Linking the HTLV-1 pre-integration complex to the chromatin

Human immunodeficiency virus type 1 (HIV-1) and human T-cell lymphotropic virus type 1 (HTLV-1) are the most notorious retroviruses since they are the cause of severe, disabling and sometimes fatal diseases. Ten to 20 million people worldwide are infected with HTLV-1. Of these individuals, about 10% will become ill. The most severe HTLV-1 induced diseases are adult T-cell leukaemia (ATL) and the neurological disorder HTLV-induced myelopathy/tropical spastic paraparesis (HAM/TSP). So far, treatment of HTLV-1 infected patients was proven to be very inefficient. Patients diagnosed with ATL, typically die within two years of presentation.

HTLV-1, like other retroviruses, are RNA viruses that reverse transcribe their RNA genome into DNA. A critical step in the lifecycle of retroviruses is the integration (or insertion) of this DNA copy into the host genome. This reaction is facilitated by the viral enzyme integrase. Where in the host genome the virus will integrate its DNA copy is not random and we know that integration in certain areas of the genome likely predispose the patients to disease. Our aim is to understand the mechanism of how the integration machinery choses where it will insert the viral DNA.

We have identified an important host protein complex, protein phosphatase 2A (PP2A), that specifically binds to HTLV-1 integrase and strongly stimulates integrase activity. We have also shown that when we use viruses mutated to lose its interaction with PP2A, infection is severely inhibited. PP2A does not bind DNA but has a wide range of substrates and binding partners that are associated with the human DNA. We hypothesize that the intasome uses PP2A as a bridging factor to bring the integration machinery to its site of integration, i.e. places in the genome that are enriched for substrates of PP2A.

We have recently identified a complex of chromatin-associated proteins that modulate chromatin compaction, influence gene expression and organisation of the cellular chromatin which (indirectly) associates with HTLV-1 integrase. We have shown that the structural integrity of the complex is essential to establish HTLV-1 infection. Here we aim to dissect which components are critical for HTLV-1 infection and integration and characterise the mechanism by which this chromatin-associated complex influences HTLV-1 infection. We also aim to solve the 3D structure of the integration machinery in complex with these host proteins.

Understanding this process will not only increase our understanding in HTLV-1 biology and possible chance of disease progression but will also propel cancer research forward since both PP2A and this chromatin complex are involved in a wide range of human diseases. Finally, identifying the mechanism by which HTLV-1 establishes infection, and structurally characterising the players involved in this process will also aid in the design and development of drugs that can specifically prevent the interaction between integrase and its host.

Human immunodeficiency virus type 1 (HIV-1) and human T-cell lymphotropic virus type 1 (HTLV-1) are the most notorious retroviruses since they are the cause of severe, disabling and sometimes fatal diseases. The most severe HTLV-1 induced diseases are adult T-cell leukaemia (ATL) and the neurological disorder HTLV-1-induced myelopathy/tropical spastic paraparesis (HAM/TSP). So far, there are no therapies for either illnesses. Patients diagnosed with ATL, typically die within two years of presentation.

A critical step in the lifecycle of retroviruses, like HTLV-1, is the integration of a DNA copy of the viral RNA genome into the host DNA. This reaction is catalysed by the viral enzyme integrase (IN). Integration is not random, and we know that integration in certain areas of the genome likely predispose the patients to disease. Our aim is to understand the mechanism of how the integration machinery choses where it will insert its viral DNA.

We identified an important host enzyme, protein phosphatase 2A (PP2A), that specifically binds to HTLV-1 IN and strongly stimulates integrase activity. We showed that viruses expressing mutant INs unable to bind to PP2A are non-infectious. PP2A does not directly associate with DNA but has a wide range of binding partners that do. We identified a chromatin binding complex that likely forms the bridge between the HTLV-1 pre-integration complex - PP2A-B56 holoenzyme and chromatin. Indeed, ablating the structural component of the complex results in significantly reduced HTLV-1 infection. Here we aim to structurally and functionally characterise the role of this complex in HTLV-1 infection. Our findings will elucidate how HTLV-1 establishes infection and could explain how HTLV-1 persists in patients. Our data will also guide the design and development of drugs that can specifically prevent the interaction between IN and its host.