PARATHYROID, VITAMIN D AND BONE G M Kellerman Pathology North Hunter Service 30/01/2015
BIOLOGY OF BONE Bone consists of protein, polysaccharide components and mineral matrix. The mineral is hydroxylapatite, of approximate composition Ca 5 (PO 4 ) 3 (OH), with some of the OH being replaced by other ions such as F or CO 3, and some Ca being replaced by Mg or Na It is the major store of Ca and P in the body and is in dynamic equilibrium with the body fluids There are at least 3 types of bone cell: Osteoblasts lay down matrix and mineral components Osteoclasts dissolve bone both mineral and nonmineral components Osteocytes may in some cases be osteoblast precursors, but many reside in the interstices of the Haversian system and interact with each other via fine processes
BONE PROTEINS The most abundant is collagen type 1, insoluble and extensively cross linked by substituted pyridinolyl groups that arise by oxidation and linking of lysine side chains. These cross links, and also hydroxyproline peptides, are released when bone is dissolved and their excretion rate in urine is an index of bone breakdown rate Smaller quantities of several proteins are found some other types of collagen, several glycoproteins or proteoglycans, sialoproteins and 2 different carboxyglutamate containing proteins. The function of most of these proteins is unknown, but at least one the plasma level of osteocalcin (or carboxyglutamate protein 1) is considered to be a measure of osteoblast activity. Level not affected by warfarin In addition there are all the proteins associated with living cells enzymes, membrane carriers and signalling receptors
CALCIFICATION Adequate Ca, phosphate and at least the enzyme alkaline phosphatase are necessary for the formation of apatite. Initially it is likely that vesicles concentrate the ions, and the ALP hydrolyses pyrophosphate, which is an inhibitor of the precipitation. (PP is a product of the activity of a number of kinase enzymes) The precipitation initially forms CaHPO 4 which rapidly changes to apatite, and once solid apatite is formed it acts as a nucleus for further crystallisation on the collagen/proteoglycan matrix. Collagen alone, in dead tissue, often forms a site for calcification The limited access of bone salt to plasma, and the activities of all the cells with their acid secretory powers, + the ability of calcium and phosphate to form supersaturated solutions without crystallising, ensure that we are able to maintain a non-equilibrium concentration of Ca and phosphate in plasma in normal health
CONTROL OF CALCIUM - 1 Calcium balance means that the net amount absorbed from the gut equals that lost in the urine. Most of the ingested calcium is lost in the stool, but approximately 150-200 mg (4-5 mmol) is absorbed Ionised and complexed Ca are filtered at the glomerulus together about 1.3 mmol/l of glomerular filtrate, or 230 mmol/day. Of this all but 4-5 mmol must be reabsorbed by the tubules Control of these processes depends on PTH and Vitamin D. VITD is made from UV (sunlight) action on 7-dehydrocholesterol in skin, or is eaten preformed in animal products or certain microbial products. It is first hydroxylated at position 25 in liver (-> 25 hydroxycholecalciferol), then at position 1 in kidney to form 1;25 dihydroxycholecalciferol, usually known by the shorter name of calcitriol (the hydroxyl in position 3 is present in all these compounds). Calcitriol is the active molecule. It can also be made in activated macrophages, e.g. in sarcoid -> hypercalcaemia
CONTROL OF CALCIUM - 2 The link between PTH and VITD is that the 1-hydroxylase enzyme in kidney is synthesised and maintained only in the presence of PTH Calcitriol induces a carrier system in the intestine that enables the absorption of Ca, and so tends to increase plasma Ca In health, the 3-member feedback loop of plasma Ca, PTH and calcitriol serves to maintain a constant plasma Ca As plasma Ca rises, PTH secretion falls As plasma Ca rises, so the kidney filters more, and both PTH and calcitriol increase tubular reabsorption so that wastage is prevented. However, with increased PTH and Ca there is a slight increase in urine Ca, sometimes to >5 mmol/day Calcitriol stimulates calcification in precursor areas of bone ( osteoid ) while PTH stimulates reabsorption of Ca from bone. Together they achieve a balance in health
CONTROL OF PHOSPHATE There is much more phosphate in a normal diet than Ca, and most of the phosphate is absorbed. In food much is in the form of phospholipids and cellular coenzymes, which are broken down in intestine to inorganic phosphate. Phosphate is freely filtered at the glomerulus and most of it is reabsorbed of the order of 90% Excretion is stimulated (reabsorption is diminished) by the action of PTH Thus a raised PTH is usually accompanied by a low plasma phosphate, which means that the increased excretion more than balances the increased mobilisation of the bone salt which gives rise to most of the raised plasma Ca In kidney failure with low GFR, filtered P is lower and often the 1- hydroxylase is deficient in spite of high PTH, so P excretion falls and plasma P rises
