We show that these variants are under strong negative selection (indicative of being deleterious), and identify a subset (that form ORFs overlapping the coding sequence) with signals of selection equivalent to coding missense variants.
We find increased signals of selection when these variants occur in the 5’UTRs of curated haploinsufficient, LoF intolerant and known dominant LoF developmental disease genes, supporting a loss-of-function effect of these variants on translation.
We identify specific genes where uORF perturbation appears to be an important disease mechanism (e.g. NF1 and IRF6), and report a novel uORF frameshift variant in NF2 that segregates with disease in two families with neurofibromatosis.
Our approach illustrates the power of using large population databases and grouping non-coding bases by functional effect, to identify subsets of variants that are highly deleterious. Although the strength of selection at the level of UTRs is equivalent to synonymous variants, we see a much stronger signal at these specific uORF-perturbing sites.
In our previous posts (June 2017 and October 2017) we explained two of our studies looking at the role of truncating variants in the Titin gene (TTNtv) in dilated cardiomyopathy (DCM). In the first of these, we found that having a TTNtv meant that a DCM patient was more likely to have had a past history of abnormal heart rhythms when they were first diagnosed with DCM. But we did not have enough data to definitively assess whether this was due to rhythms in the upper (atrial) chamber of the heart, the bottom (ventricular) chamber, or both. We also didn’t know whether this meant that DCM patients with a TTNtv were at higher long-term risk of potentially dangerous or life-threatening heart rhythm abnormalities. In our second study, we followed up a large group of patients with DCM but didn’t find an increased rate of major arrhythmic events in patients with a TTNtv. As a whole this group had relatively mild symptoms of DCM at the time of the study (the majority being in NHYA class I/II heart failure), with moderately impaired ventricular function – which puts them in a comparatively low-risk group for arrhythmias. Perhaps as a result, the overall rate of arrhythmic events in the study was low, making it harder to see subtle differences between those with and without a TTNtv.
To investigate further, we therefore started a new study on patients with DCM who also had an implantable cardiac device (ICD or CRTD). Not only are patients with these devices more likely to have an arrhythmia (they are typically given to the patients because they have worse heart function or symptoms) but, crucially, they monitor the patient’s heart rhythm 24-hours a day, 7-days a week. This allowed us to get complete coverage of each patient’s heart rhythm data over a number of years.
Our study, which was published earlier this month in JAMA: Network Open, found that TTNtv are associated with a significantly higher risk of both ventricular and atrial arrhythmia in DCM patients with an ICD or CRTD.
We had a strict definition of arrhythmia, for example only counting a ventricular arrhythmia if it was both fast (>200 beats per minute) and prolonged. These are the rhythms considered to pose the most danger to patients.
Importantly, we looked at the other major factors known to increase the risk of arrhythmia – age, male sex, level of left heart dysfunction and presence of heart fibrosis (scarring) on MRI – and even after controlling for these statistically TTNtvs were a strong predictor of arrhythmia.
Heart fibrosis, or scarring, on MRI is worth delving into a little more deeply. Fibrosis shows up as white patches on the heart in an MRI scan after a contrast agent (gadolinium) has been given via a vein. The presence of this fibrosis has long been known to be a strong predictor of arrhythmia, it’s likely that the scarred area interferes with the way electrical signals travel through the heart. DCM patients with a TTNtv don’t seem to have more fibrosis than other DCM patients – we found that in a previous study and confirmed it again here. However, a remarkable finding in our study was the apparent additive nature between fibrosis and a TTNtv in terms of arrhythmia risk. Of those patients that had both fibrosis and a TTNtv, 62% experienced a ventricular arrhythmia, compared to only 5% of those with neither fibrosis or a TTNTv.
So how might TTNtvs lead to more arrhythmia in DCM?
This study wasn’t designed to answer this question and much more work needs to be done. But one avenue worth investigating is the fact that DCM patients with a TTNtv seem to have thinner heart muscle walls than those without a TTNtv, as we discussed in one of our previous blogs from October 2017. Thinner walls lead to increased ‘strain’ on the heart muscle cells, which can potentially alter their electrical properties.
Can knowing if a patient has a TTNtv help decide whether or not they should have an ICD? This was a relatively small study and the robustness of the findings certainly needs to be tested in much larger cohorts. Such studies are under way (see our blog post from October 25th 2018) and it is hoped that in the future TTNtv may be one of several factors that can help in risk-stratifying patients with DCM.
Many researchers study a particular disease because they have a personal connection to someone who has been affected. For Dr Nicky Whiffin of Cardiovascular Genetics and Genomics group, it happened in reverse.
"I had been researching cardiomyopathies (diseases that affect the heart muscle) for a couple of years when my mum suddenly became very ill. Even walking up the stairs was a struggle, she had to pause halfway to catch her breath. ..." Read more about Nicky's story.
