As to the genetics part, its why there's rigorous screening to ensure candidates are physically compatible with the implants. The Canis Helix is also unique to the Space Wolves, and does a nice job killing off non-Fenrisians, for what are obvious reasons unless you're foolish enough to think there's wolves on Fenris. Because of that I would say it has more to do with the genetic changes inherent to Fenrisians, working to both stabilize and accelerate them as part of the creation process. Really, the Space Wolves are a type of Astartes all to themselves since no other Chapter is bound so tightly to a single world, both culturally and in terms of recruitment base.
(I could go on and on about how the Space Wolves are literally just Space Vikings, complete with Viking social dynamics between them and their serfs. You don't maintain over eight battle barges without a ridiculous amount of support infrastructure, which all means people to man that, feed them, etc. There simply has to be an entire lower class doing the jobs of Viking Freemen to support the Spess Wolves and everything else around them, unless you think Blood Claws can be trusted to go out and hunt some Asaheim elk for dinner without keeping it all for themselves. Hell, someone has to brew all that mjod.)
For the tissue and hormones, they're not normal human ones, but artificial ones designed to do things to the human body it was never meant to do. Its entirely foreign tissue, which is why outright rejection is so high even with the screening.
Christ, this shit's getting too autistic even for me, so I'll just cut things off there.
