Question:
I am trying to find out whether Juvenile Diabetes tends to run in families
and/or is hereditary. If one family member has Juvenile Diabetes, what is
the percentage that her siblings will develop type 2 (adult diabetes), and
that their children will have Juvenile Diabetes. Thanks for all the help I
can get.
Answer:
The genetics of diabetes are very complex, but before
I delve into that, you need to be aware that the terminology
has changed. What used to be called Juvenile Diabetes
is now called type 1. What used to be called Adult Onset
diabetes is now called type 2, as you note. The reason
for no longer using the terms Juvenile and Adult Onset
is that both Juvenile and Adult Onset Diabetes occur
throughout the life span. Thus, the old terms are misleading.
Type 1 and type 2 are completely different conditions which
share a common clinical end result, elevated blood glucose
levels. Actually type 2 and 1 are really families of diseases,
but we'll leave that complication out for simplicity's sake.
Yes, diabetes of both types can run in families and there
is a genetic component to the diseases which may, or may not, be
inherited. On the other hand, the majority of diabetics,
of either type, do not have a first degree relative with
the disease. The genetic predisposition to diabetes is
fairly easy to come by. In both cases, there are probably
many contributing genetic locations and the probability
you will get the disease depends on which subset of the
locations that have the disposing characteristic a particular
individual winds up possessing. There also appear to be
some genes which protect from the disease. The general
picture of the onset of diabetes is that you have a
genetic predisposition which is then triggered by some
environmental factor. Beyond this very general picture
the two types start becoming distinct.
Here are a couple general emperic risk factors that
will answer some of your questions and give you
an idea of how confusing the genetics are.
The risk of developing type 1 in the general population is
0.4%. If you have a sibling or a parent with type 1,
your risk is 5%. If that parent is your father, your
risk is 6.1%. If your mother, 2%. If your mother was
younger than 25 when you were born, 3.6%, If older,
1.1%. If your identical twin has type 1, 30-50%.
If your fraternal twin, 5%. I have never seen data
on increased risk as the result of having an aunt or uncle
with type 1, if I correctly interpreted your question.
The probability that an individual with a type 1
sibling will develop type 2 diabetes is the same
as in the general population, since they are
independent conditions. The estimates of the
risk of type 2 vary quite a bit for a couple reasons.
Type 2 incidence rises very steeply with age and
so any age cut off in your sample has a dramatic
effect on the cumulative incidence. Rates also
vary widely depending on the details of the population
you look at. Lastly, much of the type 2 diabetes
remains undiagnosed, so your calculated rate depends heavily on
your estimate of the undiagnosed population.
In round numbers, the risk of being diagnosed with type 2
diabetes during a 70 year life is 10%. The risk of
contracting type 2 is close to twice that.
