Molecular Studies of Human T-Cell Leukemia Virus and Adult T-Cell Leukemia

We describe previously published work as well as new data on the molecular biology of human T-cell leukemia virus (HTLV) and its associated disease, adult T-cell leukemia-lymphoma (ATLL). This specific kind of disease is endemic to certain areas of Japan and the Caribbean, and several isolated cases have been described also in the United States, Israel, South America, and Africa. The disease is probably also endemic to Africa and South America, but sufficient studies of these areas have not been performed. We have molecularly cloned the HTLV genome and used the viral DNA as a probe in a large molecular study of the DNAs of human hematopoietic malignancies. The results showed that HTLV sequences could be detected in the fresh leukemic cells of all cases of ATLL tested. The neoplastic cells are of clonal origin and contain one or few copies of integrated HTLV. Detailed comparative analyses by restriction enzyme mapping of the proviral DNA in U.S., Japanese, Caribbean, and Israeli cases revealed that the viruses are almost indistinguishable. The DNA from neoplastic cells of other cases of hematopoietic neoplasias analyzed were negative for HTLV sequences with the exception of two. DNA from cells of a patient (MO) with a T-cell variant of hairy cell leukemia did contain a provirus only distantly related to HTLV (less than 10% of DNA sequence homology). The virus isolated from the MO cells has been designated HTLV-II. The second case was a patient with chronic myeloid leukemia whose cells contained exogenous DNA sequences distantly related to HTLV. Different fragments of the clones HTLV genome have been used to hybridize to DNA from uninfected normal tissues of several vertebrate species, including humans, in a search for cell-derived sequences related to the HTLV genome. No homologous sequences were found except sequences distantly related to the pol and env genes, indicating that HTLV does not carry a cellularly derived one gene. Surprisingly, however, infection of normal human fresh T cells by HTLV transforms them into cells with permanent growth and with several other properties similar to neoplastic T cells. We have also studied the expression of viral and cellular genes in fresh and cultured neoplastic cells from patients with ATLL. Several species of viral mRNAs are always detected in the cultured neoplastic cells, whereas in some fresh samples expression of normal mRNA was not detected. The lack of viral mRNA in some of the fresh sample suggested that the neoplastic transformation could be maintained without the expression of viral genes. Because T cells of patients with ATLL as well as cells transformed in vitro by HTLV do have a decreased or no requirement for T-cell growth factor (TCGF; IL-2) for growth in vitro and exhibit a high number of TCGF cell surface receptors, we hypothesized that HTLV infection may involve the transcriptional activation of the TCGF gene concomitant with TCGF-induced persistent cell proliferation. However, only low-level expression of the TCGF gene was found in 2 of 5 cell lines and in none of the two fresh samples. Thus autostimulation by TCGF of one cell interacting with its own receptor cannot be the mechanism for initiation or maintenance of the abnormal growth induced by HTLV. We also analyzed the RNAs of the HTLV-infected cells for the expression of various onc genes. None of the onc genes tested seemed to be transcriptionally activated in the fresh leukemic cells, with the exception of the sis gene, which is expressed at high level in two neoplastic cell lines (HUT 102 and MO) infected with HTLV-I and HTLV-II, respectively.