月的基因:圣诞节disease

And finally it's time for our Gene of the Month, which is the gene - or rather the gene fault - responsible for Christmas disease. This was first described in a paper in the British Medical Journal in 1952, and named after the unfortunate child suffering from it - a five-year-old boy called Stephen Christmas, who bled profusely in response to the slightest injury. It's now known to be a rare disease called haemophilia B, and is due to a fault in the gene encoding a protein called Factor IX, which is involved in blood clotting - hence the unstoppable bleeding in people who suffer from it. Factor IX is part of the cascade of chemical reactions that end up chopping up a large molecule called fibrinogen to make a sturdy, insoluble protein called fibrin. This then forms a sticky net at the site of a wound, trapping blood cells and starting the clotting process.

The gene for factor IX is carried on the X chromosome - one of the two sex chromosomes. This means that the disease usually only affects boys - who have one X chromosome carrying the faulty gene and one Y chromosome - as girls have two X chromosomes, so a fault in one can be balanced out by a functional gene on the other.

In 2009, scientists in the US showed that Queen Victoria actually, carried a faulty version of the factor IX gene on one of her X chromosomes, and three generations of boys in the British Royal family were affected by the condition, sometimes known as the 'royal disease'.

Today, Christmas disease is treated with injections of Factor IX produced in genetically modified bacteria, but previously was only treatable through blood transfusions. Sadly for Stephen Christmas, he contracted HIV through one of these transfusions, before they were regularly screened for the virus, and died of AIDS in 1993. So at this time of the year, spare a moment to remember the other meaning of Christmas.