The GM1 strain of feline leukaemia virus (FeLV) was isolated from a naturally occurring case of myeloid leukaemia and induces severe haematopoietic abnormalities, including myeloblastic leukaemia, on inoculation into cats. Molecular clones of FeLV-GM1 proviruses were obtained and studied by restriction enzyme mapping, blot hybridization and partial DNA sequence analysis. Two types of clone were isolated; the first was a replication-competent FeLV of subgroup A, resembling other low or minimally pathogenic FeLV-A isolates; the second was replication-defective with extensive deletions and mutations in and , although it has an intact gene of subgroup B phenotype. Large segments of the defective proviruses, from the 5′ leader sequence upstream of the gene to the 5′ half of the gene, show structural hallmarks of endogenous FeLV-related proviruses. Infectious FeLV-GM1 viruses recovered after transfection were tested for their leukaemogenic potential in newborn cats. Early polyclonal myeloproliferative changes were observed in cats inoculated with FeLV-A/GM1 alone, although these were more pronounced in animals receiving the full FeLV-AB/GM1 complex reconstituted by cotransfection of the defective virus FeLV-B with its FeLV-A helper. Analysis of viruses in the bone marrow showed that replication of the subgroup B component is delayed and restricted to a proportion of cats. Most of the infected cats developed persistent abnormalities of haematopoiesis and one progressed to disseminated myeloid leukaemia. The defective recombinant FeLV-B/GM1 appears to play an indirect but important role in myeloid leukaemogenesis.


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