Relationship of age and body mass index to the expression of obesity and osteoarthritis-related genes in human meniscus

Abstract

Objective: Aging and obesity contribute to the initiation and progression of osteoarthritis with little information on their relation to gene expression in joint tissues, particularly the meniscus. Here, we test the hypothesis that patient age and body mass index (BMI) correlate with the expression of osteoarthritis- and obesity-related gene signatures in the meniscus.

Design: Meniscus was obtained from patients (N=68) undergoing arthroscopic partial meniscectomy. The mRNA expression of 24 osteoarthritis-related and 4 obesity-related genes in meniscus was assessed by quantitative real-time PCR. The relationship between gene expression and patient age and BMI was analyzed using Spearman's rank-order correlation. Hierarchical cluster dendrogram and heat map were generated to study inter-gene associations.

Results: Age was negatively correlated (P<0.05) with the expression of MMP-1 (r=-0.447), NFκB2 (r=-0.361), NFκBIA (r=-0.312), IκBA (r=-0.308), IL-8 (r=-0.305), ADAMTS-4 (r=-0.294), APLN (apelin) (r=-0.250) and IL-6 (r=-0.244). Similarly, BMI was negatively correlated with the expression of APLN (r=-0.328), ACAN (r=-0.268) and MMP-1 (r=-0.261). After adjusting for the correlation between age and BMI (r=0.310; P=0.008), the only independent effect of BMI on gene expression was for APLN (r=-0.272). However, age had an independent effect on the expression on ADAMTS-4 (r=-0.253), MMP-1 (r=-0.399), IL-8 (r=-0.327), COL1A1 (r=-0.287), NFκBIA (r=-0.278), NFκB2 (r=-0.312) and IκBA (r=-0.299). The gene correlation analysis identified four clusters of potentially relevant genes: transcription factors, matrix-degrading enzymes, cytokines and chemokines, and obesity genes.

Conclusion: Age and BMI were negatively correlated with several osteoarthritis- and obesity-related genes. Although the bulk of these changes appeared to be driven by age, expression of APLN was related to BMI. Inter-gene correlation analysis implicated a common role for strongly correlated genes. Although age-related variations in gene expression appear to be more relevant than obesity-related differences for the role of the meniscus in osteoarthritis development, further investigation into the role of APLN in meniscus and joint health is warranted.

Fig. 1. Arthroscopy for partial meniscectomy

Representative arthroscopic images of a traumatic meniscal tear (A) that was removed with partial meniscectomy. The rim of healthy meniscal tissue was preserved after removal of the damaged (torn) portion (B).

Fig. 2. Correlation of gene expression of candidate genes with age through qPCR studies in patients (N=68) from which meniscal tissues were resected

The gene expression for each target gene was normalized to GAPDH. Spearman’s rank-order correlation for best fitted curve (solid line) with 95% confidence interval (dotted lines) from the curve is shown for all subjects (filled circles). The expression of MMP-1, NFκB2, NFκBIA, IκBA, IL-8, ADAMTS-4, APLN and IL-6 was significantly negatively correlated with age.

Fig. 3. Correlation of gene expression of candidate genes with BMI through qPCR studies in patients (N=68) from which meniscal tissues were resected

The gene expression for each target gene was normalized to GAPDH. Spearman’s rank-order correlation for best fitted curve (solid line) with 95% confidence interval (dotted lines) from the curve is shown for all subjects (filled circles). The expression of MMP-1, ACAN and APLN and IL-6 was significantly negatively correlated with BMI.

Fig. 4. Inter-relations of gene expression levels

The gene expression values were subjected to separate different categories for their inter-relationship. For this, hierarchical cluster dendrogram was generated using Ward Minimum Variance Method and it comprised of 28 modules with four categories. The Correlations were converted to distances between 0 and 2, where d = (1 – r).The adjacent modules are related closely in terms of expression profile and similarity in correlation fades away with the increase in distance. The heat map was then manually generated from correlation matrix to see the pairwise correlations of candidate genes. In the heat map, the dark red color shows a strong positive expression correlation while dark green color depicts weak expression correlation while the rest color codes fall in between. The two gene categories were strongly correlated (red areas surrounded by white squares) while two gene categories were weakly correlated (green areas surrounded by yellow squares). TFs = transcription factors.