MBD - Biochemistry Flashcards
(66 cards)
1
Q
What are the two groups of symptoms in MBD?
A
Metabolis symptoms:
- Hypocalcaemia
- Hypercalcaemia
- Hypo/Hyperphosphataemia
Specific to bone
- Bone pain
- Deformity
Fractures
2
Q
What is caclium incorporated as in bone?
A
3
Q
What type of bone is metabolically active? How much remodelling occurs in bone at any time?
A
Cancellous bone is metabolically active and 5% is remodelling at any one time. total skeleton could be remodelled over 5 years.
There is also continuous exchange of ECF with bone fluid reserve.
4
Q
What makes a bone strong?
A
- Mass
- Material properties (matrix and mineral) - collagen, crosslinking, woven vs lamerllar, microcracks
- Microarchitecture - trabecular thickness, connectivity, cortical porosity.
- Macroarchitecture - hip axis length, diameter
5
Q
Describe the age related changes in bone mass.
A
- Peak bone mass in mid 20s
- Stable until around 40
- Men slow loss
- Women fast loss in early menopause
6
Q
How does exercise increase peak bone mass?
A
There is change in bone mass density and geometry.
7
Q
How does the tibia change during bone modelling from child to adult?
A
- More bone is placed anteriorly and posteriorly - this helps optomise deposition as not to waste mass
- Increase in bending strength ratio
- Periosteal apposition - essentially only when we are young. This is deposition of bone on the periosteal/outside surface. This reduces the elasticity of thebone.
8
Q
Describe the sexual dimorphism in bone growth.
A
- This is due to sex hormones
- Estrogens limit periosteal bone expansion but stimulate endosteal bone apposition in females, whereas androgens stimulate radial bone expansion in males.
- Men reach higher peak bone mass, greater bone size, develop a stronger skeleton than women.
- However ther skeleton is not denser as bone mineral acquisition is in proportion to the volume of the bone, so volumetric bone density does NOT differ.
9
Q
Describe the bone remodelilng cycle.
A
- Activation occurs - microcrack crosses cannaliculi, severing osteocyte processes causing osteocytic apoptosis.
- Connected surface lining cells (of osteoblastic lineage) sense this and release local factors to attract blood and marrow into the remodelling compartment
- Osteoclasts are generated locally to allow the resorption process to start. They resorb matrix and offending microcrack
- Successive teams of osteoblasts deposit new lamellar bone
- Osteoblats that are trapped in the matrix beocme osteocytes , others die or form new flattened osteoblast lining cells.
10
Q
What investigations would you typically do if you suspect bone disease?
A
Serum:
Bone profile
- Caclium
- Corrected calcium (albumin)
- Phosphate
- Alkaline phostaphatase
Renal function: creatinine
- PTH
- 25-hydroxy vitamin D
Urine:
- Clacium/phosphate.
- NTX
11
Q
Are the levels of Ca, P and Alk P high, normal or low in bone diseases:
- Osteoporosis
- Osteomalacia
- Pagets
A
12
Q
Are the levels of Ca, P and Alk P high, normal or low in bone diseases:
- Primary HPT
- Renal dystrophy
- Metastases?
A
13
Q
Describe the 3 systems involved in calcium balance.
A
GUT whats comig in; 1g day recommended intake
Kidney whats going out
BONE flux; your compensatory mechanism. This is not an inert tissue. This is where Ca is most abudant - we havea bout 1kg of Ca in bone.
14
Q
What is meant by “total calcium”?
A
- This is when we measure calcium not just “free” in the active form, but also protein bound(to albumin) and complexed to citrate and proteins.
- So the calcium result is corrected in the laboratory to complensate for protein levels. If protein levels are high they compensate down (0.02 for each g/L of albumin).
- In acid-base disturbance/ alkalosis, there is more Ca binding to proteins so free levels drop. Happens when we get tingling.
Done using the following equation:
corrected calcium = [calcium] + 0.02 (45 - [albumin])
15
Q
What are the percentages of protein-bound, complexed and free calcium in blood normally? What is the normal range for total calcium?
A
46% protein bound (but more in alkalosis)
47% FREE ionised (less in alkalosis)
7% complexed (more if high protein)
2.15-2.56mmol/L
16
Q
How does PTH regulate Ca levels?
A
- Secretion of PTH when Ca levels drop
- This acts on 2 systems:
- BONE - acute release of Ca, due to increased resoprtion of bone by osteoclasts.
