Researchers at Johns Hopkins have color-coded cells in female mice to display which of their two X chromosomes has been "silenced."
Scientists have long known that the silencing of one X chromosome in females — who have two X chromosomes in every cell — is a normal occurrence whose consequences can be significant, especially if one X chromosome carries a normal copy of a gene and the other X chromosome carries a mutated copy.
By genetically tagging different X chromosomes with genes that code for red or green fluorescent proteins, scientists say they can now peer into different tissue types to analyze genetic diversity within and between individual females at a new level of detail.
Published on Jan. 8 in the journal Neuron
, a summary of the research shows wide-ranging variation in patterns of so-called X chromosome inactivation at every level: within tissues, on the left or right sides of a centrally located tissue (like the brain), among different tissue types, between paired organs (like the eyes) and among individuals.
"Calico cats, which are only ever female, have mottled coat colors. They have two different versions of a gene for coat color, which is located on the X chromosome: one version from their mother and the other from their father," explains Jeremy Nathans, M.D., Ph.D., professor of molecular biology and genetics at the Johns Hopkins University and a Howard Hughes Medical Institute investigator. "Their fur is orange or black depending on which X chromosome is silenced in a particular patch of skin cells. X chromosome inactivation actually occurs in all cells in female mammals, including humans, and it affects most of the genes on the X chromosome. Although this phenomenon has been known for over 50 years, it couldn't be clearly visualized in internal organs and tissues until now."
Nathans adds that early in the development of most mammals, when a female embryo has only about 1,000 cells, each cell makes a "decision" to inactivate one of the two X chromosomes, a process that silences most of the genes on that chromosome. The choice of which X chromosome to inactivate appears to be random, but when those cells divide, their descendants maintain that initial decision.
In the new research, the Johns Hopkins team mated female mice carrying two copies of the gene for green fluorescent protein — one on each of the two X chromosomes — with male mice whose single X chromosome carried the gene for a red fluorescent protein. The female offspring from this mating had cells that glowed red or green based on which X chromosome was silenced. Additionally, the team engineered the mice so that not all of their cells were color-coded, since that would make it hard to distinguish one cell from another. Instead, they designed a system that allowed a single cell type in each mouse, such as heart muscle cells, to be color-coded. Their genetic trick resulted in red and green color maps with distinctive patterns for each cell and tissue type that they examined.