Novel Aspects in the pathogenesis of Chronic Kidney Disease-Mineral Bone Disorder (CKD-MBD)

Abstract

Chronic Kidney Disease – Mineral and Bone Disorder (CKD-MBD) is a clinical entity, broadly defined by (a) disturbances in mineral metabolism, (b) abnormal bone remodeling and (c) accelerated vascular calcification. CKD-MBD is ubiquitous as kidney function deteriorates and contributes significantly to increased fracture risk and excess cardiovascular morbidity and mortality. Due to the complex and multifactorial nature of this condition, there remains many unanswered questions on how to best investigate and manage all aspects of CKD-MBD.

The general aim of this thesis was to investigate the pathophysiology, clinical outcomes and novel assessment strategies of the three domains of CKD-MBD. More specifically, this thesis aimed to 1) evaluate the outcomes of calciphylaxis in Australian patients, 2) develop a greater understanding of changes in skin and subcutaneous tissue in patients with kidney disease, 3) demonstrate the consequences of severe secondary hyperparathyroidism (SHPT) in the absence of calcimimetic treatment, 4) understand the temporal relationship between calciprotein particles and biochemical markers of CKD-MBD and 5) evaluate bone microarchitecture in patients with severe SHPT using a novel imaging modality.

Although calciphylaxis is a rare disease, it can be considered a form of accelerated vascular calcification. It predominantly affects patients with chronic kidney disease (CKD) and is associated with significant morbidity and mortality due to progressive cutaneous calcification, necrotic ulceration and infection. Clinical registries have been established to better understand risk factors, optimal treatments and disease outcomes of calciphylaxis. To better understand outcomes of Australian patients with calciphylaxis, Chapter 2 investigated the five-year outcomes from the internet-based Australian Calciphylaxis Registry. Data was recorded on patient characteristics, biochemical parameters, treatments and disease outcomes. The Australian Calciphylaxis Registry highlights risk factors for calciphylaxis including diabetes, obesity and vitamin K antagonist use, whilst significantly elevated parathyroid hormone (PTH) levels were not a consistent finding at the time of diagnosis or in the preceding 12 months in this cohort. Unfortunately, resolution of calciphylaxis remains uncommon despite multimodal therapy and mortality from calciphylaxis in the first year following diagnosis is high.

Vascular calcification is well described in large- and medium-sized vessels in patients with CKD, especially in those with end-stage kidney disease (ESKD) on dialysis. Medial calcification is particularly prevalent in this population and contributes to arterial stiffness and increased cardiovascular mortality and morbidity. Apart from in the setting of calciphylaxis, few studies have assessed skin and subcutaneous calcification and associations with abnormalities of bone and mineral metabolism in patients with CKD. In Chapter 3, patients with varying stages of CKD undergoing elective surgery underwent intraoperative incisional skin biopsy to evaluate for the histological presence of vascular calcification and upregulation of pro-calcific gene transcripts. This study reports the novel finding of dermal and subcutaneous small vessel calcification in multiple anatomical locations in 38% of patients with advanced CKD or ESKD undergoing elective surgery but free from calciphylaxis. Expression of pro-calcific gene transcripts, specifically TNAP or RUNX2, was not increased in samples from patients with CKD or those with histological evidence of vascular calcification.

SHPT in patients with CKD is associated with cardiovascular and bone pathology. Measures to achieve target PTH values and control biochemical abnormalities associated with SHPT require complex therapies, and severe SHPT often requires parathyroidectomy or the calcimimetic cinacalcet. In Australia, cinacalcet prescription, in dialysis patients was reimbursed by the government from 2009 to 2015. However, after this time funding was withdrawn following publication of the EVOLVE study, which resulted in most patients on cinacalcet ceasing therapy. Changes to reimbursement of cinacalcet in Australia provided an opportunity to assess effects of medication cessation on biochemical and clinical outcomes in dialysis patients, including changes to novel biomarkers such as calciprotein particles (CPP). CPP are nanoparticles of mineral and protein in the circulation associated with increased vascular calcification in patients with CKD. Chapter 4 focused on changes to novel biochemical markers following cinacalcet withdrawal in dialysis patients with SHPT from a single centre with CPP levels assessed at four different time points over a 12-month period. Outcomes were compared with an age- and gender-matched cohort of cinacalcet-naive dialysis patients. Cinacalcet withdrawal in the real-world setting demonstrated increases in PTH, serum calcium and the novel prognostic marker CPP over a 12-month period. Following on from work in Chapter 4, Chapter 5 involved a multi-centre retrospective study of dialysis patients who ceased cinacalcet after August 2015 at 11 Australian institutions. Clinical outcomes and changes in biochemical parameters were assessed over a 12- and 24-month period from drug cessation. Significant increases in serum PTH, calcium and alkaline phosphatase occurred over a 12-month period following withdrawal of cinacalcet, without an associated increase in cardiovascular mortality and morbidity, fractures or calciphylaxis.

SHPT in patients with CKD leads to complex bone disease, affecting both trabecular and cortical bone with resulting increased fracture risk. Optimal assessment of bone in patients with CKD is yet to be determined. High-resolution magnetic resonance imaging (MRI) can provide three-dimensional assessment of bone microarchitecture, as well as determination of mechanical strength with finite element analysis (FEA). Chapter 6 evaluated bone microarchitecture in patients with severe SHPT undergoing parathyroidectomy. Correlation of MRI findings of bone microarchitecture were made with biochemical markers. MRI in this patient population demonstrated evidence of significant changes in bone microarchitecture with trabecular deterioration, low trabecular and cortical bone volume, and reduced mechanical competence of bone with overall poor bone mechanical strength.

In summary, this thesis presents insights into the vascular, biochemical and bone domains of CKD-MBD with both clinical and basic science research focusing on pathophysiology, novel biochemical markers and imaging techniques. The novel findings obtained in this thesis have broadened the understanding of all three domains of CKD-MBD, particularly with regards to CPP as a potential novel biomarker to evaluate biochemical derangement and vascular risk, and MRI as a novel imaging modality to gain a better understanding of bone microarchitecture. Further clinical and basic science studies are required to validate findings and to explore concepts established in this thesis, however, data presented here establishes important concepts for future research in this field.