- Kidney -
- increased Ca re-absorption in the distal convoluted tubule (only place where Ca is absorbed under hormonal control)
- Increases phosphate excretion by inhibiting the NA-P cotransporter - his increases free Ca as usually P forms insoluble salts with Ca
- Increased conversion of 25-hydroxy Vitamin D into 1,25-hydroxy vitamin D (calcitriol) to increase absorption of Ca in the intestines.
17
Q
What is the cause of hypocalcaemia in alcoholics?
A
Low magnesium causing low PTH and hypocalcaemia.
18
Q
What is the half life of PTH? What does this allow in the lcinical setting?
A
half life (T1/2) = 8min
allows intraoperative testing - if PTH has been too high due to a tumour then surgically removing the tumour should lower PTH levels. PTH serum levels can be measured instantly during the operation.
19
Q
Describe the molecular structure of PTH.
A
84 amino acid compound (1-84)
N1-34 (drug) is used for PTH replacement and is also used to build bone. It is more active.
20
Q
Which ion apart from Calcium, is PTH dependant on?
A
Magnesium
21
Q
What is the PTH receptor also activated by?
A
PTHrP (which is produced by some, so hypercalcaemia may be the first presenting feature)
22
Q
How does the parathyroid gland monitor serum Ca?
A
- Through Ca-sensing receptors
- Even at high Ca there is base-line PTH secretion - e.g. a hypercalcaemia of malignancy there will still be detectable PTH
- But a small change in Ca causes a large change in PTH
23
Q
Decribe how PTH drives active Ca absorption in the distal tubule of the kidney.
A
- Epithelia can absorb Ca2+ by paracellular and transcellular transport. Passive and paracellular Ca2+ transport takes place across the tight junctions and is driven by the electrochemical gradient for Ca2+ (blue arrow).
- The active form of vitamin D (1,25-(OH)2D3) stimulates the individual steps of transcellular Ca2+ transport by increasing the expression levels of the luminal Ca2+channels, calbindins, and the extrusion systems.
- Active and transcellular Ca2+ transport is carried out as a three-step process. After entry of Ca2+ through the (hetero)tetrameric epithelial Ca2+ channels, TRPV5 and TRPV6, Ca2+ bound to calbindin diffuses to the basolateral membrane.
- At the basolateral membrane, Ca2+ is extruded via an ATP-dependent Ca2+-ATPase (PMCA1b) and a Na+/Ca2+ exchanger (NCX1).
- In this way, there is net Ca2+ absorption from the luminal space to the extracellular compartment.
24
Q
How does PTH cause bone resoprtion through the RANK system?
A
- PTH stimulate osteoblasts by binding to incease their expression of RANKL and inhibit their secretion of OPg (osteoprotegerin)
- Free OPG usually competitively binds to RANKL as a decoy transport, preventing RANKL from interacting with its receptor “RANK”. Decreased OPG allows RANKL binding to RANK.
- This stimulates osteoclast precursors to fuse forming new osteoclasts which ultimately enhances bone resorption.
25
How common is hyperparathyroidism?
2% of post menopausal women develop it
Usually in 50s
Usually women 3:1 ration to men .
26
What are the most common causes of primary hyperparathyroidism?
Parathyroid adenoma - 80%
Parathyroid - hyperplasia 20%
Parathyroid CA \<1%
Familial syndromes: MEN 1 (2%), MEN 2A (rare), HPT-IT (rare) .
27
How is primary hyperparathyroidism diagnosed/ what rare the biochemical findings?
* Elevated total/ionised calcium with PTH levels elevated (in the upper half of the normal range)
* Corrected calcium \>2.6mmol/L
* PTH \>3.9 pmol/l (normal: 1.0-6.8)
1. Increased serum calcium by absorption from bone/gut
2. Decreased serum phosphate renal excretion in proximal tubule
3. PTH in the upper half of the normal range or elevated
4. Increased urine calcium excretion
5. Cr may be elevated
28
What are the common clinical features of primary HPT?
Features usually as a result of the high calcium
Clinical features: stone, abdominal moans, psychic groans.
* Thirst, polyuria
* Tiredness, fatigue, muscle weakness
* Renal colic, nephrocalcinosis, CRF
* Dyspepsia, pancreatitisConstipation, nausea, anorexia
* Depression, impaired concentration
* Drowsy, coma
* Fractures secondary to bone resorption
29
What effect will high serum calcium have on the kidney?
