Hunting for your child's DNA doppelganger

5 minute read

Maybe you believe you have an identical twin somewhere. Or if not a twin, at least someone who looks a lot like you, a doppelganger. Someone who looks like you, sure. But maybe also someone whose life has curiously paralleled your own.

There are parents who would like nothing more than such a doppelganger for their children. Amy Harmon reported last month on some of them -- parents of children with rare novel mutations.

Every person is born with a handful of new deleterious mutations. Chances are you will never notice yours. Their effects may be small, and many are recessive, needing two copies to show their bad effects -- and of course, since they're new, you only have one copy. If you have children, you will pass half your new mutations to each of them, and they will have their own new ones.

Such new mutations do not cause a large proportion of the recognized Mendelian genetic disorders. The most common disorders are caused by mutations that have been passed down in families over many generations. Some, like the X-linked hemophilia inherited by the descendants of Queen Victoria, have been recognized from pedigrees of relatives. A few are much older. For example, nearly all cases of variegate porphyria in South Africa result from an allele that arrived in Cape Town in 1688. Three hundred years of Afrikaner porphyria have been tracked through pedigrees to one woman, Ariaantje Adriaanse, who carried the mutation.

If your child has a well-known Mendelian disorder, chances are you can find a support group -- possibly locally, but certainly on the internet. These support groups have become very important resources, with everything ranging from dietary advice, results with distinct combinations of treatments, to forums for commiserating about other people's reactions to their kids' unique needs. Some of the recognized Mendelian disorders are often caused by new mutations: for example, half of all cases of neurofibromotosis (roughly 1 in 10,000 people) result from new mutations.

But there are a large set of new mutations that only occur in a tiny fraction of births. Many have only recently been discovered, as genome-wide screens for variation have become possible. Harmon's report focuses on a few of these kids.

A decade ago, these kinds of mutations were unrecognizable, and kids were lumped into broad categories like "developmentally disabled." The most obvious such category is "autism spectrum," which as the name "spectrum" implies, includes a variety of developmental challenges and a range of outcomes. As Harmon's article points out, geneticists are slowly unraveling the different causes of autism. A small fraction of cases result from these rare new mutations, such as 16p11.2.

These genetic results have started to allow families to find other children who are genetically and phenotypically similar to their own. Even if treatments or interventions are not available, this can give parents some idea about the course of their childrens' future development. That's a step forward for some families who can't find their ground easily in the large pool of children on the autism spectrum. What things may be effective with some developmental pathways may have no effect at all on others.

For three families, the impulse to find others in the same situation was immediate.
A few months before the Lanes crossed the state to meet Taygen's chromosomal cousin, Jennie Dopp, a mother in Utah, was scouring the Internet for families with "7q11.23," the diagnosis that explained her son's odd behavior and halting speech.
"I want someone to say `I know what you mean,'" Ms. Dopp told her husband, "and really mean it."
Noa Ospenson's parents flew from Boston to South Carolina for a meeting of 100 families with children who, like Noa, are also "22q13." Hoping for more information about their daughter's diagnosis, they emerged as lifetime members of what they call "Noa's tribe."
For each of them, a genetic mutation became the foundation for a new form of kinship.

This formulation of kinship is more real than many that anthropologists study, because here there is a real connection between gene sequences, although not a "genetic" one in the traditional sense of origins. But there are many forms of analogical kinship in human societies. It is natural, perhaps, the extent to which we are adopting new gene-centered forms of kinship -- from adoptees using genetics to search for their biological relatives, to genealogy buffs sending their DNA to find their distant genealogy buff kin.

For the families described in this story, it is really a case of trying to find a lost part of their own child's biological story. The very things that make their children different from their kin may make them similar to someone else's children. In many cases, finding such connections can be an enormous relief.

The results are not always happy, though:

And then they went to the biennial meeting of 22q13 families in July 2006. But that first day, in Greenville, S.C., they wondered if they had made a mistake.
Few of the children, even the handful of teenagers, were toilet trained. Some had never gained the use of their hands, which had stiffened into a claw-like shape. Many were chewing on rubber tubes or "chew rags," to keep them from shredding their clothes.
Ms. Perlson, a communications consultant, and Mr. Ospenson, a computer analyst, attended sessions on one of the genes that Noa is missing, which codes for a protein crucial to neurological development. They learned about the health problems, like seizures and kidney failure, that Noa might face in her 20s. The window onto her future was hard to digest.

It's a good article, following a number of these stories. I think it's an important view of today's human genetics -- not only are we increasing our knowledge of the origins of common mutations, but we are also increasing the number of rare ones that we know about. Ten years ago, few imagined that these structural gene variants could be an important element of human variation. Now we know that insertions and deletions of genes may account for an important fraction of phenotypic variation in humans.