Astronaut's DNA no longer matches that of his identical twin

You have misunderstood. His dna still matches perfectly. It is the gene expression which has changed. This is something that one would expect in any identical twins who live in very different environments: their phenotypes will diverge somewhat. It is not specifically a space/zero G issue.
Actually, no, @hitmouse - no misunderstanding on my part. The link as shown above was the copy/pasted headline from the original CNN article; the original article was about changes to the astronaut's DNA. In following the link now - some 16 hours after I started the thread - CNN has changed both the headline, and the article (see editorial addendum at the bottom of the story). 'Gene expression' was not the subject of the original article, and I would suspect CNN misrepresented the NASA findings in the original story...were contacted by a NASA representative...and subsequently corrected the article. Hope that clears things up.
Or if you want the shorter, easier version, this makes for a great introduction to the subject of epigenetics:

Either book is worth reading, though - epigenetics is a relatively new and exciting field that is still developing. :)
I mainly use that old maxim:
The phenotype is the product of the genome and the environment.

I would recommend the Richard Dawkins classic The Extended Phenotype.