Exploring the contribution of distal P4 promoter elements to the oncoselectivity of Minute Virus of Mice.

Minute Virus of Mice (MVM) shares inherent oncotropic properties with other members of the genus Parvovirus. Two elements responsible, at least in part, for this oncoselectivity have been mapped to an Ets1 binding site adjacent to the P4 TATA box of the initiating promoter, P4, and to a more distal cyclic AMP responsive element (CRE), located within the telomeric hairpin stem. Here the CRE overlaps one half-site for the binding of parvoviral initiation factor (PIF), which is essential for viral DNA replication. We used a degenerate oligonucleotide selection approach to show that CRE binding protein (CREB) selects the sequence ACGTCAC within this context, rather than its more generally accepted palindromic TGACGTCA recognition site. We have developed strategies for manipulating these sequences directly within the left-end palindrome of the MVM infectious clone and used them to clone mutants whose CRE either matches the symmetric consensus sequence or is scrambled, or in which the PIF binding site is incrementally weakened with respect to the CRE. The panel of mutants were tested for fitness relative to wildtype in normal murine fibroblasts A9 or transformed human fibroblasts 324 K, through multiple rounds of growth in co-infected cultures, using a differential real-time quantitative PCR assay. We confirmed that inactivating the CRE substantially abrogates oncoselectivity, but found that improving its fit to the palindromic consensus is somewhat debilitating in either cell type. We also confirmed that reducing the PIF half-site spacing by one basepair enhances oncoselectivity, but found that a further basepair deletion significantly reduces this effect.