Bokhoven, M.C.; (2008) Measurement of insertional mutagenesis by retroviral and lentiviral vectors. Doctoral thesis , University of London.

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Abstract

Retroviral vectors have been successfully used in the clinic for the correction of inherited immunodeficiencies. However, in 2 recent gene therapy trials for X-linked Severe Combined Immunodeficiency, 5 patients developed leukaemia. This was associated with vector integration near cellular proto-oncogenes, leading to their activation a process known as insertional mutagenesis. This thesis describes the development of a cell line assay that allows us to quantify insertional mutagenesis by retroviral and lentiviral vectors and to analyse the mechanisms by which this occurs. The interleukin-3 (IL-3) dependent cell line BCL15 is derived from the mouse bone marrow cell line BAF3 and over- expresses human Bcl2. The frequency at which IL-3 independent mutants are obtained following vector transduction is measured. Lentiviral and retroviral vectors transform BCL15 cells at similar integrant frequencies of 4.3 x 108 and 1.2 x 107, respectively. However, they cause insertional mutagenesis in this assay by different mechanisms. The human immunodeficiency virus-1 (HIV-1)- derived lentiviral vector HV transforms BCL15 cells by insertional activation of the growth hormone receptor (Ghr) gene. An HIV-Ghr fusion transcript was detected. It originates from the HIV-1 5'-LTR it then splices from the HIV-1 major splice donor to the splice acceptor of Ghr exon 2. The mutants express GHR and grow in response to bovine growth hormone in the foetal calf serum of the culture medium. Deletion of the HIV-1 enhancer/promoter in a self-inactivating vector prevents this mechanism of transformation. Retroviral vector transformation of the BCL15 cell line does not occur via Ghr gene activation. Retroviral vectors up- regulate expression of the cytokine IL-3, either by insertion into the IL-3 gene or by insertion into other genes, which may act as upstream activators of IL-3 expression. This assay is a general method to quantitate insertional mutagenesis. It may inform the design of safer vectors and can be used in initial safety testing of (pre-) clinical vectors.

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