5,6-dihydrouracil and 5,6-dihydroxyuracil - how they arise in DNA?
Dihydrouracil oxidase can catalyse the reduction of uracil to dihydrouracil in the presence of hydrogen peroxide. The reduction of uracil to dihydrouracil can also be catalysed by dihydrouracil dehydrogenase using NADH. 

According to this paper, 5,6-dihydroxycytosine can be formed by treating cytosine or DNA with osmium tetroxide, an extremely strong oxidising agent. Since they are both pyrimidines (and in fact cytosine often spontaneously hydrolyses to uracil), uracil will also be affected by the same oxidation. 5,6-dihydroxyuracil was also detected within the oxidised DNA sample as a result of oxidation and demethylation of thymine.


  The presence of 5,6-dihydroxyuracil (peak 11 in
  Fig. 1) can be explained analogously to the formation
  of 5-hydroxy-5-methylbarbituric acid from thymine on
  permanganate oxidation (lida &amp; Hayatsu, 1970, 1971)
  (Scheme 2). The actual oxidation products ofcytosine, i.e.
  structures (I) and (II), were not observed in DNA. As the
  calf thymus DNA used in this work did not contain uracil,
  the uracil derivatives discussed above could not have been
  formed from uracil in DNA.