By using tight binding Hamiltonian approach, the medium's effect on charge transport properties of modified poly(dA)-poly(dT) DNA molecular wire is studied. The DNA sequences used are modified poly(dA)-poly(dT) sequence by replacing 50 base pairs randomly with GC or CG base pairs. The DNA is contacted by metallic leads at both ends. The medium's effect on DNA is modeled as backbone onsite energy disorder. The DNA is taken to be at room temperature. The charge transport properties of the two sequences are studied by calculating the transmission probabilities of the charges using the scattering and transfer matrix methods simultaneously. Then the current-voltage (I-V) characteristics are calculated from the transmission probabilities using the Landauer-Büttiker Formalism. The theoretical differential conductance curve is then calculated from the I-V characteristic curve. The I-V results show that as the backbone disorder strength increases, the maximum current decreases for both sequences. However, the threshold voltage can increase or decrease with increasing backbone disorder depending on the sequence. The transmission results show that the position of transmission peaks changed with backbone disorder strength. Comparison with previous published results using poly(dG)-poly(dC) sequence shows that in contrast to that sequence, backbone disorder can decrease the threshold voltage in one of the sequences studied. This may point to a new understanding of the interplay between environmental and sequence disorder effect on charge transport in DNA.