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The human body is a very complex structure, made up of many different systems which work together to manage its over 37 trillion cells. In each individual cell, resides a complete copy of the human genome, with all the information necessary to run the entire body. If stretched out, one copy of human genome measures 2m in length, and the DNA of an average adult would stretch to the moon and back.

The human genome is the complete set of nucleic acid sequences, encoded as DNA, within the 23 chromosome pairs in all cell nuclei, along with a small DNA molecule found within cellular mitochondria. One set of chromosomes of an individual originates from each parent.

In reproduction, gametes are formed, which are cells containing only 1 set of chromosomes (23 chromosomes, no pairings). In the process of meiosis, the preparation of sex cells, mixing of our chromosome pairs occur, so that the set of genes which are represented in our germ cells (ova or sperm) are a mixture of the genotypes of both parents. Such mixing occurs through the formation of chiasmata.

 Genetics North York MedicalIn 1984, the American government planned and fully funded an initiative called the Human Genome Project, in which 20-25 major university centres from the United States, the United Kingdom, Japan, France, Germany, and China collaborated, in an effort to elucidate the sequence of the entire human genome, including multiple variations which may occur in any one gene. This required the sequencing of over 3 billion DNA nucleotides, and took nearly 14 years to complete, ending in 2003, at a cost to the U.S. taxpayer of more than $2.7 billion.

Genetics North York Medical

Moving forward from 2003, genome-based research has enabled medical science to develop highly effective diagnostic tools, to better understand the health needs of people, based on their individual genetic make-ups, with hope to design new and highly effective treatments for many diseases.

Genetics North York Medical

Today, an individual can walk into a genetics testing centre and have a DNA profile done, to learn about their risk of many diseases, and some cancers. Approximately 80% of cancers are linked to pollutants (chemical carcinogens) in the air, food and water, as well as radiation. The other 20% may be linked to genes which normally regulate cell growth and proliferation (proto-oncogenes), and under abnormal conditions, may be mutated into oncogenes, causing cell division at an accelerated rate, resulting in tumor growth.

Genetics North York Medical

Oncogenes were first identified in 1970 at the University of California, and were linked to a retrovirus. In 2002, the World Health Organization’s International Agency for Research in Cancer, estimated that 18% of human cancers were caused by infection, with 12% being caused by one of seven viruses (tumor viruses).

Genetics North York Medical

Retroviruses, such as HIV, are RNA (some retroviruses can carry DNA) carrying viruses which can infect a cell, use the cell’s machinery to produce corresponding DNA through reverse transcription, then produce biochemicals which allow its newly produced DNA to insert into the genome of the cell, and finally cause the cell to replicate the viral DNA code into many new retroviruses.

Tumor viruses come in different forms: DNA-carrying viruses, such as adenovirus, and RNA-carrying viruses, such as the Hepatitis C virus, which can cause cancers, as can retroviruses having either DNA or RNA genomes (Human T-lymphotropic virus and Hepatitis B virus).

Often, tumor viruses do not cause cancer in their hosts. For example, adenovirus does not cause cancer in humans but is instead responsible for conjunctivitis, colds and other acute illnesses. They only become tumorigenic when infected into certain rodent species, such as Syrian hamsters. Some viruses are tumorigenic when they infect a cell and persist in that cell as circular episomes or plasmids, replicating separately from host cell DNA (Epstein-Barr virus and Kaposi’s Sarcoma-associates herpes virus). Other viruses are only carcinogenic when they integrate into the host cell genome, such as polyomaviruses and papillomaviruses.

Genetics North York Medical

A direct oncogenic viral mechanism involves either insertion of additional viral oncogenic genes into the host cell or to enhance already existing oncogenic genes (proto-oncogenes) in the genome. Indirect viral oncogenicity involves chronic nonspecific inflammation occurring over decades of infection, as is the case for Hepatitis C virus-induced liver cancer.

These two mechanisms differ in their biology and epidemiology: direct tumor viruses must have at least one virus copy in every tumor cell, expressing at least one protein or RNA that is causing the cell to become cancerous. Because foreign virus antigens are expressed in these tumors, persons who are immunosuppressed such as AIDS or transplant patients are at higher risk for these types of cancers. Indirect tumor viruses, on the other hand, can be lost from a mature tumor that has accumulated sufficient mutations and growth conditions (hyperplasia) from the chronic inflammation of viral infection. In this latter case, it is controversial but at least theoretically possible that an indirect tumor virus could undergo hit-and-run, and so the virus would be lost from the clinically diagnosed tumor. In practical terms, this is an uncommon occurrence if it does occur.

Genetics North York Medical

So, now that your DNA report is ready, and there are no emergencies, the findings can be stored away. But what if there was some bad news, such as a high risk of multiple forms of cancer? What can you do with that information? If the cancer is not yet present, most people would worry, but they would do absolutely nothing.

Could we change our lifestyle? Yes, but how would that overcome the risks which are inscribed in our blueprints? Well, regular exercise is a start, but it is unlikely to be enough.

Should we wait until the tumor presents itself, hope to catch it early and plan for the burden of chemotherapy or radiation therapy? Has it presented already, but we simply have not noticed? In allopathic medicine, the use of chemotherapy for the treatment of cancer is done so with great uncertainty, and with the hope that the drug kills the cancer before killing its host. Is this the basket where all our eggs should be? Do we even have a choice? Absolutely!


Credit: Article Editing by Andre Sabungui -North York Medical Building

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