Hyper Human Mutation: 60 New Mutations in Each Genome

Human Genome Stained by FISH Each human walking this planet receives approximately 60 new mutations in our genome from our parents, according to the first-ever direct measure of new mutations coming from mother and father in whole human genomes.

The researchers have been able to answer the questions: how many new mutations does a child have and did most of them come from mother or father? The researchers measured directly the numbers of mutations in two families, using whole genome sequences from the 1000 Genomes Project. The results reveal that human genomes, like all genomes, are changed by the forces of mutation: our DNA is altered by differences in its code from that of our parents. Mutations that occur in sperm or egg cells will be 'new' mutations not seen in our parents.

New mutations are the ultimate source from which new variation is drawn. But finding new mutations is extremely technically challenging as, on average, only 1 in every 100 million letters of DNA is altered each generation.

Previous measures of the mutation rate in humans has either averaged across both sexes or measured over several generations. There has been no measure of the new mutations passed from a specific parent to a child among multiple individuals or families.

"We human geneticists have theorised that mutation rates might be different between the sexes or between people," explains Dr Matt Hurles, Senior Group Leader at the Wellcome Trust Sanger Institute, who co-led the study with scientists at Montreal and Boston, "We know now that, in some families, most mutations might arise from the mother, in others most will arise from the father. This is a surprise: many people expected that in all families most mutations would come from the father, due to the additional number of times that the genome needs to be copied to make a sperm, as opposed to an egg."

 "Today, we have been able to test previous theories through new developments in experimental technologies and our analytical algorithms," added Professor Philip Awadalla,who also co-led the project and is at University of Montreal. "This has allowed us to find these new mutations, which are like very small needles in a very large haystack."

The unexpected findings came from a careful study of two families consisting of both parents and one child. The researchers looked for new mutations present in the DNA from the children that were absent from their parents' genomes. They looked at almost 6000 possible mutations in the genome sequences.

They sorted the mutations into those that occurred during the production of sperm or eggs of the parents and those that may have occurred during the life of the child: it is the mutation rate in sperm or eggs that is important in evolution. Remarkably, in one family 92 per cent of the mutations derived from the father, whereas in the other family only 36 per cent were from the father.

This fascinating result had not been anticipated, and it raises as many questions as it answers. In each case, the team looked at a single child and so cannot tell from this first study whether the variation in numbers of new mutations is the result of differences in mutation processes between parents, or differences between individual sperm and eggs within a parent.

Using the new techniques and algorithms, the team can look at more families to answer these new riddles, and address such issues as the impact of parental age and different environment exposures on rates of new mutations, which might concern any would-be parent.

The number of mutations passed on from a parent to a child varied between parents by as much as tenfold. A person with a high natural mutation rate might be at greater risk of misdiagnosis of a genetic disease because the samples used for diagnosis might contain mutations that are not present in other cells in their body: most of their cells would be unaffected.

In another, earlier study of human mutation rate, a global team of researchers recruited two men from a remote Chinese village.  The subjects' family had lived their for over two centuries, leaving the men extremely distant relations (thirteen generations apart).  By examining sections of their Y chromosomes and screening out mutations caused by the scientific work, the team worked out the rate of change of human DNA.

One in thirty million nucleotides mutate per generation – which doesn't sound much until you realise you have twelve billion of the things.  You've got a couple of hundred errors in your cellular scripting, but luckily most don't do any damage.  And not all of these changes are errors.  The vast majority cause, "nothing", a minority cause "death", but an extremely special few can be beneficial.  Scale up over entire species and hundreds of millions of years and you can end up with almost anything. 

The discovery of this base mutation rate is an important discovery for the vital field of genetics. By comparing mutation rates between locations or even individuals, we can decode much more about our sicknesses, or our future.  It could be used to molecularly date organic remains, and don't forget that the little problem of cancer is all about mutation.  Now we've timed the ticking of the mutation clock, we'll see if we can stop or reset it.

The Daily Galaxy via Wellcome Trust Sanger Institute

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