Metabolic Bone Disease – Biochemistry Flashcards by Francesca JKH

What is metabolic bone disease?

A group of disease that cause a change in bone density and bone strength by increasing bone resorption, decreasing bone formation or altering bone structure

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What are the five main metabolic bone disorders?

Primary Hyperparathyroidism 
Osteomalacia/Rickets  
Osteporosis  
Renal Osteodystrophy 
Paget’s Disease

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What are the main components of bone strength?

Mass
Material
Microarchitecture
Macroarchitectue

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When is peak bone mass reached?

Around 25 years

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When does bone mass begin to decline?

Around 40 years

NOTE: in women, the decline in bone mass accelerates after menopause

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How are microfractures repaired?

Bone remodelling

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Briefly describe the bone remodelling cycle.

A microcrack crosses the canaliculi and severs the osteocyte processes, inducing osteocyte apoptosis
This signals to the surface lining cells, which release factors to recruit cells from the blood and marrow to the remodelling compartment
Osteoclasts are generated locally and resorb the matrix and the mitrocrack
Then osteoblasts deposit new lamellar bone
Osteoblasts that become trapped in the matrix become osteocytes

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What is the normal range for serum calcium concentration?

2.15-2.56 mmol/L

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Describe the distribution of calcium.

46% plasma protein bound (albumin)
47% free calcium
7% complexes (with phosphate or citrate)

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What is the ‘corrected’ calcium level?

This compensates for changes in protein level (if proteins are high, itcompensates down)
Corrected calcium = [Ca2+] + 0.02(45-[albumin])

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Describe the effect of metabolic alkalosis on calcium distribution.

It makes more calcium bind to plasma proteins thus reducing the free calcium levels
NOTE: venous stasis may elevate free calcium

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What are the two main targets of PTH?

Kidneys

Bone

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Describe the effects of PTH in:

a. Bone
b. Kidneys

a. Bone
Acute release of available calcium (not stored in hydroxyapatite crystal form)
More chronically, increased osteoclast activity
b. Kidneys
Increased calcium reabsorption
Increased phosphate excretion
Increased stimulation of 1-alpha hydroxylase (thus increasing calcitriol production)

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Where does the PTH-mediated increase in calcium reabsorption take place in the nephron?

DISTAL convoluted tubule

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Where does the PTH-mediated increase in phosphate excretion take place in the nephron?

PROXIMAL convoluted tubule

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How many amino acids make up PTH and which part of this is active?

Active: N1-34

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What is PTH dependent on?

Magnesium

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What is the half-life of PTH?

8 mins

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What else can the PTH receptor be activated by other than PTH?

PTHrP (PTH related protein)

This is produced by some tumours

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What does the parathyroid gland use to monitor serum calcium?

Calcium-sensing receptors

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Describe the relationship between PTH level and calcium in vivo.

Steep inverse sigmoid function

NOTE: there is a minimum level of PTH release (it can’t get below this even in the case of hypercalcaemia)

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What are the causes of primary hyperparathyroidism?

Parathyroid adenoma (80%)
Parathyroid hyperplasia (20%)
Parathyroid cancer
Familial syndromes

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What biochemical results are diagnostic of primary hyperparathyroidism?

Elevated total/ionised calcium
With PTH levels frankly elevated or in the upper half of the normal range (negative feedback should drop PTH if there is hypercalcaemia)

What are the clinical features of primary hyperparathyroidism?

Stones, Bones, Abdominal Groans and Psychic Moans
Stones – renal colic, nephrocalcinosis
Bones – osteitis fibrosa cystica
Abdominal moans – dyspepsia, pancreatitis, constipation
Psychic groans – depression, impaired concentration
NOTE: patients may also suffer fractures secondary to the bone resorption
IMPORTANT NOTE: hypercalcaemia also causes diuresis (polyuria and polydipsia)

What is the main site of action of calcitriol and what effect does it have?

Small intestine – increases calcium and phosphate absorption

Describe the effects of calcitriol on bone and in the kidneys.

Facilitates PTH effect on the DCT in the kidneys (increased calcium reabsorption) Synergises with PTH in the bone to increase osteoclast activation/maturation

Which receptors/proteins are involved in mediating the effects of calcitriol on the intestines?

TRPV6 | Calbindin

What parameter is used to determine whether a patient is vitamin D deficient?

