Teenagers make mutant sperm

A new study claims that teenage dads might be more likely to have babies with birth defects, due to higher rates of mutation in their sperm. Chris Smith asked Peter Forster from Cambridge University about the paper...

Peter - Teenage fathers have a much higher mutation rate than we have thought up to now. You would've thought young fathers have good DNA and therefore, the children should be healthy at least healthier than on average. But what you see is the opposite. It is the teenage fathers who are associated with pregnancies which turn out to have birth defects at an increased percentage.

Chris - What sort of rates are we seeing?

Peter - So, in the USA overall, we see that 1.5% of the births have birth defects. In the teenage father's cases, the birth defects increased by about 30%, so to about 2%.

Chris - Well, what do you think is going on?

Peter - Well, that is a question we didn't directly ask in our current research, but it turns out that we may have found a solution because what we see in our own research is that the teenage fathers have an unexpectedly high number of mutations. It is 6 times higher than the teenage girls and secondly, the fathers are even more highly mutated in their sperm and in the children they produce than the fathers who are 20, 25, 30 years old.

Chris - So, there's a sort of blip. When they first become potentially reproductively active post puberty, their sperm contain more genetic errors, mutations then than they do later...

Peter - I'd say so. It's the case that, to begin with, you have these hump of mutations and we don't know the reason for this yet. But there are two potential explanations. One is that, to produce a sperm cell, you need precursor cells which have to divide and each time it divides, there's the opportunity for an error to creep into the DNA. There might be cell divisions going on long before puberty during boyhood which we don't know anything about and this is causing this accumulation of errors. The alternative explanation is that you have the cells which have been quiet for 10, 15 years have not divided, have not done anything during boyhood. And now, these precursor cells suddenly have to start producing sperm as puberty sets in and they're not quite ready yet. And they have a higher error rate than would normally be the case.

Chris - How did you track this down? How did you find this relationship?

Peter - What we have are our clients who come into our institute who want a paternity test. And so, we generally have the mother, the father, and the child in question. And also, we have asylum seekers who wish reunite their families and therefore, they want their children and their wife tested to prove to the authorities that they are indeed one biological family and have a right to be granted asylum as a whole. It is these clients who, over the past 20 years have generated us with a nice database of 24,000 parents with their children from which we can see how often these mutations happen and how these mutations relate to the ages of the parents.

Chris - How old does a father have to be then in order not to have this increased risk of passing on more genetic changes?

Peter - Well roughly, the best time to be a father, so the minimal risk of having a child with defects is between 20 and 35 years of age. But to put it into perspective, even a 40-year-old father or 15-year-old father will, with more than 97% probability have a healthy child.