5th October 2019

How does lead cause CVD?

Following my last blog, several people asked the question. How does lead cause CVD – or atherosclerotic plaques? What is the mechanism of action? It’s a good question, one that I think I have answered before, at least in part. However, I think there is real value in going over it again.

First, I want to highlight some of the more general thinking about causes of CVD, I believe this is important as well, in order to see how lead fits in, and where my interest in lead came from.

For many, many, years now I have been trying to create a unified hypothesis about cardiovascular disease. A journey I thought I would never finish. Mainly, I now realise, because I kept coming across ever more ‘factors’ that had a role in CVD. This meant that – although I couldn’t quite work out why at first – I was running into the impossible, and unsolvable, problem.

246 factorial

The unsolvable problem is a direct result of the number of possible interactions between all the risk factors that have been identified.

To try and explain this further, I shall start with the latest UK risk factor calculator which is called Qrisk3. The previous one was Qrisk2. Qrisk3 can be found on-line here https://qrisk.org/three/ You can play with it to your heart’s content. Qrisk3 has moved on considerably from Qrisk1 and 2. It now incorporates twenty different factors. If you strip them out of the algorithm they are, in no particular order:

Age

Sex

Smoking

Diabetes

Total cholesterol/HDL ratio

Raised blood pressure

Variation in two blood pressure readings

BMI

Chronic kidney disease

Rheumatoid arthritis

Systemic Lupus Erythematosus (SLE)

History of migraines

Severe mental illness

On atypical antipsychotic medication

Using steroid tablets

Atrial fibrillation

Diagnosis of erectile dysfunction

Angina, or heart attack in first degree relative under the age of 60

Ethnicity

Postcode

As a quick side-track, it amuses me that LDL is not in there – yet HDL is.

Now, you may think that this appears to be a relatively short list. At first sight it does not appear a complex task to fit these factors together into a coherent model. A twenty-piece jigsaw puzzle – at most?

Not so. The reality is that, if you view the model of CVD as twenty independent and unconnected risk factors, the number of possible interactions, or pieces, you need to analyse becomes mind-boggling.

Just to give you an idea of the scale of the maths involved here. You have twenty different risk factors, and you do not know how the connections between them work. Every risk factor can, potentially, interact with all the others – independently. This means that the possible combinations you must analyse is twenty factorial.

Calculating factorials is, on one hand, very straightforward. You simply multiply each factor, by all of the other factors, in turn. Thus, twenty factorial = 20 x 19 x 18 x 17 x 16 etc.

The result of multiplying 20 x 19 x 18 x 17 x 16 etc. is you end up with the following number: 432,902,008,176,640,000. Which is the number of different possible combinations between twenty factors. Rounding this figure up slightly, that equates to four hundred and thirty-three quadrillion. Which is a lot. And it gets far worse than that.

As far back as 1981, a paper was published outlining 246 different risk factors involved in CVD 1. Today, there would be far more, several thousand at least. However, even by 1981 the number of possible combinations was already incomprehensibly huge. I say this because 246 factorial is:

980360372638941007038951797078339359751464353463061342202811188548638347461066010066193275864531994024640834549254693776854464608509281547718518965382728677985343589672835884994580815417004715718468026937051493675623385569404900262441027874255428340399091926993707625233667755768320823071062785275404107485450075779940944580451919726756974354635829128751944137276448671023801110260206915547825809239994946405007360000000000000000000000000000000000000000000000000000000000

Yes, to my amazement, there is a website which will calculate factorials for you. You don’t think I worked that out myself do you? I would have definitely got bored and made several mistakes on the way. Therefore, I have no idea if this figure is right or wrong, but it seems to be in the right sort of ballpark.

There is no way to even describe a number that big, and it would certainly make for some jigsaw puzzle. In truth what we have here represents a figure so huge that you cannot possibly do anything with it. It has fifty-seven zeros before you even get to another number. At least I think it is fifty-seven, I may have lost count.

How long would it take to feed in the data on all these risk factors, run the combinations, and see if you can establish how they all fit together? That would take as close to an infinite amount of time as makes no practicable difference. Even with the latest Google quantum computer.

Thinking about things in this way, I came to realise that unearthing risk factor, after risk factor, after risk factor, was not going to make it easier to work out the cause of CVD. It was making it impossible.

The other word for impossible, I came to realise, is ‘multifactorial’.

Multifactorial = a word commonly used by cardiologists to prevent any discussion as to the real causes of CVD. In conversation on this issue, I silently change the word multifactorial to ‘246-factorial’, just to remind myself what a stupid concept it is to call a disease multifactorial. Then to believe that, by doing so, you have explained anything. ‘So, how do the factors all fit together?’ I mutter, imaging a number so vast that it is beyond comprehension.

Then I may quote Poincaré at them.

‘Science is built up of facts, as a house is built of stones; but an accumulation of facts is no more a science than a heap of stones is a house.’ Henri Poincaré.

