While the research does not necessarily mean that the developmental defects are relevant to human disease, it hints that mutant huntingtin trips up the brain’s formation in ways that render neurons more susceptible to the environmental stressors that come with aging. If true, “it really reorients our thinking about Huntington disease,” said Christopher Ross of Johns Hopkins University in Baltimore, Maryland, who was not involved with the study. Ross compared the theory to the “two-hit hypothesis” from cancer research: the first hit could be minor developmental defects caused by mutant huntingtin, which do not cause problems until a second hit, such as the oxidative damage associated with aging, triggers neurodegeneration and disease.

Mehler first theorized, nearly 10 years ago, that the proteins associated with neurodegenerative disease, such as huntingtin, presenilins, and amyloid-β precursor protein (APP), might cause developmental problems leading to eventual diseases of aging (Mehler and Gokhan, 2000; Mehler and Gokhan, 2001). He selected Huntington’s as the first disease to investigate because of the clear genetic cause and availability of good animal models.

Since the initial theory papers, several other reports have suggested that mutant huntingtin affects the body before full-blown Huntington’s appears. Researchers in the multicenter PREDICT-HD trial, studying healthy people who carry glutamine-expanded huntingtin, have found issues such as motor problems (Biglan et al., 2009), changes in brain anatomy (Klöppel et al., 2009), and behavioral symptoms (Beglinger et al., 2008) before the onset of what are considered characteristic HD symptoms. Others have observed cognitive problems before the disease asserts itself (Robins Wahlin et al., 2007). Huntingtin’s role in development is also apparent in knockout mice, where early embryo patterning is altered (Woda et al., 2005).

It took Mehler and Molero seven years to assemble their data. They knew that if their theory was correct, any defects caused by mutant huntingtin were likely to be subtle. The researchers compared two knock-in mouse lines in which the first exon of endogenous huntingtin was replaced with the equivalent human exon with either 18 or 111 glutamine repeats. Hdh-18 represents the wild-type and Hdh-111 the mutant form of the protein. Molero and colleagues analyzed several aspects of the formation of medium spiny neurons in embryos, focusing on the striatum, a part of the brain involved in motor function that is affected in HD.

The researchers found that there were many, many things wrong with the Hdh-111 embryos. “The range of deficits was extraordinary,” Mehler said. “We saw deficits across the board.” Formation of the striatum appeared to be delayed. Under the microscope, the researchers saw reduced expression of medium spiny neuron markers such as Islet1 and NeuN, compared with Hdh-18 embryos of the same age. DARP-32 and mGluR1, which showed a patchy distribution in the Hdh-18 embryos, were more diffuse in the Hdh-111 embryos. To analyze cell division, they tagged the embryos’ DNA with BrdU, which is diluted with each passing cell division. Progenitor cells in the striatal region were slow to exit the cell cycle and differentiate. These cells also overexpressed pluripotency markers such as Sox2 and Nanog, which may have caused the delay in maturation. “One of the things that is most remarkable is, given how many changes we find, the fact that homeostasis can be maintained,” Mehler said.

The authors suspect that minor developmental changes make medium spiny neurons in the striatum extra susceptible to environmental stressors later in life. “Adult life is essentially a toxic state for the brain,” Mehler said. Cellular stresses that normal cells can handle—such as DNA damage, oxidative stress, or dysregulation of the proteosomal pathway—might be enough to tip these cells toward degeneration.

“The fact that we have discovered these abnormalities doesn’t necessarily say that they relate to pathogenesis,” Mehler said. “We have in no way established that there is a causative link.”

Stan Lazic of Roche Pharmaceuticals in Basel, Switzerland, was skeptical of the link between early developmental defects and eventual neurodegeneration. “If these differences occur so much earlier than the onset of symptoms (about 40 years earlier, if we extrapolate to human disease), then these aren’t the important differences!” he wrote in an e-mail to ARF. “Young pre-symptomatic individuals with the HD gene are relatively normal, so if developmental differences exist, their role is likely to be relatively minor in light of the massive cell death that these patients experience later in life.”

The clear next step is to examine striatal development in other HD models, in adult animals as well as in people. “If the same results are found in other animal models, then it is more likely that similar differences would be found in the human condition,” Lazic wrote.

Mehler is also looking into the possibility that there are developmental defects in other models of neurodegenerative disease. APP and PS1, which cause early-onset familial Alzheimer disease when mutated, have both been linked to neurodevelopment in mammals (see ARF related news story and Eder-Colli et al., 2009).—Amber Dance.

Reference:
Molero AE, Gokhan S, Gonzalez S, Feig JL, Alexandre LC, Mehler MF. Impairment of developmental stem cell-mediated striatal neurogenesis and pluripotency genes in a knock-in model of Huntington’s disease. PNAS 2009 Dec 2. Abstract