Human T-Lymphotropic Virus Type 1: (HTLV-1)

Introduction

The human T-lymphotropic virus type 1 (HTLV-1) was the first oncogenic human retrovirus to be discovered. It was first studied in 1977. The virus can cause adult T-cell leukemia/lymphoma (ATL) and progressive nervous system condition known as HTLV-1-associated myelopathy or tropical spastic paraparesis (HAM/TSP)along with other neurodegenerative diseases. Although HTLV is known for being associated with lymphoma and leukemia, it more commonly causes a range of neurological disorders due to the types of cells that are infected. Human T-Lymphotropic Virus can attack a variety of cells including T cells, B cells, dendritic cells, monocytes, and endothelial cells but this virus can only edit or transform T cells and B cells. This transformation is what causes cancer in rare cases. HTLV is most commonly associated with Tropical spastic paraparesis.Tropical spastic paraparesis (TSP), is a medical condition that causes weakness, muscle spasms, and sensory disturbance resulting in weakness of the legs. However, most HTLV carriers are asymptomatic and transmit the virus to others. HTLV type 1 is less commonly associated with B cell chronic lymphocytic leukemia and T-prolymphocytic leukemia.

Structure

This is the immature and mature structure of HTLV [1].

Human T-Lymphotropic Virus is part of the Delta-type retrovirus group. HTLVs are enveloped viruses with a diameter of approximately 80–100 nm. The HTLV virions contain two covalently bound genomic RNA strands, which are complexed with the viral enzymes reverse transcriptase, integrase and protease, and the capsid proteins. The outer part of the virions consists of a membrane-associated matrix protein and a lipid layer intersected by the envelope proteins. The genetic information is enclosed by a membrane or capsid.

Life Cycle

The most common type of transmission of HTLV is through blood, sexual transmission, or from mother to child via breastfeeding. After transmission Human T cell Lymphotropic Virus Type-1 is dependent on outside forces to initiate replication. A new viral cell binds to a receptor on a target cell and is integrated into the cell by a process called fusion. This begins the process of infection and replication. The virus depends on the host cell for the initial stages of transcription. In order to replicate, the viral genome must be reverse transcribed into double stranded DNA instead of RNA. The viral genome encodes for many essential genes required for the function of the virus including regulatory genes tax and rex. Tax regulates cell cycle control, increases rate of transcription, regulates cell proliferation and differentiation, and regulates DNA repair. Rex is a post transcription stabilizer and acts as an export for viral mRNAs from the nucleus to the cytoplasm. When these genes are damaged or fail to do their job these two genes are most commonly the cause of cancerous or oncogenic cells.

This is the cycle of infection, replication, and creation of new viruses.[2].

Diagnosis

The diagnosis of this virus is tricky considering that most infected are asymptomatic however, when symptoms are present they begin very subtly but end up being chronic very dangerous. Initial symptoms can include gait problems, unexplained falls, low back pain, constipation, urinary urgency, and numbness or pain in the lower limbs. As the illness progresses symptoms can look like progressive leg weakness and failure of the urinary system as well as possible neurological damage.Diagnosis of HTLV is usually the detection of antibodies in the blood or spinal fluid. Another way of diagnosis is the cell shape. After cells are infected they can take on a flower shape that can be seen after a biopsy is taken and a blood smear is examined. There are many treatments or therapies available, mostly in the form of a medication to get rid of the virus or a medication to relieve symptoms. As for the cancer that results in rare cases, there are many investigative therapies available.

This map shows areas of the world where HTLV-1 is commonly diagnosed. Note that the areas are not divided by country, just the specific areas where HTLV has the most effects.[3].

Conclusion

Although HTLV is associated with being the first virus found to cause cancer in T cells and B cells, it is not the most common effect of HTLV type-1. This specific type of Human T-Lymphotropic Virus most commonly causes progressive degenerative neurological disorders. Diagnosis of this virus is loosely based on symptoms and more dependent on antibody tests and biopsies.

Edited by [Sydney Srnka], student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2020, Kenyon College.