In an effort to better understand Parkinson’s, researchers have repeatedly analysed data from large epidemiological studies in order to gain insight into factors that could have a possible causal influence in the development of the condition.
This week a manuscript was made available on the preprint website BioRxiv that provided us with a large database of information about aspects of life that are associated with increased incidence of Parkinson’s.
Some new associations have been made… and some of them are intriguing, while others are simply baffling!
In today’s post, we will have a look at what has been learnt from epidemiological research on Parkinson’s, and then discuss the new research and what it could mean for Parkinson’s.
What are the differentiators? Source: Umweltbundesamt
What makes me different from you?
Other than my ridiculous height and the freakishly good looks, that is. What influential factors have resulted in the two of us being so different?
Yes, there is the genetics component playing a role, sure. 7,500 generations of homo sapien has resulted in a fair bit of genetic variation across the species (think red hair vs brown hair, dark skin vs light skin, tall Scandinavians vs African pygmies, etc). And then there are aspects like developmental noise and epigenetics (factors that cause modifications in gene activity rather than altering the genetic code itself).
And over-riding all of this, is a bunch of other stuff that we generally refer to simply as ‘life’. Habits and routines, likes and dislikes, war and famine, etc. The products of how we interact with the environment, and how it interacts with us.
But which of all these factors plays a role in determining our ultimate outcome?
It is a fascinating question. One that absorbs a large area of medical research, particularly with regards to factors that could be influential in causing a specific chronic conditions.
What does this have to do with Parkinson’s?
At the bottom of our previous post, we mentioned that Japan is the only country where women have a higher incidence of Parkinson’s disease than men.
We also suggested that we have no idea why this difference exists. Well, a study presented at the Cardiovascular, Renal and Metabolic Diseases conference in Annapolis City (Maryland) last week may now be able to explain why this is.
The prevalence of Alzheimer’s disease is significantly higher in women compared to men. One recent estimate suggested that almost two-thirds of individuals diagnosed with Alzheimer’s disease are women (More information here). One possible reason for this is that Alzheimer’s disease is a condition of the elderly and women live longer.
So why is it then is the exact opposite true in Parkinson’s disease???
Source: The Telegraph Newspaper
Men are approximately twice as likely to develop Parkinson’s disease as females (More information here)
In addition, women are on average diagnosed 2 years later than men (More information here)
This gender difference has long puzzled the Parkinson’s research community. But now a group from the University of North Texas Health Science Center think that they may have the answer.
The researchers – lead by Shaletha Holmes from Dr Rebecca Cunningham’s lab – observed that when they stressed dopamine neurons, adding the male hormone testosterone made the damage worse. Interestingly, they found that testosterone was doing this by acting on a protein called cyclooxygenase 2 (or COX2). When they blocked the actions of COX2 while stressing dopamine neurons, they found that they also blocked the damaging effect of testosterone. The researchers concluded that testosterone may exacerbate the damage (and death) in dopamine neurons that occurs in Parkinson’s disease, thus possibly explaining the sex differences described above.
Now, there are several interesting aspects to this finding:
Firstly, the use of Ibuprofen, the nonsteroidal anti-inflammatory drug used for relieving pain, has long been associated with reducing the risk of Parkinson’s disease (More information here).
Ibuprofen is a COX2 inhibitor.
But more importantly, several years ago it was shown that Japanese men have lower levels of testosterone than their Western equivalents. Here is the study:
Title: Evidence for geographical and racial variation in serum sex steroid levels in older men.
Authors: Orwoll ES, Nielson CM, Labrie F, Barrett-Connor E, Cauley JA, Cummings SR, Ensrud K, Karlsson M, Lau E, Leung PC, Lunggren O, Mellström D, Patrick AL, Stefanick ML, Nakamura K, Yoshimura N, Zmuda J, Vandenput L, Ohlsson C; Osteoporotic Fractures in Men (MrOS) Research Group.
Journal: Journal of Clinical Endocrinol. Metab. 2010 Oct;95(10):E151-60.
The study suggested that total testosterone levels (while similar in men from Sweden, Tobago and the US) were 16 per cent higher in men from Hong Kong and Japan. BUT – and here’s the catch – Japanese men also had higher levels of a testosterone-binding hormone (Sex hormone-binding globulin or SHBG), so there is less of the testosterone floating around free to act. As a result, Japanese men had the lowest levels of active testosterone in the study.
Intriguingly, the researchers found that Japanese men who emigrated to the US had similar testosterone levels to men of European descent, suggesting that environmental influences may be having an effect of testosterone levels. Diet perhaps?
If testosterone is found to play a role in the gender difference found in Parkinson’s disease, the lower levels of free testosterone observed in Japanese men may explain why women in Japan have a higher risk of Parkinson’s disease than men.
