A Family’s Shared Defect Sheds Light on the Human Genome

A Family’s Shared Defect Sheds Light on the Human Genome
By NATALIE ANGIER
Jan 9 2017
https://www.nytimes.com/2017/01/09/science/dna-tads.html

They said it was their family curse: a rare congenital deformity called syndactyly, in which the thumb and index finger are fused together on one or both hands. Ten members of the extended clan were affected, and with each new birth, they told Stefan Mundlos of the Max Planck Institute for Molecular Genetics, the first question was always: “How are the baby’s hands? Are they normal?”

Afflicted relatives described feeling like outcasts in their village, convinced that their “strange fingers” repulsed everybody they knew — including their unaffected kin. “One woman told me that she never received a hug from her father,” Dr. Mundlos said. “He avoided her.”

The family, under promise of anonymity, is taking part in a study by Dr. Mundlos and his colleagues of the origin and development of limb malformations. And while the researchers cannot yet offer a way to prevent syndactyly, or to entirely correct it through surgery, Dr. Mundlos has sought to replace the notion of a family curse with “a rational answer for their condition,” he said — and maybe a touch of pioneers’ pride.

The scientists have traced the family’s limb anomaly to a novel class of genetic defects unlike any seen before, a finding with profound implications for understanding a raft of heretofore mysterious diseases.

The mutations affect a newly discovered design feature of the DNA molecule called topologically associating domains, or TADs. It turns out that the vast informational expanse of the genome is divvied up into a series of manageable, parochial and law-abiding neighborhoods with strict nucleic partitions between them — each one a TAD.

Breach a TAD barrier, and you end up with the molecular equivalent of that famous final scene in Mel Brooks’s comedy, “Blazing Saddles,” when the cowboy actors from one movie set burst through a wall and onto the rehearsal stage of a campy Fred Astaire-style musical. Soon fists, top hats and cream pies are flying.

By studying TADs, researchers hope to better fathom the deep structure of the human genome, in real time and three dimensions, and to determine how a quivering, mucilaginous string of some three-billion chemical subunits that would measure more than six-feet long if stretched out nonetheless can be coiled and compressed down to four-10,000ths of an inch, the width of a cell nucleus — and still keep its operational wits about it.

“DNA is a super-long molecule packed into a very small space, and it’s clear that it’s not packed randomly,” Dr. Mundlos said. “It follows a very intricate and controlled packing mechanism, and TADs are a major part of the folding protocol.”

For much of the past 50 years, genetic research has focused on DNA as a kind of computer code, a sequence of genetic “letters” that inscribe instructions for piecing together amino acids into proteins, which in turn do the work of keeping us alive.

[snip]

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