Because multifactorial also effectively = a pile of stones. Well done, you have found thousands of stones, and carefully piled them ever higher, but this gets no nearer to constructing a house. To build a house you need to know how all the stones join up. You need a plan my friend.

Which starts to bring me, in a roundabout way, back to lead.

As regular readers of this blog will know, I ripped up the multifactorial model of CVD and tried to replace it with a process model. The plan of the house, if you like. I was no longer interested in finding endless risk factors, then chucking them on the pile. I wanted to know the process – or processes – involved.

In the end I stripped it down to three main elements. Basement, walls, roof.

Endothelial damage (damage to the lining of artery walls)

Formation of a blood clot

Repair

Or at least I stripped it down to three main processes – going wrong. Because this triad is all quite normal, and healthy. It is only when endothelial damage and clot formation accelerate, or repair is sub-optimal, that CVD/atherosclerosis will develop.

So, the simplest possible model is: rate of damage > rate of repair = CVD

Using the three-process model I began a different search. Starting with things that could damage the endothelium. I cast the net far and wide. Of course, it is more difficult to do the searching this way. Where do I begin? Do you just start thinking of things that might be damaging, and hope for the best? You will find yourself wandering all over the place. At least I did. Although it is quite an interesting journey – for a geek.

Bringing some structure into my search strategy, I decided that heavy metals were something that could not be doing any good to the human body. Mercury, lead, cadmium, and suchlike. Gold? Gold doesn’t seem to do much, one way or another. It was used to treat rheumatoid arthritis at one time. As for the others – not great. Lots of damage to health.

But do they damage the endothelial lining of the artery wall? Well, yes, they do. Looking at lead, here is a passage from the paper: ‘Mechanisms of lead-induced hypertension and cardiovascular disease.’

I admit that it is far too jargon heavy for most people. But I enjoyed it and I reproduced it in full because, what we have here, is the perfect storm. Many mechanisms I have previously mentioned in my long and winding series on what causes heart disease can be found here. Including many I did not mention because they were just too technical. I have put in bold some of the more important mechanisms:

‘Lead is a ubiquitous environmental toxin that is capable of causing numerous acute and chronic illnesses. Population studies have demonstrated a link between lead exposure and subsequent development of hypertension (HTN) and cardiovascular disease. In vivo and in vitro studies have shown that chronic lead exposure causes HTN and cardiovascular disease by promoting oxidative stress, limiting nitric oxide availability, impairing nitric oxide signaling, augmenting adrenergic activity, increasing endothelin production, altering the renin-angiotensin system, raising vasoconstrictor prostaglandins, lowering vasodilator prostaglandins, promoting inflammation, disturbing vascular smooth muscle Ca2+ signaling, diminishing endothelium-dependent vasorelaxation, and modifying the vascular response to vasoactive agonists. Moreover, lead has been shown to cause endothelial injury, impede endothelial repair, inhibit angiogenesis, reduce endothelial cell growth, suppress proteoglycan production, stimulate vascular smooth muscle cell proliferation and phenotypic transformation, reduce tissue plasminogen activator, and raise plasminogen activator inhibitor-1 production.’ 2

So, there you go. Not just one mechanism of action, but twenty-one different processes that lead can cause CVD. Fifteen ways of damaging the endothelium, four that inhibit repair, and two mechanisms for making blood clots more difficult to get rid of, as highlighted in the final passage… reduce tissue plasminogen activator, and raise plasminogen activator inhibitor-1 production.’

Tissue plasminogen activator (TPa) is the enzyme that activates the breakdown of blood clots. TPa converts plasminogen to plasmin, and plasmin then chops fibrin to bits, thus shaving down blood clots. Plasminogen activator inhibitor-1 is a substance that inhibits the action of tissue plasminogen activator (TPa = the clot buster, often given to patients after a heart attack or stroke).

Clearly, if you reduce TPa, and increase TPa inhibition, you end up with a blood clot that is very difficult to get rid of and is thus more damaging.

So, when people ask, have you got a mechanism of action to explain how lead causes CVD I say (rather smugly), no, I have got twenty-one. In truth, I have found quite a few more, but twenty-one is probably enough to be getting on with. One thing I have found is that once you start looking at all the potential processes, there seems almost no end to this stuff. It stretches in all directions.

Big fleas have little fleas upon their backs to bite ’em,

And little fleas have lesser fleas, and so, ad infinitum.

And the great fleas, themselves, in turn, have greater fleas to go on;

While these again have greater still, and greater still, and so on

Anyway, getting back on track, I started to look for factors, causal agents, whatever is the best name for them, that can impact on one of three processes:

Endothelial damage (damage to the lining of artery walls)

Formation of a blood clot

Repair

This is how I got to lead, only to discover that there was a huge body of research linking lead to CVD… that I had been completely unaware of. My analogy was that of a round the world sailor bumping into Australia and wondering why no-one had bothered to tell him it was there. ‘It’s pretty big, you know.’

