How a LHON gene is inherited differently by different parts of the body
A LHON Gene being copied different amounts in different parts of the body as a fertilised egg develops into a full-sized body
This diagram shows a fertilised egg dividing, eventually to form every cell in the body.
When a gene is on one of the chromosomes in the nucleus, the fertilised egg has two copies of it. However, when a gene is in the mitochondria, there are thousands of
copies of it. Each copy might be the 'normal' version, or might be the LHON mutation.
We all start life as one cell - the fertilised egg. This contains thousands of mitochondria, each with their own genes.
When this cell divides to form two, the mitochondria in the cell divide as well. If this did not happen, every new cell would have fewer mitochondria than its parent.
When the cell divides, some mitochondria go one way, and some go the other. The new cells are not guaranteed to have the same mix of mitochondria as their parent. The mix of mitochondria each cell gets is a random chance.
The fate of a cell is decided very early in the life of an embryo. That is, a few cells in the tiny growing embryo will eventually produce all the cells in one part of the body, or one organ. This means that, if these few cells get a large number of mitochondria with LHON affected genes, then the part of the body they become will have mostly affected mitochondria.
There is a theory that one factor which decides who will have symptoms and who will not is this random distribution of affected mitochondria in the developing embryo. If the cells which will eventually form the optic nerves and their blood vessels get mostly
LHON-affected mitochondria, then that person is likely to develop symptoms.
