Retroviruses have influenced the human genome for millions of years. However, significant infectious diseases are also due to retroviruses.
What are retroviruses?
A virus is an infectious particle that is not capable of independent reproduction. Viruses also do not have their own metabolism. Therefore, viruses are not counted as living organisms, even though they exhibit individual characteristics of life. A retrovirus is a virus without its own DNA (deoxyribonucleic acid), a molecule found in all living organisms as well as in some viruses, which consists of a double helix and contains all hereditary information. In contrast, the genetic material (genome) of retroviruses, which have a diameter of approximately 100 nm, contains only one strand of RNA (ribonucleic acid), which is surrounded by a packaging (“capsid”) consisting of proteins. The outer envelope of retroviruses is formed by largely water-insoluble molecules (made of “lipid” substances) into which viral proteins are incorporated.
Significance and function
“Endogenous retroviruses” (“XRV”) were integrated (“provirus”) into the genome of a host cell in the germline of the host organism many generations ago and are passed on from generation to generation. Research has shown that approximately 9 percent of the human genome consists of viral RNA. The vast majority of this RNA entered the genome of our ancestors an estimated 40 to 70 million years ago. Partial building blocks of retroviruses still recognizable in the human genome became part of the genetic material 100 million years ago. The genomes of some endogenous retroviruses even have a protective function for life: Human pregnancies, for example, are only possible because a certain ancient retrovirus prevents the rejection of the embryo. “Exogenous retroviruses” (“ERV”), on the other hand, enter the host organism by infection. Retroviruses, which infect vertebrates in particular, infect specific animal somatic cells to which they are specialized. Within the cell they infect, they incorporate their genetic material into the host cell genetic material. After retroviruses reproduce within the host cell, the viruses are released into the bloodstream and can thus infect other cells. The DNA of a cell ensures the production of RNA, which itself functions as part of the genome and also transmits information as “messenger RNA” (mRNA, messenger RNA), which is necessary for the formation of proteins. The name “retrovirus” results from the fact that this form of virus reverses the initial procedure for RNA formation within a cell: it is not the original DNA of the host cell that now triggers the instructions for RNA formation. Rather, the retrovirus alters the DNA of the host cell, which after infection gives instructions for the production of new retroviruses. The so-called “reverse transcriptase” (RT), a special “enzyme” of the retrovirus enables the insertion of the retrovirus RNA into the DNA of the host cell. Enzymes are substances that can trigger certain biochemical reactions.
Hazards, disorders, risks, and diseases
The best-known retrovirus is the HI virus (human immunodeficiency virus), which causes immunodeficiency in humans. HIV specializes in so-called “T helper cells” (also called “CD4 lymphocytes“), which are responsible for coordinating defenses against pathogens and foreign substances in the human body. Lymphocytes belong to the white blood cells (“leukocytes“). T helper cells represent a subgroup of the “T cells”. The term “T-cell” refers to the “thymus“, which is part of the so-called “lymphatic system” and thus of the immune system. The thymus is an organ composed of two lobes, which in humans is located above the heart. The “T cells” (“T lymphocytes“) produced in the bone marrow and migrating from there to the thymus are responsible for the immune defense after their maturation in the thymus. Worldwide, 34 million people have been infected with the HIV virus. SIV (simian immunodeficiency virus) is a group of viruses from which HIV is thought to have evolved. “Simian” means “ape-like” and refers to the carriers of SIV. HTLV-1 virus (human T-lymphotropic virus 1), which also infects CD4 T lymphocytes in humans and related primates, is also a retrovirus. A small number of infected humans develop neurological diseases such as “Tropical Spastic Paraparesis” or “T-cell Leukemia“. The symptoms of Tropical Spastic Paraparesis are similar to multiple sclerosis.T-cell leukemia leads to malignant (“malignant”) tumors originating from the lymphocytes. The infection rate with the HTLV-1 virus is low in Europe: in Western Europe, probably 6,000 people are infected, of whom about one percent develop Tropical Spastic Paraparesis. Worldwide, however, it is estimated that up to 20 million people have been affected by HTLV-1. Weakening of the immune system by decreasing the number of T cells leads to increased susceptibility to infection. The fight against diseases caused by retroviruses is complicated by a high mutation rate: one in every thousand to ten thousand reverse transcriptases results in a retrovirus mutation. The development of drugs for the therapy of retrovirus diseases is also aimed primarily at influencing reverse transcriptase.
