No abstract

Adult T-cell leukemia (ATL) is associated with prior infection with human T-cell leukemia virus type 1 (HTLV-1); however, the mechanism by which HTLV-1 causes adult T-cell leukemia has not been fully elucidated. Recently, a functional basic leucine zipper (bZIP) protein coded in the minus strand of HTLV-1 genome (HBZ) was identified. We report here a novel isoform of the HTLV-1 bZIP factor (HBZ), HBZ-SI, identified by means of reverse transcription-PCR (RT-PCR) in conjunction with 5 and 3 rapid amplification of cDNA ends (RACE). HBZ-SI is a 206-amino-acid-long protein and is generated by alternative splicing between part of the HBZ gene and a novel exon located in the 3 long terminal repeat of the HTLV-1 genome. Consequently, these isoforms share >95% amino acid sequence identity, and differ only at their N termini, indicating that HBZ-SI is also a functional protein. Duplex RT-PCR and real-time quantitative RT-PCR analyses showed that the mRNAs of these isoforms were expressed at equivalent levels in all ATL cell samples examined. Nonetheless, we found by Western blotting that the HBZ-SI protein was preferentially expressed in some ATL cell lines examined. A key finding was obtained from the subcellular localization analyses of these isoforms. Despite their high sequence similarity, each isoform was targeted to distinguishable subnuclear structures. These data show the presence of a novel isoform of HBZ in ATL cells, and in addition, shed new light on the possibility that each isoform may play a unique role in distinct regions in the cell nucleus. Adult T-cell leukemia (ATL) is an aggressive and lethal CD4ϩ T-cell malignancy with characteristic nuclear irregularity. Human T-cell leukemia virus type 1 (HTLV-1) is a singlestranded RNA virus belonging to the subfamily Deltaretrovinae and containing reverse transcriptase. The RNA of the retrovirus is transcribed into DNA by reverse transcriptase, and is then inserted into the host genome by an integrase, forming the provirus. Since ATL is associated with prior infection with HTLV-1 (11, 27), although the mechanisms by which tumorigenesis occurs are not fully defined, the viral proteins from HTLV-1 genome have been thought to be essential for the process of leukemogenesis in ATL.The HTLV-1 genome encodes common structural and enzymatic proteins (Gag, Pol and Env) and regulatory and accessory proteins (Tax, Rex, p12 I , p13 II and p30 II ) (13). Among these HTLV-1 viral proteins, Tax protein is considered to play a central role in the early stage of leukemogenesis (8,14,21,24,25,29). However, leukemic cells frequently lack the expression of Tax due to genetic and epigenetic changes of the HTLV-1 provirus (5,23,26), suggesting that while Tax may be a necessary prerequisite for the malignant transformation of infected cells, it is not essential for the maintenance of ATL cells in vivo. In the final stage of leukemogenesis, other continuously expressed viral proteins from the HTLV-1 genome are likely to be involved in the maintenance of ATL cells, because ATL is a uniqu...