In partnership with Cardiomyopathy UK, we are looking for ways to help people with heart muscle disease participate in research to move science forward and improve healthcare and quality of life. Patients often want to take part in research, but opportunities are concentrated around universities and major hospitals or involve long distance travel preventing many patients from getting involved.
The heart hive will make research accessible to everyone. Patients can upload their own health information to our secure web portal and decide which projects can use it. Genetic information can be collected via saliva kits sent out through the post. Check out http://thehearthive.org for more information.
The largest ever study of the heart condition dilated cardiomyopathy (DCM) is underway.
There is fresh hope for hundreds of thousands of people in the UK with the potentially deadly heart condition dilated cardiomyopathy (DCM), as the largest ever study of the poorly-understood disease begins led by Professor Stuart Cook of the National Heart and Lung Institute. The research is funded by an investment of over 2 million pounds by the British Heart Foundation (BHF).
In our last post you heard a bit about the role of titin truncating variants (TTNtv) in dilated cardiomyopathy, but we had lots more questions at the end.
Well, we’ve now completed a major study of the clinical outcomes of patients with TTNtv. In the research, recently published in the Journal of the American College of Cardiology, 716 patients with DCM, including 83 patients with TTNtv, were followed up for a primary composite outcome of cardiovascular death, major heart failure events and major arrhythmic events. We found that there was no difference in outcomes between patients with and without TTNtv (5 year event rate for TTN positive/negative patients: 10.4%/13.6%, p=0.65).
So, what does this mean?
This study suggests that having a TTNtv doesn’t affect your outcome in DCM. It shows that TTNtv DCM doesn’t have the adverse event profile associated with other genetic cardiomyopathies such as lamin cardiomyopathy. This is potentially really useful information for patients and clinicians.
But does that mean there is no role for titin in risk stratification in DCM? Not quite.
The first point to note is that the patients in the study were largely in NYHA class I/II, with moderately impaired ventricular function. Secondly, whilst this was the largest single center study of patients with TTNtv DCM with independently adjudicated outcomes and CMR phenotyping, the overall follow up time was 3.9 years (median). This is considered medium term follow up. Maybe we were a little early and longer term follow up will reveal differences that this study did not. Finally the number of the events within the cohort was lower than previous studies, not just from registry data but also historic studies in our institution. This suggests an improvement in DCM outcomes overall (note – this is a good thing for patients). Therefore it is possible that longer follow up in patients with a higher risk for adverse events could reveal subtle differences between titin positive and negative patients.
Some previous studies have suggested that there is a difference in outcome in male patients with TTNtv compared to female patients with TTNtv. TTNtv may well have a different effect in some groups e.g. women, patients with fibrosis, but the number of events in our study meant that we could not definitively analyse this. Multicenter studies will be the way forward.
On a general point, I would advise caution when interpreting studies that analyse data in only a subset of their cohort (e.g. survival analysis in only the patients with TTN and not compared to the entire DCM cohort – you may end up just picking up a DCM effect and not a TTN specific effect) or those that present survival analysis from birth (this works for hard outcomes e.g. death, but doesn’t work for outcomes such as heart failure hospitalisations, as you can’t be sure the patient didn’t have an event before they enrolled in your study).
Another important finding of the study was that prognostic indicators for all-cause DCM also predict outcome in TTNtv DCM. This means that the factors that predict outcome in DCM such as LVEF, mid wall fibrosis late gadolinium enhancement, and left atrial size, are still valid in patients with TTNtv DCM. This information is important as we move towards multi-modality risk stratification in DCM – the next part of the DCM jigsaw…
Dilated cardiomyopathy is a heart muscle disease whereby the main pumping chamber of the heart (the left ventricle) becomes bigger and weaker. It affects 1 in 250 people. In the vast majority of cases we don’t know why the heart becomes damaged in the first place. Research has found that abnormalities in a giant gene, called titin, are found in approximately 10-20% of people with the condition.
In research we recently published in the Journal of the American College of Cardiology, we found that these abnormalities in the titin gene are associated with an increased risk of abnormal heart beats (arrhythmias) in patients with dilated cardiomyopathy.
We studied 572 patients with dilated cardiomyopathy who had all had genetic sequencing (to study the titin gene) and MRI scans of the heart (to get detailed information about the structure of their hearts). We also looked at their medical records leading up to the time of their diagnosis.
We found that patients with the abnormal titin gene have a 2.90 fold increased risk of abnormal heart rhythms either before their diagnosis or in the early stages of disease (atrial fibrillation – from the top chamber of the heart or ventricular tachycardia- from the bottom chamber of the heart). Crucially, this was after taking into account all the usual factors that cause abnormal heart rhythms such as being male, being older, or having a more damaged heart to start with.
Going forwards, we now need to figure out if patients with the abnormal titin gene have a higher risk of dangerous heart rhythms in the longer term. If so, we need to know if the risk is of abnormal top chamber (atrial) or bottom chamber (ventricular) arrhythmias, or both. Finally, we also need to understand how the titin gene is linked to the electrical system of the heart. Plenty of questions to keep researchers in this area busy!