BIOCHEMICAL TESTS We can measure plasma inorganic phosphate, total plasma Ca, ionised Ca (with a Ca electrode as in blood gas machine, but it MUST NOT be permitted contact with air as CO 2 is lost and ph rises). We also measure Mg; Mg deficiency inhibits PTH secretion We can measure intact PTH with a 2-site immunoassay (breakdown products of PTH are in much higher concentration in plasma and are inactive). This hormone is unstable and its assay needs precautions We can measure osteocalcin and ALP as index of osteoblast activity We can measure collagen breakdown products in urine hydroxyproline peptides on collagen-free diet, pyridinoline peptides We can measure 25-hydroxyVITD as index of availability We can measure calcitriol in special cases, but the 25hydroxyVITD is usually an adequate indicator
COMMON PROBLEMS - 1 Advancing renal failure, especially severe. There is inability to make calcitriol, so Ca falls and PTH rises. This is even more so as the P rises and that stimulates PTH secretion. The secondary hyperpth leads to bone reabsorption with weakness, fractures etc renal osteodystrophy. In many cases in late stages the parathyroids get out of control and become independently hyperplastic with even higher PTH and often need parathyroidectomy. Therapy aims at enough calcitriol dose to maintain Ca slightly above normal, to suppress PTH, and phosphate binders to diminish absorption in gut Hypovitaminosis D leads to childhood rickets and adult osteomalacia. Indoor living, air pollution, rigorous covering of exposed skin (religious, cultural, sun cancer scare) mean that this is relatively common especially in the old and at high latitudes but even in Newcastle at 33ºS many people are borderline or low. VITD low, Ca normal or low, P low, PTH variably raised
COMMON PROBLEMS - 2 Hyperparathyroidism about 1:1000 of population but most are mild. Diagnose by raised ionised Ca, low P and raised PTH Hypoparathyroidism is now very rare, but it used to be a not infrequent complication of poor surgery of thyroid gland. Anti-thyroid drugs and radio-iodine treatment have stopped this. Ca low, P high Metastatic bone disease, or bone marrow malignancy especially myeloma. Some seem to do it by direct invasion, some excrete cytokines, some excrete PTHrP (a fetal hormone that improves Ca uptake through placenta and can be derepressed in tumours). This peptide can be assayed by special laboratories. In these cases Ca is high, P is variable and PTH is usually suppressed Hyperthyroidism all metabolism is faster but bone breakdown > synthesis. Ca may be quite high, and pyridinolines are high
COMMON PROBLEMS - 3 Hypercortisolism either from adrenal tumour, pituitary tumour, ectopic ACTH secretion or most commonly chronically administered steroid for its anti-inflammatory and immunosuppressive activities. Cortisol inhibits VITD activation, inhibits Ca absorption in gut and inhibits osteoblast activity. No regular pattern of test results, except bone breakdown products in urine usually increase Intestinal malabsorption syndromes such as coeliac disease etc. The fat soluble VITD is not absorbed (if taken) and Ca is not well absorbed in the presence of steatorrhoea. VITD low, Ca low Hypervitaminosis D is rare, usually from self medication. However, prescribed calcitriol must be carefully controlled as it readily leads to an increased Ca if the dose is too large and needs can vary over time in a given patient. Ca high, P low, PTH low unless independent
OSTEOPOROSIS This is the commonest and most intractable of all bone diseases. Unfortunately the laboratory findings are of little help as in the absence of other complications such as simultaneous osteomalacia or inappropriate therapy, they are all normal. Osteocalcin has been a disappointment, and bone breakdown products are often within the rather wide normal range. We cannot assess oestrogen replacement levels as most of the commonly used synthetics do not register on our immunoassays. Therefore this is a clinical problem, and the major assessment (bone mineral density studies by dual wavelength X-ray absorption) is in the radiologist s domain We can certainly judge excessive bisphosphonate dosage by a falling Ca below normal, but this is most frequent with malignancy
ALKALINE PHOSPHATASE As mentioned earlier, this enzyme is needed for proper calcification. In rare inborn errors it is missing and bony development is poor or absent With increased bone turnover this enzyme is raised. We usually measure total ALP, but the bone isoenzyme can be specifically measured if essential but it is tedious. Total ALP is usually enough in absence of cholestasis when liver ALP is raised Levels are normally raised in infants, raised less in children, then increase in the pubertal growth spurt, or following fractures especially of large bones High levels are found in Paget s disease, especially if it is extensive, and pyridinoline levels are also raised due to the increased bone turnover. Both fall as the disease is controlled (bisphosphonate)
RANKL AND ANTI-RANKL I apologise for mentioning a substance still at the research stage, but two articles in NEJM (Vol 361, 20 Aug 2009, pp745-765) make it interesting. See Nature 2003, Vol 423, pp337-342 for the basis RANKL stands for Receptor Activator of Nuclear Factor-KappaB Ligand, and this is a cytokine that is essential for the function and survival of osteoclasts. Inhibition of this factor will therefore stop bone reabsorption and in theory permit the osteoblasts to lay down more bone hopefully in a strengthening way and not brittle Inhibition could therefore act in the same way as bisphosphonates without some of their side effects especially oesophageal It is too early to say whether inhibition of osteoclasts in this way also has undesirable side effects such as jaw necrosis etc