Background: Investigation of the prognostic value of the expression of mRNA for extracellular matrix (ECM) components with respect to deterioration of kidney function in patients with renal disease requires an evaluation of the basal expression of ECM mRNA in healthy individuals and of the reliability of ECM mRNA measurements. In the current study, the collagen alpha 1(IV)/GAPDH (C4:G) and collagen alpha 1(I)/GAPDH (C1:G) mRNA ratios and the accumulation of collagen IV and collagen I protein were investigated in renal cortices of individuals of various age. Furthermore, we examined whether the C4:G mRNA ratio measured in a renal biopsy is representative of that in the rest of the kidney.
Methods: To investigate the effect of age on collagen expression, kidneys obtained at autopsy from patients with a normal renal function (N = 18; age 19 to 92) were used. C4:G and C1:G mRNA ratios were measured by real-time polymerase chain reaction (PCR) analysis. Accumulation of collagen IV and collagen I protein was measured by quantitative image analysis on immunohistochemically stained sections. To determine whether the site at which a biopsy is taken affects the C4:G mRNA ratio, this ratio was measured in cortical biopsies taken from different locations from each of four kidneys: one without renal disease, one with diabetes mellitus type I, and two with diabetes mellitus type II. C4:G mRNA ratios were measured by using real-time PCR.
Results: The C4:G mRNA ratio, but not the C1:G mRNA ratio or collagen IV protein accumulation, increased significantly with age (r = 0.55, P < 0.03). Collagen I protein accumulation increased with age (r = 0.85, P < 0.001) and correlated with the extent of interstitial fibrosis (r = 0.50, P < 0.05). The C4:G mRNA ratio did not differ significantly within a kidney.
Conclusions: This report shows, to our knowledge for the first time, that in the aging, normally functioning human kidney, there is a dissociation between the levels of mRNA for collagen IV and collagen I and the accumulation of these proteins. The levels of mRNA for collagen IV in a single renal biopsy can be regarded as representative of those in the rest of the kidney. These observations should be taken into account when ECM mRNA levels are used for diagnostic purposes.