How can we identify environmental risk factors for ALS?

Author: Dr James Rooney^1,2

¹Academic Unit of Neurology, Trinity Biomedical Sciences Institute, Trinity College Dublin

²Institute and Clinic for Occupational, Social and Environmental Medicine, University Hospital, LMU Munich, Munich, Germany

What are risk factors anyway?

We often hear about certain diseases as being caused by, or resulting from, environmental exposures[1]. The most common and well-established example being that of lung cancer caused by cigarette smoking. Other well-known examples include cirrhosis of the liver caused by excessive alcohol consumption, and skin cancer caused by excessive sun exposure (or UV light more specifically). In scientific research it is typical to refer to the exposure in question as a ‘risk factor’[2]. Researchers will typically say something like: ‘UV light exposure is a risk factor for skin cancer’.

These examples have several features in common:

1. The effect of cigarette smoke, alcohol, UV light on the lung, liver or skin retrospectively is a strong one. That is – the chances of getting any of these diseases are heavily impacted by the particular exposure associated with each disease.
2. The diseased organ gets a high dose of the exposure in question – i.e. cigarette smoke is inhaled directly into the lung, the liver breaks down almost all the alcohol in the body, and the skin is directly impacted by UV light.
3. The diseases in question and also the corresponding exposures are all relatively common within the population.

These features mean that if you are carrying out a study on one of these diseases, and your study includes the risk exposure in question, and assuming the study is otherwise well designed, then there is a reasonably good chance that your study will correctly identify the dangerous exposure for the disease in question!

However, what happens when the effect the exposure has on a disease is not as strong as that of cigarettes on lung cancer, or the organ in question is not directly exposed to possible risky exposures, or the disease, the exposures, or both, are less common than those discussed above? Unfortunately, in this case, finding the risky exposure (or exposures) for the disease in question can become extremely difficult. This is where we find ourselves when trying to identify risk exposures for ALS.

Environmental risk factors for ALS

In terms of the causes of ALS the picture is somewhat complicated. A subset of ALS cases are known to be caused by genetics. The most common of these is called C9orf72[3] and it explains about 10% of cases in Ireland. A large number of other genetic causes have been identified that explain smaller numbers of cases. Recently, highly detailed studies of Irish families showed that there may be inherited risk in up to 50% of ALS cases (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6646974/). So, if that is true, what is causing the other 50% of cases? Well, there are a variety of opinions on this. Some believe that all cases are genetic, but that we have not yet discovered all the causative genes yet. Others believe that is caused by multiple genes acting in concert (i.e. polygenic). Still others believe that a wide range of environmental exposures might be responsible for the unexplained cases. And finally, others believe that genes and environmental exposures acting in combination are the root cause.

In summary -> its complicated! But we won’t let that stop us. Given all the above, if we want to try to identify environmental exposures that might be causing, or contributing to the risk of getting ALS, how would we do it? The first thing we must do is have a way to measure people’s exposures. There are many approaches to this, but some examples include the following strategies:

• Suppose we want to know whether cigarette smoking causes a disease, how might we do it? Well one approach is to survey people with the disease, and people without the disease on their smoking habits. This works reasonably well, because cigarettes come in fixed packet sizes, and as they are expensive and people need to budget for that, people tend to be quite aware of how much they smoke – although in general people do underestimate this. We know this because another way we can measure people’s cigarette smoking habits is by measuring a chemical in the blood called cotinine that will only be found in smokers and is found in concentrations proportional to the amount of cigarettes smoked. Such a chemical is a biomarker of exposure. So, we have now identified at least two methods of assessing environmental exposures – by survey of people’s consumption habits, and by measurement of biomarkers in the blood (or we could use urine, or hair, or nails – depending on the exposure of interest).
• However, suppose we wanted to know if exposure to diesel fumes was an important environmental exposure for a disease? People generally have no idea how much diesel fumes they are exposed to, and it is not something you can easily measure in a blood sample. But we do know that people in certain occupations have higher exposure than others. For example: farmers, truck drivers and mechanics will have higher diesel fume exposure on average than people working in an office. This approach has been used extensively in the past for respiratory diseases in particular, and industrial hygienist and epidemiologists have developed job exposure matrices to link many thousands of jobs with various exposures. Therefore, by surveying people’s employment history, they can generate estimates of their exposures to a variety of chemicals or radiation.
• What about exposures like air pollution? Again, this is something people are generally unaware of on a daily basis (unless air pollution levels are extremely high). Although one can now purchase their own air pollution monitor, and wearable devices are becoming available to measure such things in real time, this doesn’t help us as researchers if we want to know about someone’s exposure 5 or 10 years ago. However, extensive satellite air pollution data is available from NASA (https://airquality.gsfc.nasa.gov), and Copernicus (https://atmosphere.copernicus.eu). Therefore, it is possible to look up a given persons home GPS coordinates and estimate their exposure over a time period to different air pollution measurements (i.e. particulates or various gases).

