There's no cure and only a few palliative treatments for Huntington's disease (HD), a devastating, degenerative, inherited brain disorder that slowly diminishes a person's ability to walk, talk and reason. More than a quarter million Americans either have or are at risk for the disease (Huntington's Disease Society of America). However, hope is on the way. Researchers at the California Institute of Technology (Caltech) have found that a highly specific intrabody (iAb) – an antibody fragment that works against a target inside a cell - attenuates the development of Huntington’s disease in a variety of HD mouse models (Southwell, Ko, and Patterson 2009).
Huntington's is caused by a mutation in the huntingtin (Htt) gene. Normally, Htt has from 10 to 30 repeats of a CAG nucleotide trio, which encodes the amino acid glutamine. In contrast, mutant Htt (mHtt) may have more than 120 of these repeats. As a result, the abnormally long mHtt protein is cleaved into smaller pieces that accumulate in and are toxic to brain cells. Generally, obvious HD symptoms begin between the ages of 30 and 45, but, in rare cases, they may begin as early as 2 years.
HD mouse models
Although the laboratory mouse has been an important model of Huntington’s disease, no model perfectly reflects human HD. In some models, such as the C57BL/6J (000664) lentiviral model, HD is induced by viral vectors that encode mHtt. Like humans with Huntington’s disease, these models lose striatal neurons, but they have only a few motor deficits, do not lose weight and have a normal lifespan.
Other transgenic models encode human mHtt fragments, which are much more toxic than full-length mHtt. Two such models are the R6/2 and N171-82Q mouse strains – such as B6CBA-Tg(HDexon1)62Gpb/1J (002810) and B6C3-Tg(HD82Gln)81Dbo/J (003627) respectively. These two models exhibit early onset of progressive motor and cognitive deficits, lose weight, form mHtt inclusions, and develop enlarged ventricles, but do not lose striatal neurons. Additionally, they cannot be used to study therapeutic strategies directed at the C-terminus of mHtt.
Transgenic models that encode full-length mHtt include the BACHD and YAC128 lines – such as FVB/N-Tg(HTT*97Q)IXwy/J (008197) and FVB-Tg(YAC128)53Hay/J (004938) respectively. These two models have a more human-like phenotype, with slower, progressive cognitive and motor deficits, and they exhibit striatal atrophy, develop enlarged ventricles, and lose some striatal neurons as the disease progresses. However, they gain rather than lose weight as disease progresses. Additionally, their FVB background results in retinal degeneration and blindness, which confounds the ability of researchers to test behavior in older mice.
So, how did the Caltech team deal with the lack of a single representative Huntington’s disease mouse model? They tested the therapeutic efficacy of two iAbs in the four different HD JAX® Mice models mentioned above and in the lentiviral model.
The iAb strategy
The Caltech team reasoned that, being an autosomal dominant disease, Huntington’s disease might be better treated by preventing mHtt-induced brain damage instead of managing or repairing the damage after it was done, the current strategy of most HD-directed therapies. Intrabodies seemed perfectly suited for this purpose. They can recognize different epitopes of the same protein, distinguish between highly homologous proteins, discriminate among different conformations of the same protein, and target proteins in particular cellular compartments. Two iAbs, VL12.3 and Happ1, were known to reduce the accumulation of toxic Htt protein in cell culture and fruit-fly models. The Caltech team wanted to know if the two intrabodies would have similar effects in HD mouse models.
Happ1 may be a silver bullet
The Caltech team was not impressed with the effects of the VL12.3 iAb. Although it improves behavior and attenuates neuropathology in the lentiviral model, it has no effect on the YAC128 model and actually exacerbates HD severity in the R6/2 model. In contrast, Happ1's effects are remarkable. It restores normal motor performance in N171-82Q, YAC128, BACHD, and R6/2 mice in rotarod and beam-crossing tests, in YAC128 and BACHD mice in climbing tests, in N171-82Q mice in the clasping test, and in the lentiviral model in the amphetamine-induced rotation test. It restores normal performance of YAC128 and BACHD mice in open field tests and of YAC128 mice in cognitive tests involving novel object location and preference. Happ1 also dramatically reduces striatal neuron loss in the lentiviral model and Htt aggregation in the lentiviral and R6/2 models. It also normalizes ventricle size in R6/2, YAC128, and BACHD mice. Moreover, Happ1 significantly increases body weight and life span of N171-82Q mice.
The iAb Happ1 normalizes the appearance of mice in the N171 HD model. On the left is an N171 mouse given a striatal injection of the adeno-associated viral vector encoding green fluorescent protein (GFP; control); on the right is an N171 mouse given an injection of the vector coding for Happ1. The control mouse is thinner and has ruffled fur, indicative of disease. (Photo courtesy of Dr. Paul Patterson)
Happ1 likely works by binding to a very specific part of the mHtt protein. Its extremely specific action may be particularly effective for treating Huntington’s disease while producing very few side effects. As Dr. Paul Patterson, senior author on the Caltech study states, "Our studies show that the use of intrabodies can block the parts of mutant huntingtin that cause its toxicity without affecting the wild-type, or normal, Htt – or any other proteins." Thus, Happ1 may be the "silver-bullet therapy" that many medical researchers dream of (Science Daily 2009). Happ1 might be even better if its stability and mHtt binding efficiency are improved. Says Dr. Patterson, "We need to improve the efficacy of the iAb, and we need to build a viral vector that can be controlled – induced and turned off – in case of unexpected side effects."
Science Daily. 2009. Gene Therapy Stalls Development Of Huntington's Disease In Mice. Science Daily, Nov. 9 http://www.sciencedaily.com/releases/2009/10/091031002310.htm
Southwell AL, Ko J, Patterson PH. 2009. Intrabody gene therapy ameliorates motor, cognitive, and neuropathological symptoms in multiple mouse models of Huntington's disease. J Neurosci 29:13589-602.