Bone metabolites

Question 1

Are PTH fragments produced intracellularly?

Answer 1

Yes, in chief cells, under the influence of high calcium levels, PTH will degrade into fragments within secretory vesicles

Question 2

What is the mechanism of Vit D downregulation of PTH secretion?

Answer 2

1,25 OHD inhibits PTH gene expression

Question 3

What is the mechanism of FGF-23 downregulation of PTH secretion?

Answer 3

Downregulation of PTH gene expression and release of secretory vesicles

Question 4

What is the mechanism of calcium downregulation of PTH secretion?

Answer 4

Reduced gene expression Reduced release of secretory vesicles
Enhanced PTH degradation to fragments within secretory vesicles

Question 5

Are preproPTH or proPTH detectable in serum?

Answer 5

No

Question 6

Which is active N-terminal or C-terminal of PTH? Where does it act?

Answer 6

N-terminal binds to Type 1 PTH/PTHrp receptor

Question 7

Active and inactive forms of PTH and their half lives?

Answer 7

Active PTH is full 1-84 aa peptide hormone and half life is 2 minutes.
Inactive forms are non(7-84) and C-terminal PTH fragments, with half lives of 20-30 minutes
Other forms include oxidised PTH (in dialysis pts and other pts undergoing strong oxidative stress) and amino PTH. The latter is active and seen in parathyroid carcinoma and hyperplasia in CKD pts.

Question 8

Define 2nd generation PTH assay

Answer 8

AKA “intact PTH assay”. Has an antibody that is raised against an epitope within the first 34 aas of PTH.
Reacts to 1-84 and 7-84PTH

Question 9

Ways to assess renal vs GI losses of Mg

Answer 9

1. FEMg
2. 24 hour urine Mg
3. Mg loading test (30-40mmol Mg infused over 7-8 hrs with 24 hour urine collection from start of infusion)

Question 10

Pre-analytical advantages of P1NP

Answer 10

1. Low within-subject biological variability
2. Negligible affect of food intake or circadian rhythm, allowing non-fasting sampling at any time of the day
3. Measurements equivalent in serum and plasma
4. Stable in serum and EDTA plasma for 5 days at 4degC

Question 11

Disadvantages of urine bone turnover markers

Answer 11

1. High biological variability of urine samples
2. Creatinine correction for dilution may accentuate inter-individual variation due to differences in muscle mass

Question 12

3 commercial immunoassays available for P1NP?

Answer 12

1. Roche chemiluminescence immunoassay
2. IDS iSYS chemiluminescence immunoassay
3. Orion Diagnostical radioimmunoassay

Question 13

Disadvantage of the Roche P1NP assay compared to the others?

Answer 13

“Total” P1NP assay detects both intact P1NP and P1NP fragments. P1NP fragments accumulate in renal disease. Therefore Roche assay can give misleadingly high results in renal failure.
However, in patients with normal renal function, the IDS iSYS and Roche Cobas assays produce similar enough results that RIs are now harmonised across Australia for P1NP

Question 14

Indications for P1NP measurement

Answer 14

1. Fracture risk assessment
2. Monitoring of osteoporosis therapy
3. Paget’s disease of bone
4. Mineral bone disease of chronic kidney disease

Question 15

How is P1NP useful in fracture risk assessment?

Answer 15

There is a modest but significant association of P1NP with fracture risk, HR 1.23 (1.09-1.39) per SD of P1NP for both men and women. However, this was unadjusted for BMD and therefore currently there is no evidence for P1NP as an independent marker of fracture risk. No BTMs are currently included in fracture risk scores as a result.

Question 16

How is P1NP useful in monitoring osteoporosis therapy?

Answer 16

Useful for assessing efficacy, adherence and persistence with treatment, long before a LSC in BMD results would be appreciable.
In antiresorptive therapy, bone resorption increases first, followed by P1NP and other markers of bone formation.
In anabolic therapy, bone formation increases first, followed by resorption.
P1NP has shown to perform well for monitoring both antiresorptive and anabolic therapy.
Utility for monitoring new dual action agents eg romosozumab remain to be ascertained, however, it is likely that an initial transient increase in P1NP and more sustained decrease in CTX may be useful markers of effective uncoupling and efficacy of romosozumab.
Short-term, antiresorptive therapy-related changes in ALP and P1NP strongly predict vertebral fracture treatment efficacy.
Effectiveness of therapy initiated after denosumab to prevent rebound fracture can be monitored using P1NP or CTX aiming for < premenopausal median.
Offset of bisphosphonate action after a drug holiday can be monitored with P1NP or CTX - an increase to above the LSC or premenopausal median could be used as an indication to reinitiate therapy.

Question 17

How are BTMs useful for diagnosing/monitoring Paget’s disease?

Answer 17

The rise in ALP in Paget’s is usually sensitive enough for diagnosis and for monitoring of recurrence after treatment.
Aim of treatment is to maintain ALP/P1NP in the lower half of the reference interval.
Pts in remission are monitored with 6 monthly BTMs.
Recurrence is diagnosed with an increase in ALP/P1NP by their LSC.
In monostotic Pagets, ALP may not be sensitive enough - P1NP can be useful in this setting.
Urine NTx is an alternative or addition to the bone formation markers.

Question 18

Utility of P1NP in mineral bone disease of chronic kidney disease?

Answer 18

Total P1NP assays have limited utility due to accumulation in renal failure. However, many BTMs suffer from the same flaw and hence are not recommended for use in CKD-MBD. Bone-specific ALP is one exception. The other marker that is recommended to be monitored is PTH.

However, intact P1NP could be useful as it is not increased in renal failure. Further study is required.

Question 19

What is P1NP?

Answer 19

Procollagen type 1 N pro-peptide

Question 20

What is P1NP a marker of and why?

Answer 20

Bone formation. Procollagen is synthesised by osteoblasts and P1NP is cleaved off when collagen is laid down to form the new osteoid matrix during bone formation.

Question 21

Where is P1NP metabolised?

Answer 21

In the liver and the fragments produced are inactive and excreted by the kidney.

Question 22

Gold standard for assessment of bone turnover?

Answer 22

Bone histomorphometry

Question 23

Effect of glucocorticoids on P1NP

Answer 23

Reduces P1NP

Question 24

How does hypomagnesaemia cause hypoparathyroidism?

Answer 24

Hypomagnesaemia results in failure of chief cells to release PTH.
Hypocalcaemia will not resolve unless hypomagnesaemia treated.

Question 25

Types and locations of phosphate in the body?

Answer 25

Phosphate exists as organic and inorganic phosphate. Only inorganic phosphate in plasma is measured. Inorganic phosphate in plasma exists as either HPO4(2-) or H2PO4(-) anions, bound to protein (10%), other cations (35%) or free (55%). The ratio of H2PO4(-) to HPO4(2-) is pH dependent. The more acidic the solution, the more H2PO4(-) there is. The more basic the solution, the more HPO4(2-) there is.
Inorganic phosphate exists in bone as part of hydroxyapatite and organic phosphate exists in soft tissues and cellular components of blood.

Question 26

Payne formula for calculating adjusted calcium?

Answer 26

adjusted calcium (mmol/L) = total calcium (mmol/L) + 0.02x(40 - serumalb (g/L))