* Ca sensory receptor shuts down when Ca is too high
* You no longer recycle calcium by the countercurrent mechanism
* **It acts as a diuretic**
* Serum Ca above 3 causes this and so you get more and more dehydrated
30
Why should under 50s have an operation to remove the parathyroid in hyperparathyroidism?
If Ca stays too high for long periods of time it will cause calcification of the vasculature and damage the kidneys
31
What risks are associated with chronically high PTH?
* Increased risk of stones in kidneys - due to diuresis causing hypercalcuria
* Subperiosteal bone resorption - cortical\>cancellous
* Increased fracture risk
32
Why are rickets/osteomalacia becoming common again?
80% of our Vitamin D comes from conversion of dehydrocholesterol by UV light to cholecalciferol
Also mean sources: Vit D3
Vegetable sources: Vit D2
People do not get enough sunlight. People eat less meat.
33
Summarise the metabolism of Vitamin D.
Vitamin D is metabolised by the liver and kidney.
Dehydrocholesterol gets converted to vitamin D by UV light. It can be broken down if there is too much of it. It can also come from dietary sources e.g. vegetables (D2) and meat (D3).
The half life is long (3 days) so levels remain stable. Levels of protein bound vitamin D (Vit D binding proteinDBP) can be measured. Vit D is filtered by the kidney.
It becomes deactivated in forms 24,25.
Vitamin D becomes activated when Ca or P levels are low. Vitamin D is converted in the proximal tubule of the kidneys and the activated form acts on...
* ... the small intestine - 1,25(OH)2Vitamin D activates Ca and P absorption;
* ...bone - synergises with PTH to increase osteoclastic osteolysis, differentiation agent for osteoclast precursors
* ...kidney - facilitates PTH action in distal t. to increase Ca reabsorption
34
What are the effects of vitamin D on calcium regulation?
* ... the small intestine - 1,25(OH)2Vitamin D activates Ca and P absorption;
* ...bone - synergises with PTH to increase osteoclastic osteolysis, differentiation agent for osteoclast precursors
* ...kidney - facilitates PTH action in distal t. to increase Ca reabsorption
35
How does Vitamin D affect gut Ca absorption?
Increases Ca absorption in the duodenum, jejunum and colon.
???
20-60% load duodenum, jejenum also colon.
Passive = paracellular linear
Active = up to 40% saturable duodenum 1,25Vit D
1,25(OH)2VitD activates Ca and P absorption from dodenume (through TRPV6, calbindin)
36
What is the definition of vitamin D deficiency?
30-50% of European and US population are vitamin D insufficient or deficient with levels of 10-55ng/ml
PTH levels start to platea at their nadir when 25(OH)D levels are between 30-40ng/mL.
**Deficiency should be defined as 25(OH)D of 21-29ng/ml (50-72.5nmol/L)**
Preferred level of Vit D is now \>30ng/mL (75nmol/l)
37
What would VitD levels have to be to cause intoxication?
\>150-200ng/mL
Not even lifeguards have ever been recorded to get vit D intoxication (even though their VitD levels are often 100-125ng/mL)
38
Define rickets.
Inadequate Vit D activity leading to a defective mineralisation of the cartilagenous growth plate (before a low calcium)
39
What are the signs and symptoms of rickets in children?
An imporant sign is that they will not want to play in school.
Symptoms
* Bone pain and tenderness (axial)
* Muscle weakness (proximal)
* Lack of play
Signs
* Age dependent deformity
* Myopathy
* Hypotonia
* Short stature
* Tenderness on percussion
40
What are the common causes of rickets/osteomalacia?
**Usually vitamin D related.**
* Dietary
* Gastrointestinal - small bowel malabsorption/bypass, pancreatc insufficiency, liver/biliary disturbance, drugs such as phenytoin, phenobarbitone.
* Renal - chronic renal failure
* Rare hereditary - vitamin D dependent rickets: type I (deficiency of 1alpha hydroxylase) and type II (defective VDR(vit D receptor) for calcitriol)
41
What is the biochemistry in rickets/osteomalacia?
Serum :
* Calcium - N/low
* Phosphate - N/low
* Alkaline phosphate - High
* 25(OH)VitD - Low
* PTH - High (secondarily to compensate)
Urine
* Phosphate - high
* ?Glycosuria, aminoaciduria, high pH, proteinuria.
