The biggest problem I see with the MODY-2 diagnosis is that too many doctors buy into the idea that they can ignore MODY-2 because it doesn't cause the classic diabetic complications. And they tell patients that they can't lower blood sugar, so why bother.
But what I have heard from the people with MODY-2 diagnoses who have replied to my data collection request (on my web site) is that cutting carbs DOES lower their fasting blood sugar, even though they were told it won't.
And I am also hearing that their families get those early onset fatal heart attacks, like the ones that run in my family. (I don't have MODY-2 but probably MODY-3 or -1 since my insulin sensitive diabetes responds dramatically to tiny doses of Sulf.) Since we know that heart disease starts ramping up with exposure to blood sugars over 155 mg/dl, its quite understandable that MODY-2 would lead to heart disease untreated.
But I urge people with that diagnosis NOT to accept the argument "There's nothing you can do about it" because there is. Find a doctor who will work with you to lower your post-meal sugars and do keep those carbs down. It will help!
My observations, which are anecdotal but might be worth considering suggest that children exposed to higher than normal blood sugars in mid-100s develop very strong intellects. All the bright people in our family seem to have the gene. So perhaps that's the trade off.
And if you commit to tight control you can live a very long life. My dad made it to age 100 with my gene (and heart disease) through controlling his diet extremely tightly.