EDITOR’S NOTE: WHILE WE HAVE NO DOUBTS REGARDING THE RESEARCH OF DR CUNNINGHAM AND HER GROUP, WE ARE TAKING A LEAP IN THIS POST BY APPLYING THE TESTOSTERONE RESULTS TO THE GENDER DIFFERENCE IN JAPAN. THIS IS PURE SPECULATION ON OUR PART. WE HAVE SIMPLY SAT DOWN AND TRIED TO NUT OUT POSSIBLE REASONS AS TO WHY THERE IS A REVERSED GENDER DIFFERENCE FOR PARKINSON’S DISEASE IN JAPAN. OUR THEORY IS YET TO BE TESTED, AND MAY BE COMPLETELY BONKERS. WE PRESENT IT HERE PURELY FOR DISCUSSION SAKE AND WELCOME YOUR THOUGHTS.
In 1950, Dr Tavia Gordon noticed that while the overall mortality rates for men in the USA and Japan were very similar, the incidence of heart disease was significantly lower in Japan. This observation resulted in three longitudinal studies – one of which became known as the Honolulu Heart Study.
Dr Travis Gordon. Source: JSTOR
The original purpose of the study was to determine whether there was a difference in heart disease incidence between Japanese people living in Japan and individuals of Japanese ancestry living in Hawaii.
The subjects recruited for the study were “non-institutionalized men of Japanese ancestry, born 1900-1919, resident on the island of Oahu.” In all, 12,417 men were identified as meeting the criteria. Of those contacted, 1,269 questionnaires were ‘return to sender’, 2,962 men declined to participate in the study, and 180 died before the study commenced. That left 8,006 participants who would be studied and followed for the rest of their lives.
From October 1965 onwards, the participants were interviewed and given physical examinations every few years. The interview processed asked for:
- Family and personal history of illness
- Sociological history
- Smoking status
- Physical activity level
The physical examination was very thorough, looking at:
- ECG (Electrocardiography – electrical activity of the heart)
- Urine analysis
- Measurements of weight, height, skinfold thickness, etc.
- Blood pressure and serum cholesterol
As a result, the study built up a HUGE amount of epidemiological information regarding these 8,006 individuals.
So, what does this have to do with Parkinson’s disease????
Given the enormous number of individuals involved in the study and the length of time that they were followed, it was inevitable that a certain percentage of them would develop Parkinson’s disease as the study progressed. As a result, the Honolulu Heart Study represents one of the largest epidemiological study of Parkinson’s to date. In 1994, a group of research involved in the study, published some very interesting findings relating to Parkinson’s disease. That published article was:
Title: Epidemiologic observations on Parkinson’s disease: incidence and mortality in a prospective study of middle-aged men.
Authors: Morens DM, Davis JW, Grandinetti A, Ross GW, Popper JS, White LR.
Journal: Neurology, 1996 Apr;46(4):1044-50.
In total, 92 of the 8006 individuals enrolled in the study developed Parkinson’s disease. The incidence of Parkinson’s cases was registered between 1965 and November 30th 1994. The majority of the cases were diagnosed between 55 and 79 years of age (n=80). Diagnosis after the age of 80 was very rare. It is interesting to note that when the researchers divided the group into those ‘born before 1910’ and those ‘born after 1910’, the older group (born before 1910) had a lower risk of Parkinson’s disease.
In another study, the same group of investigators noted
Title: Prospective study of cigarette smoking and the risk of developing idiopathic Parkinson’s disease.
Authors: Grandinetti A, Morens DM, Reed D, MacEachern D.
Journal: American Journal of Epidemiology 1994 Jun 15;139(12):1129-38.
In this study the authors found that men who had smoked cigarettes at any time prior to their enrollment in the study in 1965, had a reduced risk of developing idiopathic Parkinson’s disease (relative risk = 0.39). That is to say, smoking reduced the chance of developing Parkinson’s disease. And a few years later the authors published a follow up paper which rejected the possibility that smoking was killing people before they could develop Parkinson’s disease (selective mortality representing a false positive). That follow up report can be found here.
EDITOR’S NOTE: THIS DOES NOT MEAN THAT EVERYONE SHOULD RUSH OUT AND START SMOKING. THERE DOES, HOWEVER, APPEAR TO BE SOME INGREDIENT IN CIGARETTES THAT REDUCES THE INCIDENCE OF PARKINSON’S DISEASE. A LOT OF RESEARCH IS CURRENTLY TRYING TO IDENTIFY THAT INGREDIENT.
This finding was made alongside other interesting correlations (Note: coffee and alcohol reduce the risk of Parkinson’s disease):
From Grandinetti et al (1994).
It should be noted that many of these associations (smoking in particular) had been reported before, but the Honolulu Heart Study was the first epidemiological study providing definitive proof. And it should be noted that subsequent epidemiological studies have found similar results.
INTERESTING FACTS ABOUT THE JAPANESE:
- The Japanese as a population have a lower incidence of Parkinson’s disease (much like most of the Asian nations) than their western equivalents, despite living longer.
- Japan is the only country in the world where females have a higher incidence of Parkinson’s disease than men (and we have no idea why!). Look here for more on this.