Genetic clock ticks for men

Genetic clock ticks for men Les Sheffield

June 12, 2008 12:00am
MOST men would have been surprised to read that overseas researchers had found the death rate of young adults was higher if they had been born to older fathers.

This is no surprise to me. It has been scientifically established that genetic changes occur more often in the sperm of older fathers than younger fathers.

As men age there is a higher chance of changes in the genes in the sperm.

These changes can cause genetic conditions in their offspring, such as birth defects, autism and schizophrenia.

Their partners can also have an increased risk of miscarriages.

The presumed reason for the increase occurrence of all of these conditions is that they are all due to a new genetic change in the sperm of the older father.

Genetic changes are occurring all the time. Sometimes they have a beneficial effect, such as making the individual stronger, taller or smarter.

This is part of the concept of "survival of the fittest".

Sometimes, when the gene change is in a non-coding part of the genome, they have no effect. At other times, they can be harmful.

The problem is that these harmful effects are extremely varied because they can affect any one of the 20,000 or so human genes.

For example, they often change the structure of the body. One example is dwarfism, where the arms and legs are short due to a genetic change. The commonest type of dwarfism is achondroplasia.

An individual with this condition will have a 50 per cent risk of having an affected child themselves.

Indeed, about 20 per cent of the parents of achondroplastic babies have one of the parents with this condition, but the remaining 80 per cent do not.

If you look at the parents of babies with achondroplasia, who do not have the condition themselves, you find their average age is older than other people having babies in the population.

Significantly, statistics show it is the father's age which is important and not the mother's.

Achondroplasia is rare and it is only one of the many genes that can go wrong. Collectively, any of the 20,000 genes can change and this causes an increase in risk from about the age of 40.

The risk in men for any single gene change is one in 200 at age of 40, 20 at age 50 and rises steeply after that.

This increase in risk with paternal age is no surprise to me, but it is a surprise to practically everyone else.

The increase risk for older mothers for Down syndrome is well-known.

As part of my work as a clinical geneticist, I see couples every week who come to ask about the risk of having babies because of the age of the mother.

We talk about this and often, as the male partner is also older, we talk about the risk of his age. Most of the partners are quite surprised and even taken aback with this news.

In today's society, delaying pregnancy until later is often done for career and other purposes but usually only the age of the mother is taken into account in planning when to start a family. Why is the increased risk in relation to a father's age not widely known?

There are many possible reasons. Some of the information - such as increased death rates of adults - is new.

But information about single gene changes, such as achondroplasia, has been around for many years.

I think the real reason for the lack of knowledge is the conditions that can be caused are varied and can't really be prevented by a screening program like the one offered for Down syndrome.

In fact, most of the conditions, such as achondroplasia, can't even be picked up by the normal ultrasound scan for abnormalities done at 18-20 weeks of a pregnancy.

So, if you're a male, the only way not to be exposed to this increased risk of genetic defects in your offspring is to plan your children early and regard the increasing risks of the woman in her late 30s and early 40s as also applying to you.

In other words, stop your child bearing at the same sort of age that women stop child bearing. This may not be what older men want to hear, but they need to seek information about what the risks actually are before making child-bearing decisions.

We hear about the positive sides of parenthood in some older celebrity fathers but the story last week about the increase in death rates of the offspring brings out the hidden risks associated with fathering children at an older age.

Associate Professor Les Sheffield is a clinical geneticist with the Victorian Clinical Genetics Services