42
Desribe how FGF-23 can cause PCT phosphate loss. What are the similarities and differences between it and PTH?
* FGF-23 is a hormone (32KD protein) produced by osteoblasts
* FGF-23, like PTH, causes phosphate loss.
* Unlike PTH, FGF-23 inhibits activation of Vit D by 1-alpha-hydroxylase when PTH levels are low.
* When Ca levels are low, PTH rises to increase reabsorption of Ca which with it causes phosphate reabsorption.
* When there is sufficient Ca absorbed, phosphate is probably in excess due to Ca absorption. To get rid of this, negative feedback to the PTH means that PTH cannot help. But FGF-23 can deactivate VitD to cause phosphate loss in the kidneys.
(NB phosphate is usually fully fiilteres and only reabsorbed in the PCT)
43
How can FGF-23 lead to rickets?
When there is an FGF-23 producing tumour, it can lead to rickets.
Wasting of phosphate in urine can cause hypophosphateamic rickets.
44
When vitamin D and Calcium levels are normal, what other parameter could be the cause of rickets/osteomalacia?
Renal phosphate loss due to...
* isolated hypophosphataemia (genetic)
* oncogenic osteomalacia (mesenchymal tumours)
45
Describe the pathophysiology of isolated hypophosphataemia.
Who is affected?
How common is it?
What are the clinical signs?
**= X-linked hypophosphataemic rickets**
* 1;20,000
* mutations in PHEX; high levels of FGF-23
* toddlers with leg deformity, enthesopathy, dentin anomalies
= **Autosomal dominant hypophosphataemic rickets (ADRR)**
* variable age of onset; may improve
* cleavage site for FGF-23 mutated, so high FGF-2
46
Describe the pathophysiology of oncogenic osteomalacia.
* mesenchymal tumours
* produce FGF-23, causes phosphaturia and stops 1α OHase
47
What are the 3 ways in which an excess of FGF-23 can cause rickets/osteomalacia?
* Tumour producing an excess of FGF-23
* Lack of breakdown of FGF-23 due to inactive Phex
* Point mutation in R179 (--\>lack of breakdown?)
48
What can damage to kidney proximal tubule cause in relation to levels of phosphate?
Phosphaturia and stops 1-alpha-hydroxylation of Vit D
This is called Fanconi Syndrome
49
What is Fanconi Syndrome? What are its causes?
Syndrome of inadequate reabsorption in the proximal tubule of the kidney.
Causes:
* multiple myeloma
* heavy metal poisoning: lead, mercury
* drugs: tenofovir, gentamycin
* congenital disease: Wilsons, glycogen storage diseases
50
What are the diffferent causes of osteoporosis according to probable mechanism?
HIGH TURNOVER - inc. bone resorption \> inc. bone formation
* oestrogen deficiency
* hyperparathyroidism
* hypogonadism in young men and women
* cyclosporine
* heparin
LOW TURNOVER - dec, bone formation \> dec. bone resorption
* liver disease - primarily 1o biliary cirrhosis
* heparin
* age above 50 years
Inc bone resorption and decreased bone formation
* glucocorticoids
51
What is the effect of oestrogen deficiency on menopausal bone loss?
* Increase in remodelling units and remodelling imbalance
* deeper and more resorption pits
* ...lead to trabecular perforation, cortical excess excavation
* 90% bone resorption and 45% bone formation
* enhanced osteoclast survival and activity
* decreased osteocyte sensing
* remodelling errors
52
Describe this graph of cancellous bone loss.
* Peak bone mass is reached in early 20s then starts degenerating.
* There is a fast phase loss in early menopause, then slower loss.
* Men only show slow loss - about 1% loss per year is normal. So there is sexual dimorphism - there is **disproportionate loss of cancellous bone in women.**
53
Describe the biochemistry associated with osteoporosis.
**Serum biochemistry should all be normal if _primary._**
1. Check for Vit D deficiency
2. Check for secondary endocrine causes
* Primary hyperparathyroidism = PTH high
* Primary hyperthyroidism = free T3, TSH supressed
* Hypogonadism = testosterone low
3. Exclude multiple myeloma
4. May have high urine calcium
54
How do we assess osteoporosis?
* **Bone denisty (BMD)** is the best predictor of fracture risk - represents 70% of total risk
* **DEXA - dual X ray absorptiometry** - measures transmission through body of X-Rays of two different photon energies and enables densities of two different tissues to be inferred.