Looking at things, by starting with one of the three processes, is also how I came across the evidence on sickle cell disease (SCD). I reasoned that sharp pointy red blood cells (sickled cells) hammering through the blood vessels would create serious damage to endothelial cells.

When I started looking, I found that, in some studies, SCD increases the (relative risk) of CVD by fifty thousand per cent. Yes, you did read that right. Fifty thousand per cent. With none of the other ‘established’ risk factors present.

Which makes SCD a ‘sufficient’ cause of CVD. In fact, it is the only sufficient cause I have ever found – in that it can lead to atherosclerosis in the blood vessels in the lungs, where the blood pressure is pretty low.

Which means that, with SCD, you don’t even need a high blood pressure. SCD can cause CVD all by itself. If you can find any other factor that can do that – let me know. For a more in-depth discussion on causation, and the concept of ‘sufficient’ see this article: https://jech.bmj.com/content/55/12/905.long

Then, I thought, what else causes damage to the endothelium. I ended up looking at a group of diseases known as ‘vasculitis’. Itis means, inflammation, as in tonsillitis, appendicitis. So, vasculitis means inflammation of the vascular system, by which I mean inflammation of the lining of the blood vessels. By which I mean, damage (and repair) to the lining of the blood vessels. Remember, inflammation = repair.

There are many different forms of vasculitis, most of which are not really thought of as being ‘vasculitis.’ For example, Rheumatoid arthritis, and Systemic Lupus Erythematosus. These conditions cause inflammation in many different places, but they also cause vasculitis. You may have noticed that both also appear on the Qrisk3 calculator.

Other forms of vasculitis, or diseases where vasculitis is an important part of the spectrum of abnormalities include:

Scleroderma

Sjogren’s

Erythema nodosum

Takayasu’s arteritis

Kawasaki’s disease

I think they all have great, evocative names:

All these forms of vasculitis are associated with a greatly increased risk of CVD. You can look this up yourself, if you want. In fact, children who suffer Kawasaki’s can die of myocardial infarctions (MIs), aged five. They have a brief, super-accelerated, form of endothelial damage that lasts a few weeks. Some can then end up with large aneurysms (balloon-like swellings) in their coronary arteries. These can burst, causing a MI.

So, not an entirely conventional MI. Nor conventional atherosclerosis. However, if you think of an aneurysm as a late stage abnormality in atherosclerosis [which most are] then in Kawasaki’s we can get from endothelial damage, to an aneurysm, in a month. Something that normally takes about sixty years to develop.

Three stones to make a wall

Lead, sickle cell disease, vasculitis.

In one sense it could be said that I have been discussing three completely unrelated things here. Lead, sickle cell disease, vasculitis. In another sense I hope you can see that these three ‘factors’ are related to CVD. Not by what they are, but by what they do. The damage they cause… the process.

They all fit very neatly into the walls of the house that are called ‘endothelial damage’. How else can you explain how three such disparate things can possibly cause exactly the same disease.

I must admit that this breakthrough in my thinking, from causes to process, was not mine. It was entirely due to one man. Professor Paul Rosch. We were discussing stress (strain) and CVD and he was critiquing a presentation I had given.

I shall paraphrase his comment. ‘Very good, you have given us the what, but not the how.’ Yes, very simple, when you think of it that way. What he was really asking was, what is the process? Since then, I have often wondered why have others not gone down this route?

I then realised that the problem, the great problem in all research into CVD, is that very early on it was decreed that LDL/cholesterol causes CVD. Therefore, all thinking, and any hypothesis on CVD required that LDL sat at the centre.

To my mind this is like opening a two-thousand-piece jigsaw puzzle and deciding, straight away, that one big piece – LDL/cholesterol – sits at the centre, and the other pieces must be made to fit around it.

Well, perhaps it does have a (small role) to play in CVD, but it most certainly does not sit and the centre. But if you keep it there, you distort the entire puzzle and make it impossible to complete. The pieces must be forced into place. Hammered down, or twisted into extreme shapes.

This, though, is where CVD research currently sits. Thousands of pieces are lying about the board. A few of them have been fitted together, here and there. As for the whole puzzle, it is doomed to failure, because the wrong piece is taking up the key position. Unfortunately, if you are a ‘serious’ CVD researcher, who wants to get grants for research, you can’t move it.

What I found if that you chuck that piece away, and start again, then everything becomes clear. The puzzle can be made to fit into one of these three processes:

Endothelial damage (damage to the lining of artery walls)

Formation of a blood clot

Repair

Lead, for example. Lead makes no sense as a significant risk factor using conventional thinking. It doesn’t raise BP, it doesn’t raise LDL, it doesn’t cause diabetes, it simply does not fit. So, it has become, essentially, ignored. But how can you ignore something that may be responsible for four hundred thousand deaths per year, in the US alone? Most of them CVD deaths?

The answer is that you cannot.

1: https://www.atherosclerosis-journal.com/article/0021-9150(81)90122-2/abstract

2: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2519216/