Calico Cats Inspire Research Into X Chromosome Inactivation
February 19, 2014

Unlocking The Mysteries Behind X Chromosome Inactivation, A Trait Found In Calico Cats

April Flowers for - Your Universe Online

Calico cats are renowned and beloved for their funky orange and black patchwork or "tortoiseshell" fur. Cat Breeds Encyclopedia reports that these felines — considered to be good luck in several countries — are 90 percent female, suggesting that the color pattern is related to the gender of the animal. New research from the University of California San Francisco (UCSF), has further linked this unique color pattern to X chromosome inactivation or "silencing."

The study, which will be presented at the 58th Annual Biophysical Society Meeting, aims to unlock the mystery of how one X chromosome can be rendered nearly completely inactive.

Female mammals contain two copies of the X chromosome, one from each parent, in their cells. However, since cells only require one active X, the other is "turned off." The gene for orange color fur is on one X chromosome and the gene for black color fur is on the other. The random silencing of one of the X's in each cell creates the distinctive patchwork coat of a calico. Researchers have observed such manifestations of chromosome silencing for quite a while, but how a cell deactivates one chromosome remains a mystery.

To unravel this puzzle, the team first found a way to image the X chromosome in its natural position within an intact cell. "A cell's nucleus contains the genetic code, its DNA. But while the structure of the DNA was determined more than 50 years ago, and we're rapidly determining the position of specific genes on chromosomes, no one had visualized the DNA within an intact nucleus -- an unfixed, hydrated whole cell," explained Elizabeth Smith, a postdoctoral fellow working in Carolyn Larabell's lab in the Anatomy Department at UCSF. "We decided to try."

The development of the new instrument is taking place at the National Center for X-ray Tomography, where Larabell is the director.

The research team anticipates their work will help future scientists better understand how many different kinds of genes can be turned on or off without altering the underlying DNA sequence. "The inactivation of one out of two X chromosomes in females is an enormously important epigenetic process," said Smith. "Uncovering how only one X chromosome is inactivated will help explain the whole process of epigenetic control, meaning the way changes in gene activity can be inherited without changing the DNA code. It can help answer other questions such as if and how traits like obesity can be passed down through generations."

Smith and her team turned to a new imaging technology, soft x-ray tomography, to visualize the DNA within an intact nucleus. "We obtained high-resolution, 3-dimensional views of the intact nucleus and, by using a prototype cryo-fluorescence microscope along with the x-ray microscope, we were able to identify one specific chromosome, the inactive X chromosome of female cells," Smith said.

The researchers imaged and analyzed the inactive X chromosome in a number of different cells. They were surprised by the wide variation in the structural organization adopted by the chromosomes. "We were able to show a remarkable sub-structural organization of this chromosome, which consists of three distinct domains of differing amounts of chromatin," said Smith.

The team developed a new "correlated imaging" technique to obtain their results, which could have a wide range of possible future applications. Specifically, the team sees it being used to identify the position of specific molecules within the densely packed environment of the nucleus. "With new fluorescent probes, we can start identifying the position of specific genes in context -- inside the tangled network of DNA within the intact nucleus," Smith explained.

This work is still at the basic research stage, however, it shows the potential to have a significant impact on understanding, diagnosing, and treating X-chromosome-linked diseases in the future.