Deficient < 20 ng/M (50 nmol/L) | Normal > 30 ng/M (75 nmol/L)

What is Rickets?

Inadequate vitamin D activity leads to defective mineralisation of the cartilaginous growth plate (before a low calcium)

State some signs and symptoms of Rickets.

``` Symptoms:  Lack of play  Bone pain and tenderness (axial)  Muscle weakness (proximal) Sign:  Age dependent deformity  Myopathy  Hypotonia  Short stature  Tenderness on percussion ```

State some Vitamin D related causes of Rickets/Osteomalacia.

``` Dietary deficiency Malabsorptoin Drugs – e.g. enzyme inducers such as phenytoin Chronic renal failure Rare hereditary ```

For each of the following state whether it would be high, low ornormal in the serum of a Rickets patient: a. Calcium b. Phosphate c. Alkaline Phosphatase d. 25-OH cholecalciferol e. PTH f. URINE phosphate

``` a. Calcium Normal or Low b. Phosphate Normal or Low c. Alkaline Phosphatase High d.25-OH cholecalciferol Low e. PTH High f. URINE phosphate High ```

Other than PTH, what else can cause increased phosphate excretion?

FGF23

What effect does this factor have that is unlike PTH?

It inhibits 1 alpha-hydroxylase, thus inhibiting calcitriol production

Which cells produce this factor?

Osteoblast lineage cells

Other than Vitamin D deficiency, what else can cause Rickets/Osteomalacia?

Phosphate deficiency

State some phosphate-related conditions that cause Rickets/Osteomalacia.

X-linked Hypophosphataemic Rickets (mutation in Phex (this cleaves FGF23)) Autosomal Dominant Hypophosphataemia Rickets Oncogenic Osteomalacia (mesenchymal tumours can produce FGF23)

What can cause osteoporosis due to increased bone resorption and decreased bone formation?

Glucocorticoids

How does oestrogen deficiency lead to a decrease in bone mineral density?

It increases the number of bone remodelling units | It causes an imbalance in bone remodelling with increased bone resorption compared to bone formation

Describe the biochemistry of someone with osteoporosis.

Everything should be normal if the cause is primary

What is the single best predictor of fracture risk?

BMD

What is used to measure BMD?

DEXA scans

Which bones are used when measuring BMD and why?

Vertebral bodies  Commonest fracture  Good measure of cancellous bone  It is a highly metabolically active bone so it is quick to respond to treatment Hip – second commonest fracture NOTE: fracture risk assessment tool (FRAX) uses hip BMD

Which chains make up type 1 collagen?

2 x alpha 1 | 1 x alpha 2

What can be used as a marker of bone formation that is linked tocollagen production?

Procollagen type 1 N-terminal propeptide (P1NP)

What can be used as a measure of bone resorption that is linked to collagen production?

C-terminal telopeptide (CTX) – serum N-terminal telopeptide (NTX) – urine 3 hydroxylysine molecules on adjacent tropocollagen fibrils condense to form a pyridinium ring linkage These can be measured

After how long do bone resorption markers fall?

4-6 weeks

What are the problems with cross-linking collagen, with regards to measurement of bone markers?

Reproducibility Positive association with age Need to correct for creatinine Diurnal variation in urine markers

What bone formation marker is commonly in use?

Alkaline Phosphatase

What is it used in the diagnosis and monitoring of?

Osteomalacia Paget’s Bone Metastases

What is P1NP being used for now?

Used as a predictor of response to anabolic treatments

What are the two forms of alkaline phosphatase?

Liver | Bone

Which bone diseases will cause a rise in ALP?

Osteomalacia Bone metastases Also hyperparathyroidism and hyperthyroidism

How does alkaline phosphatase change with age?

Increases markedly during puberty reaching its highest levels Remains relatively constant following puberty (potential small rise after the age of 50)

What biochemical changes occur in renal osteodystrophy?

Increased serum phosphate | Reduction in calcitriol

Describe the sequelae of renal osteodystrophy.

Secondary hyperparathyroidism This is unsuccessful and hypocalcaemia develops This leads to excessive stimulation of the parathyroid glands, leading to parathyroid hyperplasia The parathyroid cells begin to reduce expression of calcium-sensing receptors (CSR) and Vitamin D receptors (VDR) and become autonomous (tertiary) This causes hypercalcaemia