# Novel involvement of LMTK2 and EML6 in rheumatoid arthritis: potential biomarkers for disease activity and seronegative patients

## TL;DR

This study identifies LMTK2 and EML6 as potential biomarkers for rheumatoid arthritis, particularly in patients with low antibody levels and varying disease activity.

## Contribution

The study introduces LMTK2 and EML6 as novel RNA-based biomarkers for rheumatoid arthritis disease activity and seronegative patients.

## Key findings

- LMTK2 LINOUT expression decreases with higher disease activity and distinguishes RF-negative RA patients from healthy controls.
- EML6 LINOUT expression increases with disease activity and complements traditional markers like ESR for improved diagnostic accuracy.
- Combining LMTK2 and EML6 LINOUT with ESR achieves high diagnostic accuracy (AUC = 0.923).

## Abstract

Rheumatoid arthritis (RA) is a chronic, multifactorial autoimmune disease characterized by symmetrical polyarthritis, joint pain, and swelling, which can lead to disability and premature death. Increasing attention has focused on epigenetic mechanisms, including non-coding RNAs in circular and linear forms, in RA pathogenesis.

This study aimed to identify novel supportive RNA-based biomarkers associated with RA disease activity.

The discovery cohort included 18 RA patients and 10 healthy controls (HCs). Peripheral blood mononuclear cells were analyzed using targeted RNA sequencing, encompassing both linear (LINOUT) and circular (CIRC) RNA forms, to assess differences between RA patients and HCs, as well as between high (HDA; DAS28 > 5.1; n = 10) and low disease activity/remission (LDA/REM; DAS28 ≤ 3.2; n = 8) disease activity groups. The results were validated in cohort of 45 patients with RA, divided into a high disease activity group (HDA; DAS28 > 5.1; n = 22) and a non-high disease activity group (non-HDA; DAS28 ≤ 5.1; n = 23) along with 24 control subjects, using quantitative PCR (qPCR).

LMTK2 LINOUT expression correlated negatively with disease activity
(rs = -0.30) and distinguished RF-negative patients (n = 17) from HCs (p = 0.027). Expression was significantly lower in the high activity group (n = 22) versus the non-high activity group (n = 23; p = 0.022). Post-hoc ANOVA showed significant differences among HDA, non-HDA, and HCs (p = 0.001), with reduced expression in HDA versus non-HDA (p = 0.038) and increased expression in non-HDA versus controls (p = 0.001). EML6 LINOUT expression exhibited activity-dependent changes (p = 0.034) and positively correlated with disease activity
(rs = 0.303). Integration with erythrocyte sedimentation rate (ESR) improved discriminative performance. Combining LMTK2 LINOUT + EML6 LINOUT + ESR yielded the highest accuracy (AUC = 0.923).

LMTK2 and EML6 show disease activity-dependent expression in RA and provide complementary information to conventional inflammatory parameters such as ESR. Their integration may improve diagnostic performance, highlighting their potential as novel supportive molecular biomarkers in RA.