The risk of bone fracture and subsequent high morbidity increases with the progression of type 2 diabetes (T2D), and the clinical assessment of bone mineral density (BMD) is not particularly effective in diagnosing this risk. Moreover, lowering fracture risk among patients with T2D requires an understanding of the changes in the bone that affect fracture resistance. Addressing these unmet needs, the proposed project aims i) to determine the primary biomechanical reason for the increase in fracture risk with T2D, ii) to identify molecular changes in the bone matrix that can significantly affect fracture resistance, and iii) to ascertain the ability of matrix-sensitive tools to assess significant differences in bone quality between age-matched non-diabetics and diabetic individuals.
In Aim 1, cadaveric bone from non-diabetic and T2D donors matching for age and gender will be comprehensively analyzed to determine whether the primary T2D-related decrease in fracture resistance is lower bone toughness, lower fatigue resistance, and/or lower fracture toughness as opposed to lower strength at the whole-bone- and material-level. Moreover, the ability of matrix characteristics to explain T2D-related differences in these mechanical properties of bone will be assessed in relation to volumetric BMD and micro-structure (assessed by micro-computed tomography) as well as the degree of mineralization and osteonal density (assessed backscattered electron imaging). Matrix characteristics will include secondary structure of collagen I as determined by sub-peak ratios of the Amide I band from Raman spectroscopy (RS), bound and pore water volume fractions as determined by proton nuclear magnetic resonance (NMR) relaxometry, and resistance to microindentation properties as determined by both cyclic and impact reference point (RPI).
In Aim 2, the focus will be on the contribution of novel molecular mechanisms to the underlying diabetic changes in matrix-bound water and collagen structure. Specifically, emphasis will be placed on post-translational modifications (PTMs) of collagen I and non-collagenous bone matrix proteins as a mechanism that affects hydrogen bonding between water and the matrix. Implicated in diabetes, these PTMs will include non-enzymatic advanced glycation end products (AGEs) as well as enzymatic hydroxylation, glycosylation, and carboxylation.
In Aim 3, matrix-sensitive, clinical techniques will determine whether bone quality is significantly different between patients without diabetes and patients with established T2D. By matching age and BMD-derived T-scores between the groups, we will ascertain whether bone material strength index from impact RPI, bound and pore water concentrations from ultra-short time-to-echo magnetic resonance imaging (UTE-MRI) of NMR signals (T2), and Amide I sub-peak ratios from spatially offset, transcutaneous RS, all acquired at the tibia mid-shaft, will potentially add value to the clinical assessment of fracture risk. The completion of the Aims of this proposal will lay the foundation for use of matrix-sensitive tools in clinical diagnostics of diabetic bone disease and potentially identify specific targets within the bone matrix for lowering fracture risk among diabetics.
The elevated fracture risk among those with type 2 diabetes is not necessarily a problem of low bone mineral density and strength, the basis for current clinical diagnosis of osteoporosis. The proposed research therefore assesses whether clinically viable, matrix-sensitive techniques can meaningfully assess diabetes-related differences in bone. Moreover, underlying molecular modifications to the matrix that contribute to reduced fracture resistance will potentially be identified. In doing so, this research could provide new targets for improving bone quality as well as new diagnostic techniques for improved fracture risk prediction among individuals with type 2 diabetes.