Christmas disease, until now, has been considered incurable – a disease for life.
In the healthy, cut fingers and bruised blood vessels release chemicals, which then start chain reactions. These chemicals activate the first steps in sequences of protein machinery, with each then activating more in turn. Many amplified steps later, the last protein works to form the fibres of a clot and stem bleeding.
But what if a link in this chain is missing or broken? In Christmas disease, the critical step provided by the enigmatically named protein Factor IX is lacking. A Christmas disease patient still has the right signals to start clotting, but lacks one of the protein messengers needed to complete the chain. As a result, a tiny nick or knock can cause dangerous bleeding.
Christmas disease carries the name of the first known patient: Canadian Stephen Christmas. Like all others affected, Stephen bled far too easily. To reduce the risk of a dangerous bleed, he needed injections of Factor IX gathered from the blood of others. The missing protein needed replacing several times per week and despite this, each day there was the risk that he might still bleed. This could be a stroke if the bleeding was in his brain or, more likely, but agonisingly, into his joints after a tiny knock.
But would it not be better if, instead of replacing the protein, the source of the protein could be replaced? This is indeed what a team from London have recently achieved. Their discovery came too late for Stephen Christmas, who died in 1993 from HIV transmitted by one of his many injections, but gives hope to many others who suffer the disease carrying his name.
Each protein is manufactured by the cells of the body according to the many blueprints of the genes in our DNA. Most often, one gene creates one protein. In Christmas disease, the genetic blueprint for Factor IX has a mistake and the protein it makes does not form properly. So why not replace the gene?
This is harder than it sounds. Imagine sneaking a few extra words into a manuscript that is repeatedly being copied out. And then this needs to be on a microscopic scale, thousands of times at once. And then it must escape the sentinel that is the immune system, which tries to destroy foreign genetic material.
The solution came through hijacking an ape virus and taking advantage of its unique properties. The Trojan horse in question – adeno-associated virus 2 – more commonly affects chimpanzees and gorillas but has several key points in its favour. First, the virus itself causes little illness: it is no good if the cure is worse than the disease. Second, because the virus homes in on liver cells, it naturally ends up in a part of the body only lightly patrolled by the immune system, even when injected in the back of a hand. A header of genetic code was also added to that for Factor IX. This header was only ever read – and translated into active protein – in the liver and so this ensures that the new Factor IX is only produced there too. Third, the virus is known to be effective at keeping its genetic code within the cells it infects and hijacks the protein-making machinery. In this way, when the body makes proteins according to its original code, it makes the virus’ proteins too.
So, the genetic code for Factor IX was first inserted into adeno-associated virus 2 and the new virus injected into volunteers. At first however, only a little Factor IX was produced. It seemed that body was recognising and attacking the virus and that the new code was not integrating well enough.
Two more cunning tweaks were required. A second Trojan horse was employed: virus 2 was put into the outer coat of its cousin, virus 8. Fewer human immune systems recognise number 8 and so less is destroyed. Next, a second complementary code sequence for Factor IX gene was added to the virus. Not complimentary in a polite society sense, but the mirror code of the intended signal. The two copies bound together protecting each other until unravelling inside a liver cell (Figure 1).
This new gene in a virus in another virus’ clothes was a tremendous success. When given to six people with Christmas disease, all needed far less Factor IX and several needed none: they were effectively cured. This meant fewer injections, less risk of catching unwanted viruses like HIV and, critically, less risk of bleeding.
As well as helping Christmas disease patients, the most exciting aspect of this gene therapy technique is its potential use in other genetic diseases: cystic fibrosis, sickle cell anaemia and more are possible targets. However, further tweaking of the system is required to create these more complicated proteins.
It might be that European royal families pay particular attention to these developments too: the genetic defect that causes Christmas disease runs through the (ex-)Russian, Greek and British monarchies, and was carried by Queen Victoria. Some will recognise Christmas disease better through its alternative name – haemophilia B – and their history lessons at school.
Nathwani, A. C., et al. (2006). “Self-complementary adeno-associated virus vectors containing a novel liver-specific human factor IX expression cassette enable highly efficient transduction of murine and nonhuman primate liver.” Blood 107(7): 2653-2661.
Nathwani, A. C., et al. (2014). “Long-Term Safety and Efficacy of Factor IX Gene Therapy in Hemophilia B.” New England Journal of Medicine 371(21): 1994-2004.
Rogaev, E. I., et al. (2009). “Genotype Analysis Identifies the Cause of the ‘Royal Disease’.” Science 326(5954): 817-817.
By Gwilym Webb; PhD Student, University of Birmingham