A number of studies into environmental exposures and risk of ALS have been carried out in Ireland. The EURALS study (2008 to 2012) surveyed ALS patients and age and gender matched healthy volunteers (commonly called ‘controls’ in studies) about their lifetime exposures. This was followed by the EuroMOTOR study (2011 to 2014) which built on the design of EURALS and expanded the survey to include additional possible exposures including a broader medical history and very detailed occupational history. Both studies were carried out across multiple European countries to include as many people as possible. Some of the findings from these studies included:

• EURALS found coffee consumption was associated with decreased risk of ALS (https://academic.oup.com/aje/article/174/9/1002/168671)
• EURALS found that repeated trauma and severe trauma were associated with increased risk of ALS (https://www.tandfonline.com/doi/abs/10.1080/21678421.2017.1386687?journalCode=iafd20)
• EuroMOTOR found that occupational exposure to a range of chemicals was associated with increased risk (http://www.tara.tcd.ie/handle/2262/85062 – Chapter 5.5)
• EURALS found that physical activity was not associated with a higher risk of ALS (https://onlinelibrary.wiley.com/doi/10.1002/ana.24150)
• EuroMOTOR found that physical activity was associated with increased risk for ALS (https://jnnp.bmj.com/content/89/8/797.long)

So given such findings, why do we not tell ALS patients to drink lots of coffee for example? Well, you might have noticed that the findings of EURALS and EuroMOTOR disagree with regard to physical activity. EURALS found that physical activity was not associated with increased risk for ALS, while EuroMOTOR found the opposite. How can this be? There are several possible answers here, including random chance, measurement errors or even differences in study design. Another possibility is that something else might be associated with both exercise and ALS, such as a gene that is associated with being a good athlete and could also be involved in ALS. However, the take home point here is that we cannot rely on a single study to identify a risky exposure, no matter how well designed or how many participants are involved. Because of this, every few years experts perform ‘systematic reviews’ of the research and consider all the papers on a given topic. This should be a highly organised process involving grading each paper by its design, number of participants and so on. Then it is possible to make a more general decision about a risk factor using special statistical methods to combine study results. Currently, the only environmental exposure widely recognised to increase the risk of getting ALS is cigarette smoking, adding to the list of reasons why everyone who smokes should take action to cut down and quit the habit.

The MetALS study

Currently, a study on environmental exposures known as the MetALS study is running in Ireland. This study is designed to further evaluate the role of metals as risk factors for ALS. The metals being studied are: aluminium, arsenic, cadmium, chromium, copper, lead, manganese, mercury and selenium. Some of these metals such as lead and mercury are always considered toxic to humans, while others such as copper and selenium are nutrients that are required by the body in low doses, but also can be dangerous at too high a dose. In MetALS, we are combining a survey approach with the collection of blood and urine samples from volunteer patients and controls. Using these samples, together with samples from German and Italian patients and controls collected by the MND-Net organisation in Germany, and the PARALS register in Turin, we are measuring the concentrations of blood and urine metals, standard hospital blood tests such as liver and kidney function tests, and the measurement of heat shock proteins. Heat shock proteins are special proteins that help our cells to cope with stressful events. For a human cell, stressful events can mean many things, but includes exposure to heat (hence the name heat shock proteins), and also chemical exposures to the cell, such as that of toxic metals. The heat shock proteins function to help the cell continue its normal functions in spite of such stresses. We also hope that some of our volunteers will return to give repeated blood samples so that we can determine trends over time of our measurements.

When we have completed all of the measurements, we will use statistical methods to make sense of all this data. We are interested in seeing whether metal or heat shock protein concentrations differ between people with ALS and volunteer controls, to see if there might be associations between metal concentrations and heat shock protein concentrations, and to see if any of these measurements might be correlated with disease progression (measured via the ALSFRS). Right now (September 2021), we are carrying out measurements on samples from the first recruits to our study. However, recruitment of volunteers for MetALS is still ongoing through the ALS clinic at Beaumont Hospital. We are really grateful for the interest of the ALS community in this work, and we would be really happy to hear from any volunteers wishing to take part in the MetALS study, either with or without a diagnosis of ALS.  I hope that this overview has been of interest for the wider ALS community. If you’d like to hear more information about our results so far or taking part in future research, please email me at rooneyj4@tcd.ie or Gráinne Geoghegan at Grainne.Geoghegan@tcd.ie.

I’d like to thank Professor Orla Hardiman and Magdalena Kotalla, MSc, for feedback on this article.

[1] In scientific research “environmental exposures” can refer to air pollution, chemical exposures, radiation exposure, or even behavioural qualities such as diet and exercise

[2] Risk factor is a somewhat controversial term in that it can have different meanings in different contexts, however in this article we take it to mean an environmental exposure that can cause a disease. More on this topic can be read here: https://www.bmj.com/content/355/bmj.i6536 

[3] Technically, an expansion of the C9orf72 is responsible. Everyone carries the C9orf72 gene, but in some individuals it is extra-long (hence ‘expansion’), and it is the expansion that is associated with ALS and FTD (fronto-temporal dementia).

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 846794