* Beng 1SD below that of your age group gives you a x2.5 increased fracture risk
* usually hip or vertebrae
* Radiation dose - 1-10 μSv
* Background - 7 μSv
* CXR - 100 μSv
55
Which bones in the body are used to investigate osteoporosis?
Vertebral - MOST COMMON fracture, increasing incidence after 60yr, measure of cancellous bone, metabolic bone (qulckest response to teratment)
Hip - MOST IMPORTANT, 2nd commonest fracture, costs and mortality high, FRAX uses hip BMD.
56
What is FRAX?
Fracture risk assessment tool (FRAX)
Uses hip BMD
57
Describe how bone markers can be used to assess bone disease.
In bone disease cycle is disrupted. There are markers of bone of formation and resorption. These markers are dynamic.
* These can be measured to give an insight into activity or to check if anti-resorptive treatment is working.
* BMD should change in 18 months
* bone resorption markers fall in 4-6 weeks
* expect a 50% drop of urine NTx by 3 months
58
What markers of bone formation can be measured in blood?
* **Markers of bone formation - P1NP (procollagesn type 1 N-terminal propeptide)**
* *In collagen synthesis, 2 "alpha 1" and 1 "alpha 2" collagen type I chains are produced by osteoblast joining. Extension peptides are cut off to produce measurable markers and tropocollagen .*
* P1NP is being used as a predictor of response to anabolic treatments - with PTH teratmnet sit rises to peak in 3 months. Predicts response.
* **Markers of bone resorption - serum CTX, urine NTX**
* 3 hydroxylysine molcules on adjacent tropocollagen fibrils condense to form a pyridinium ring likage. These can be measured to show bone resorption
* **Only one in common use today is alkaline phosphatase** - to diagnose and monitor Pagets, osteomalacia, boney metastases (prostate with PSA).
59
What are the types, roles and uses of BSAP?
Types:
* tissue-specific form, liver vs bone
* intestine, germ cell, placental forms
Role
* essential for mineralisation
* regulates concentrations of phosphocompounds
Uses:
* consistent with an individual; t(1/2) =40hours
* increased in: Paget's. osteomalacia, bone metastases, hyperparathyroidism, hyperthyroidism.
BSAP = bone-specific alkaline phosphatase.
60
How do alkaline phosphatase levels change with age?
* BSAP represents around 75% of serum pre-puberty
* Increased markedly in growth
* 1:1 with liver in adults
* Increases over age 50 especially in menopausal women, levels go up by 75% but not used as a marker in osteoporosis.
* placental isoform increases during pregnancy, gradullay in first 2 trimesters and rapidly in last trimester.
61
What is the biochemistry in renal osteodystrophy?
1. Increasing serum phosphate
2. Reduction in 1,25 Vit D (calcitriol)
62
What compensation systems kick in in renal osteodystrophy?
1. increasing serum phosphate
2. Reduction in 1,25VitaminD (calcitriol)
So s**econdary hyperparathyroidism** develops to compensate
But unsuccessful and so **hypocalcaemia** develops
Later, parathryoids become autonomous (**tertiary**) causing **hypercalcaemia**
63
Describe the progression of secondary HPT to tertiary HPT in chronic kidney disease.
* Progressive decline in renal function causes parathyroid hyperplasia
* Constant deman on parathyroid glands (due to hypocalcaemia) drives cell proliferation
* Initially parathryoid glands respond by increasing proportion of secretory/chief cells and then by increasing total number f cells, resulting in hyperplasia of the gland
* Cell growth is polyclonal but with CaR and VDR down-regulation
* As CKD progresses to stage 5 (end stage renal disease), parathyroid hyperplasia evolves further. Monoclonal abnormalities lead to nodular hyperplasia of the glands. with reduced expression of CaRs and VDRs.
* Parathyroid glands with nodular hyperplasia become less responsive to serum calcium levels and resistant to medical treatment of SHPT.
64
Using a diagram, summarise how CKD leads to these bone disorders:
* Demineralisation
* Osteomalacia
* Increased bone resorption
* Metastatic clacification
65
What is shown on the image below?
Heterotopic calcification/ossification
Formation of true bone in extraskeletal soft tissues.
66
How do you calculate the T-score?
T-score = (measured BMD - young adult mean BMD0/ young adult standard deviation
how many standard deviations are you off the average for a 25 year old?
T-score = -2.5 =osteoporosis
-1 to -2.5 =osteopenia
\>-1 =normal
