From what I understand, any tumor supressor mutation that is dominant is most likely to be a case of hemi-insufficiency, like the second poster said. any decrease in the levels of the tumor supressor (like 1 allele getting mutated) causes enough decrease in activity that the oncogene it is supposed to be keeping in check starts running wild
of course, there are a few cases I know of where tumor supressor mutations are dominant by another mechanism, such as lynch's syndrome. The mutation is in a tumor supressor gene that repairs DNA bp mismatches, and if you are born with 1 mutated allele, the odds are simply overwhelming that an exogenous or endogenous mutation will lead to a hit on the second allele in at least one of the 10 trillion cells of your body, at which point the mutations in that cell will run wild. So the disease appears dominant--due to the overwhelming odds of a second hit--even though it is really recessive (because until that second hit happens, the one normal allele is sufficient to keep the mutations in check)
please correct me if I'm wrong! : )
