Last October I reported an incredible story in which researchers used an HIV chimeric virus to cure leukemia. Here's another success story.
Hemophilia B is a blood clotting disorder caused by spontaneous mutations in the Factor IX gene, leading to a deficiency of Factor IX, an enzyme essential in blood coagulation. The gene is expressed mostly in the liver, where the enzyme is produced and then sent into circulation in the blood. Less than 1% of normal levels of Factor IX lead to severe hemophilia and require a lifetime treatment of intravenous injections of FIX protein concentrate 2-3 times a week.
"Somatic gene therapy for hemophilia B offers the potential for a cure through continuous endogenous production of FIX after a single administration of vector, especially since a small rise in circulating FIX to at least 1% of normal levels can substantially ameliorate the bleeding phenotype ."Unfortunately, previous gene therapy experiments have been unsuccessful, showing only transient expression of Factor IX (effects weaned off after a while). It is possible that the patients' immune system produces a T-cell response against the infused cells. In its essence, gene therapy is the transfer of a healthy gene in the cell line affected by the defective genes. This transfer is usually obtained through a modified virus (the "vector") because viruses have the innate ability to attack a cell and inject it with their own genetic material. When choosing a viral vector, therefore, it is essential to establish that the patient's immune system does not "recognize" the viral vector. That's often the problem with gene therapy: basically, you are trying to "fool" the immune system by injecting extraneous genetic material and hoping that it successfully replaces the defective one. However, the immune system is not so easy to fool.
Nathwani et al.  tried a new gene therapy approach. The vector typically used is a modified adenovirus, but the researchers used a self-complementary vector so it would yield higher efficiency in transgene expression. They also used a subtype of virus that has a lower prevalence in humans, and thus lower chances of patients having developed humoral immunity against it. Lastly, whether previous approaches would infuse the virus directly to the liver, in this study patients were administered the vector in the peripheral vein, which is a less invasive and safer approach.
The results were quite promising: "AAV-mediated expression of FIX at 2 to 11% of normal levels was observed in all participants. Four of the six discontinued FIX prophylaxis and remained free of spontaneous hemorrhage; in the other two, the interval between prophylactic injections was increased."
The difference in responses to the therapy was partly due to the fact that patients were given different doses of the genetically modified virus (low, moderate, and high), which in fact were dose dependent. However, it also suggests that individual immune responses and different exposures to the virus greatly affect the outcome. These factors need to be better understood and it will require a larger number of participants in future studies. Furthermore, as with any other gene therapy treatment, there are risks associated: the six participants are currently monitored for hepatic dysfunction, but the researchers are optimistic that, even with such potential risks, this kind of therapy
"has the potential to convert the severe bleeding phenotype into a mild form of the disease or to reverse it entirely."
Edited to add a cool comment from antisocialbutterflie:
Intriguingly enough my grad lab worked on AAV capsid proteins (not my project but I could give the first 15 minutes of a seminar from memory). It's a great gene therapy vector assuming (a) there isn't the preexisting immunity that you mentioned and (b) the gene you are therapifying (word?) is small enough.
The engineering of non-natural variants that exhibit the appropriate tissue specificity while escaping the existing immune response is pretty cool. It all boils down to a series of surface loops (variable regions) that mediate both the receptor-binding but also present the antigens that antibodies are developed against. Swapping out the antigenic residues are likely to disrupt the receptor binding and mess up the tissue specificity making it useless as a therapy vector.
There is also a finite amount of DNA it can package. If I remember correctly the max is around 8 kb. If it's bigger you have to turn to things like adenoviruses or the previously mentioned HIV.
 Nathwani, A., Tuddenham, E., Rangarajan, S., Rosales, C., McIntosh, J., Linch, D., Chowdary, P., Riddell, A., Pie, A., Harrington, C., O'Beirne, J., Smith, K., Pasi, J., Glader, B., Rustagi, P., Ng, C., Kay, M., Zhou, J., Spence, Y., Morton, C., Allay, J., Coleman, J., Sleep, S., Cunningham, J., Srivastava, D., Basner-Tschakarjan, E., Mingozzi, F., High, K., Gray, J., Reiss, U., Nienhuis, A., & Davidoff, A. (2011). Adenovirus-Associated Virus Vector–Mediated Gene Transfer in Hemophilia B New England Journal of Medicine DOI: 10.1056/NEJMoa1108046
Photo: sculpture by Joshua Tobey, Santa Fe, NM. Shutter speed 1/40, F-stop 7.1, focal length 75mm, ISO speed 100.