Human retroviruses and paediatric disease.

Retroviruses are a distinct group of RNA viruses which replicate via a double stranded DNA intermediate, synthesised from the virion by an RNA dependent DNA polymerase (reverse transcriptase). Thus unlike other viruses, exogenous RNA codes for DNA. The intermediate, known as the provirus, is integrated into host cell DNA and acts as a template for the production of viral genomic and messenger RNA by the host cell. Retroviruses are known to cause a wide range of diseases in numerous animals. (Table). In spite of an intensive search for similar viruses in man the first human retrovirus was not reported until 1980. The human T cell leukaemia/lymphoma virus type I (HTLV-1) has since been shown to be the causal agent of the adult T cell leukaemia syndrome, and more recently to be closely linked to tropical spastic paraparesis. ' A second human retrovirus HTLV-2 is similar to HTLV-1 but has yet to be associated with disease. The third human retrovirus previously known as lymphadenopathy virus (LAV-1), or human T cell lymphotropic virus type 3 (HTLV-3), and now collectively referred to as human immunodeficiency virus (HIV), is the causative agent of the acquired immune deficiency syndrome (AIDS) and its related disorders. All three viruses have animal counterparts. Retroviruses are divided into three groups: oncoviruses, lentiviruses, and spumaviruses. Oncoviruses are associated with transformation either by transporting an 'oncogene' into the host cell, or by inserting themselves next to cellular homologies of oncogenes and 'turning them on' by promotor activity. HTLV-1 and HTLV-II, although capable of causing transformation, do so by an alternative method by activating genes not necessarily next to them but at a