One Person – Several Genomes

You probably think you possess a single genome – one set of genes.  Mom meets Dad, sperm penetrates egg, nine months later you appear.  And your life is marked by a single set of genetic instructions that let you enjoy cartoons and  ten food commercials every day,  master calculus and before you realize what you’re doing start the whole process all over.

Guess again.

Excellent science writer Carl Zimmer in his NY Times article “DNA Double Take” explains that another scientific dogma is now writhing in the dust – though most scientists and medics have yet to notice the dying patient.  Let’s talk about gender.  In one recent study Canadian pathologists doing autopsies on the brains of 59 women found Y chromosomes present in 63% of those silent cerebri. Talk of raging hormonal imbalances!  Another study at DNA Farber Cancer found 56% of women had Y chromosomes lurking about– in sample biopsies from their breasts.

The majority of women are thus showing up with male cells. And things are sometimes far more extreme.  You might  discover that you’re not the mother of your own children – even though they were conceived and carried in your womb.

The Mother Who Couldn’t Be

Not very long ago a woman came in to hospital with chronic kidney disease.   She requested to have a kidney transplant.  At age 52, the standard procedures were performed – including DNA testing.

Much of the time donors for kidneys are relatives.  Offspring and siblings are a prime consideration.  Her children were DNA tested.

The patient was told two out of three kids could not be hers.  They did not have her genes.

This was potentially bad news as far as transplants were concerned.  But it was a greater shock to the mother.

She was fully aware that neither of the two kids were immaculately conceived.  Nor was she a surrogate mother.  That pair of children had been conceived in her womb and stayed there until birth.

They were her children, alright.

And despite the tests, they were.

She was a chimera.  Some 53 years ago two eggs had been fertilized simultaneously in her mother’s uterus.  And those two eggs had fused.  The result had been her – a woman possessing two complete genomes.

One genome had gone into a single child. Another genome had been used to create the eggs of her other two kids.

Chimeras are now created routinely in research labs and IVF facilities.  But such mutations also take place in humans – all the time.  And the cells that result are now genetically different.

Don’t Trust Appearances

It’s been known for a long time mutations cause many tumors.  They’re particularly prominent in cells – like those of the gut lining – that reproduce themselves quickly.  Lots of new cells means lots of possibilities for mutation.

But mutations still occur in cells that don’t reproduce much.

Researchers are now finding loads of mutations in functioning human cells.  Some have turned out to be cancerous.  But many are not.

Some are missing big chunks of DNA.  Or they have multiple copies of different genes.

And on autopsy and pathology they usually look the same – under the microscope.  But genetically they’re really different.

So parts of you may be male as well as female.  Parts of one organ may be made up of cells with thoroughly different instructions than the others – even though they’re all “spleen” or “liver” cells.  And some of you may even possess totally separate chromosomes.

Which produces some pretty wild implications:

The Other Human Inside You

1. Immunity is more open – even more ecumenical – than we thought.

If the majority of women have elements of a Y chromosome in breast and brain tissue, immune cells seem to be perfectly happy with very foreign interlopers in their midst.  Which may explain how we’ve come to  live together with hundreds of trillions of viruses, bacteria, fungi, rickettsia, prions, mycoplasma and who knows what else – which may include hundreds of different tumor cells that came and go every day.

2. DNA changes all the time.  And many of the mutations which might have been thought to provoke severe immune reaction seem to survive.  They change us in ways that have not yet been looked at carefully – or researched at at all.

And, yes, large parts of human DNA come from viruses and bacteria. Eight to twelve percent of human DNA comes from retroviruses, the viral group that includes AIDS.  For a long time that DNA has been thought to be “just” basic housekeeping stuff.   You know, cell physiology, membrane proteins, that sort of thing?

Wrong.  Our DNA is probably far more open to change  – and epigenetic control – than we’ve given it credit for.  When you start to realize that toxoplasma infections of mice – even after they’re eradicated – can turn mice instinctually terrified of cats into mice unthreatened, even friendly to felines, you start to see how much human information might be manipulated by other organisms.

3. Humans are a system of information that never stands still.  Our inner information – from proteins to DNA – is constantly modified and remade.

And much of those modifications occur by chance.

So it’s time to think of yourself differently.  You’re a living system of information, powered by chance.  The information instructions, down to your “basic” genes, change all the time.

Including the time it took to read this sentence.  Chance changes make and break you.

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