Anti-citrullinated protein antibodies (ACPA), at the heart of both rheumatoid arthritis and cardiovascular disease? The statistical analysis plan

Statistical analysis plan for our study regarding anti-citrullinated protein antibodies in patients with coronary heart disease, without concomitant rheumatoid arthritis. Drafted and published before the start of data analysis. 

Abstract

Background: Rheumatoid arthritis (RA) is a prevalent autoimmune disease leading to joint damage, disability, and increased mortality. Approximately 69% (95% confidence interval (CI) 39 to 94%) of patients with RA are positive for anti-citrullinated protein antibodies (ACPA). ACPA are very specific for RA (specificity of 97% (range 81–100%)) and are seen as markers for underlying autoimmune pathophysiology. ACPA is detected in approximately 1% of healthy individuals. Most risk factors known for RA, such as smoking, or the presence of HLA shared epitope alleles are exclusively associated with autoantibody-positive disease. ACPA-positive RA patients have increased cardiovascular mortality. ACPA have been described in 10% of patients with coronary heart disease (CHD) without concomitant RA, a much higher prevalence than would be expected. Furthermore, ACPA were independently associated with poor cardiovascular outcomes and significantly increased mortality in these patients.

Objectives: To investigate the ACPA response in CHD by examining both the clinical phenotype of ACPA-positive CHD patients and the immunological characteristics of the ACPA in these patients. These findings will be compared to the existing data on the ACPA response in RA patients to gain more insight in the mechanisms underlying ACPA development and the specific ACPA features important in the pathophysiology of both diseases.

Part 1 ACPA in coronary heart disease

1. How many and which patients with CHD are ACPA-positive? Can we replicate the findings that around 10% of patients with CHD without concomitant RA is ACPA-positive?

Part 2 Clinical phenotype, risk factors and disease outcomes

1. Do ACPA-positive CHD patients differ from CHD-patients without ACPA regarding clinical phenotype and risk factors, such as female sex, smoking and low-grade inflammation?
2. Do ACPA-positive CHD patients differ from CHD-patients without ACPA regarding disease outcomes, such as cardiovascular disease and all-cause mortality?
3. Is ACPA-positivity in CHD associated with the same risk factors and clinical outcomes as ACPA-positivity in RA?

Part 3 Immunological characteristics of ACPA

1. What are the distinct immunological characteristics of ACPA in ACPA-positive CHD patients? We will focus on characteristics such as isotype usage, fine-specificity, avidity and glycosylation.
2. Do the ACPA features in CHD differ from the ACPA features found in RA patients?
3. Are certain ACPA features associated with clinical (CHD) outcomes, such as cardiovascular disease and all-cause mortality, in patients with and without RA?

Methods: The specific patient populations and methods that will be used for each objective are stated in the table below. Stored samples and data from the following studies will be used: the clarithromycin for patients with stable coronary heart disease (CLARICOR) trial cohort containing CHD patients without RA ; the Leiden Early Arthritis Clinic (EAC) cohort containing RA patients ; and a cohort of healthy controls. The findings in the CLARICOR trial will be replicated in another CHD cohort: the LUdwigshafen Risk and Cardiovascular Health (LURIC) study . The flow of the analyses will be as following: First, we will determine the percentage of ACPA positive patients in the CLARICOR and LURIC studies. If the prevalence of ACPA is indeed increased in those patients compared to healthy donors, we will proceed with part 2 and 3.

Discussion: ACPA are considered specific for rheumatoid arthritis. The findings that ACPA can also be present in patients with coronary heart disease offers a unique opportunity to learn more about pathophysiology of both diseases. With this study we aim to perform an investigation of the ACPA response in coronary heart disease (CHD) by examining both the clinical phenotype, risk factors and disease outcomes of ACPA-positive CHD patients and the immunological characteristics of the ACPA. By investigating whether ACPA-positivity in CHD is associated with the same risk factors, clinical outcomes and ACPA characteristics as ACPA in RA patients, we hope to gain more insight in the mechanisms underlying ACPA development and the specific ACPA features important in the pathophysiology of both diseases.