Over the course of the past decade, research has emerged pointing to potential genetic commonalities between those people who develop Alzheimer’s disease (AD), and those who have Parkinson’s.
In particular, scientists have homed in on so-called “genetic regions” within the human genome that may increase an individual’s risk for both diseases. Knowing more about the potential genetic similarities between the two diseases could help scientists within both fields to work together to find new treatments for patients.
This week, however, a new study from researchers in Wales suggests that AD and PD may have less in common than recent science had hoped.
According to MedPage Today, a large-scale comparative analysis from a team of researchers at Cardiff University School of Medicine “found no evidence of common genetic regions that increase the risk of both illnesses.”
More than 25,000 individuals from previous genome wide association research into PD and AD were studied. Principal Investigator Valentina Moskvina and her team found that “that loci (the specific location of a gene) that increase the risk of both (diseases) are not widespread.”
One major point of weakness from the study, acknowledged by its authors, is that it did not take into account people with Lewy Body Dementia, an umbrella term referring to those who have been diagnosed with either PD dementia, or dementia with Lewy Bodies.
"It's a good study, but it's premature to say that there is no genetic relationship between these two diseases," explains Matt Farrer, PhD, of the University of British Columbia, and an MJFF scientific advisor. "By excluding Lewy body dementia the authors may have inadvertently removed an overlapping genetic signal between AD and PD."
Post-mortem studies of Parkinson's brains, Farrer explains, show Lewy body disease in the brainstem and often in the cortex. Similar post-mortem studies of Alzheimer's brains (in as many as two-thirds of cases) show similar Lewy body pathology in cortical regions of the brain. This Lewy pathology might influence the rate of cognitive impairment in both AD and PD, and how and when it manifests, for which there are likely to be genetic determinants yet to be discovered.
It’s still possible that AD and PD have much in common, both genetically, and otherwise. On the whole, research continues into pathological (or underlying biological) similarities of the two diseases. In 2010, The Wall Street Journal reported on a study from the University of Washington that found that a significant number of Parkinson’s patients with cognitive impairment had decreased amounts of amyloid in their spinal fluid. Amyloid is a protein that has been linked to AD.
The Michael J. Fox Foundation (MJFF) is supporting work into another protein, called tau, implicated in AD, which has become a point of focus for the Parkinson’s field as a genetic risk factor of the disease. In addition, scientists from the Parkinson's Progression Markers Initiative (PPMI) are currently investigating whether levels of tau in a person's cerebrospinal fluid might also provide a biomarker for who might be more likely to go on to develop Parkinson's. Very early studies suggest that lower levels of tau in CSF may in fact be representative of PD.