result for human genome project

Gene Mutations

There are often errors in the DNA of a gene that cause mutations. A gene mutation can affect the cell in numerous ways. Some mutations stop a protein from being made, reduce the proteins output, turn off the protein completely, cause a gene to turn on, or have no noticeable effect [1].

 

How Genes Mutate

Genes undergo mutation either from an outside source or randomly. Some acquired mutations can be caused by things that we are exposed to in our environment, including cigarette smoke, radiation, hormones, and diet [1]. Cell division can also cause forms of genetic mutation. In order for a cell to divide and to make two new cells, it has to copy all of its DNA. With so much DNA, sometimes mistakes are made in the new copy (like typos) which lead to DNA mutations [1]. When a cell divides, it is another chance for mutations to occur. As we age, the numbers of gene mutations build up over time and increases the chance of getting cancer.Image result for dna mutations

How Mutations Cause Cancer

Some mutations can lead to certain diseases such as cancer. These mutations can be inherited from one or both parents or occur on their own. Inherited mutations are much less common for causing cancer than acquired mutations which are also called sporadic or somatic [2]. Cells become cancer cells largely because of mutations in their genes. Often many mutations are required before a cell becomes a cancerous. The mutations may affect different genes that make a cell grow and divide. Some changes can affect just one unit of DNA known as a nucleotide, replace a nucleotide, or can be missing entirely [3]. Some of these genes are called tumor suppressor genes while others affect normal genes to become cancer-causing genes known as oncogenes [1]. When a mutation occurs usually, the cell detects the change and repairs it [1]. If it can’t be repaired, the cell will undergo what is called apoptosis, which is the death of a cell. But if the cell doesn’t go through apoptosis and the mutation is not fixed, it can increase the chance of an individual getting cancer [1]. This is more likely if the mutation affects a gene involved with cell division or a gene that normally causes a defective cell to die [1].

 

The Cancer Genome Atlas (TCGA)

The Cancer Genome Atlas (TCGA) is an global and group effort to help understand, on a molecular level, cancer through the application of analyzing genomes, including large-scale genome sequencing [4]. So far, TCGA researchers have analyzed large divisions of over 30 human tumors through large-scale genome sequencing and integrated multi-dimensional analyzes [5].

 

Am I At Risk for Cancer?

Unfortunately, there is no way to prevent cancer.  Everyone has some risk of developing the disease, some more than others. The individuals at a greater risk are ones who carry hereditary mutations. They do not necessarily get cancer, but their risk of developing the disease at some point during is higher than average [1]. In most cases the disease develops by chance. These people have a higher risk of developing the disease than those in the general public [2].

 

Hereditary Cancers

Inherited mutations play a serious role in about 5 to 10 percent of all cancers [3]. Researchers have connected mutations in certain genes with more than 50 hereditary cancer types, which are disorders that may lead individuals to developing certain cancers [3]. The chart below shows some types of cancers along with their mutated genes.

 

Type of Cancer Mutated Gene Related Cancers
Hereditary breast cancer and ovarian cancer syndrome BRCA1, BRCA2 Female/male breast, ovarian, prostate, and pancreatic cancer
Li-Fraumeni syndrome TP53 Breast cancer, soft tissue sarcoma, osteosarcoma, leukemia, brain tumors, and adrenocortical carcinoma
Cowden syndrome (PTEN hamartoma tumor syndrome) PTEN Breast, thyroid, endometrial, and other cancers
Lynch syndrome (hereditary nonpolyposis colorectal cancer) MSH2, MLH1, MSH6, PMS2, EPCAM Colorectal, endometrial, ovarian, renal pelvis, pancreatic, small intestine, liver and biliary tract, stomach, brain, and breast cancers
Familial adenomatous polyposis APC Colorectal cancer, multiple non-malignant colon polyps, and both non-cancerous and cancerous tumors in the small intestine, brain, stomach, bone, skin, and other tissues
Retinoblastoma RB1 Eye cancer, pinealoma, osteosarcoma, melanoma, and soft tissue sarcoma
Multiple endocrine neoplasia type 1 (Wermer syndrome) MEN1 Pancreatic endocrine tumors, parathyroid, and pituitary gland tumors
Multiple endocrine neoplasia type 2 RET Medullary thyroid cancer and pheochromocytoma
Von Hippel-Lindau syndrome VHL Kidney cancer and multiple non cancerous tumors, including pheochromocytoma

Data from [3]

 

Genetic Testing and Counseling

For some hereditary cancers genetic testing is available. If you have a family history of cancer, or if you would like to find out whether you have an increased likelihood of developing cancer, you are encouraged to consult with a specialist who has experience in the area of hereditary cancer, cancer risk assessment, and genetic counseling and testing [2]. Genetic testing involves a blood test that could be used to obtain a more precise estimate of your cancer risk or, if available, stored tissue samples from deceased relatives [2]. Genetic counseling includes a detailed review of the individual’s personal and family medical history related to possible cancer risk [3]. It is usually recommended for individuals or families with multiple cases of cancer diagnosed in the family, at unusually young ages, and specific racial and ethnic groups at higher risks for cancers [3].

 

What do The Test Results Mean?

Genetic testing can have many different possible outcomes each with a different meaning and course of action. A “positive test result” means that the laboratory found a specific genetic alteration (or mutation) that is associated with a hereditary cancer syndrome [3]. Since they have tested positive to the hereditary mutation gene individuals have an increased chance of developing cancer. A “negative test result” means that the laboratory did not find the specific alteration that the test was designed to detect [3]. This means that the hereditary mutation was not passed onto the offspring. A “true negative” means a negative test result but a known genetic mutation is present [3]. Cancer is still possible for the individual but it is the same as the general population. An uninformative negative means that a strong family history of an associated hereditary cancer is present but the test result is negative [3]. A “false negative” means that the test results were inconclusive or the person does not have an altered cancer causing gene [3]. A “variant of unknown significance” means that the test shows a gene mutation that has not been recorded in other types of cancers [3]. Finally, if there is a genetic mutation amongst the general non-cancer public it is considered “polymorphism” [3]. Each of these results can be interpreted by a trained genetic counselor or trained professional specializing in genetics.

 

References

  1. “Family Cancer Syndromes.” American Cancer Society, The American Cancer Society Medical and Editorial Content Team, 4 Jan. 2018, www.cancer.org/cancer/cancer-causes/genetics/family-cancer-syndromes.html.
  2. “Hereditary Cancer & Genetics.” Memorial Sloan Kettering, Memorial Sloan Kettering Cancer Center, www.mskcc.org/cancer-care/risk-assessment-screening/hereditary-genetics.
  3. “Genetic Testing for Hereditary Cancer Syndromes.” National Cancer Institute, National Cancer Institute, 11 Apr. 2013, www.cancer.gov/about-cancer/causes-prevention/genetics/genetic-testing-fact-sheet.
  4. “The Cancer Genome Atlas.” National Human Genome Research Institute (NHGRI), National Cancer Institute, 31 May 2018, www.genome.gov/17516564/the-cancer-genome-atlas/.
  5. Tomczak, Katarzyna, et al. “The Cancer Genome Atlas (TCGA): an Immeasurable Source of Knowledge.” Advances in Pediatrics., U.S. National Library of Medicine, 20 Jan. 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4322527/.