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D1 Fall - Biochem > 11/15: Hormonal Control of Calcium and Phosphorus II > Flashcards

11/15: Hormonal Control of Calcium and Phosphorus II Flashcards

1
Q

What is the main way that the Ca and Pi homeostasis can be disrupted?

A

Chronic kidney disease

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2
Q

How does chronic kidney disease affect Ca/Pi homeostasis?

A

impaired kidney function interferes with Ca2+ and Pi reabsorption

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3
Q

What is hypocalcemia?

A

Low serum calcium

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4
Q

What are serum calcium levels in hypocalcemia?

A

(low serum calcium)
< 8.5 mg/dL total calcium or
<4.4 mg/dL ionized calcium

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5
Q

What are causes of hypocalcemia?

A
  1. Inadequate PTH production (hypoparathyroidism)
  2. Syndromes with hypoparathyroidism
  3. PTH resistance
  4. Inadequate vitamin D
  5. Vitamin D resistance or synthesis defects
  6. Miscellaneous
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6
Q

What is hypoparathyroidism?

A

Hypocalcemia with serum PTH inappropriately low for hypocalcemic state

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7
Q

What is the most common cause of hypoparathyroidism?

A

autoimmune destruction of
parathyroids/loss of parathyroids due to thyroidectomy

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8
Q

What does the loss of PTh producing tissue cause?

A

hypocalcemia due to decreased Ca 2+ uptake in gut/kidney, decreased Ca2+ release from bone

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9
Q

What is a congenital disease with complete lack of parathyroids at birth?

A

Di George Syndrome

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10
Q

What mutations is hypoparathyroidism associated with?

A

Activating CaSR mutations

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11
Q

How does activating CaSR mutations cause hypoparathyroidism?

A

CaSR is hypersensitive to extracellular Ca2+ and suppresses PTH
production, even though even though Ca2+ levels are low (i.e.
parathyroid “misreads” Ca2+ levels as high and inappropriately
suppresses PTH)

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12
Q

A decrease in Ca2+ reabsorption in kidney (more excreted in urine) and decreases release from bone, uptake in gut, etc causes?

A

Low serum calcium

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13
Q

What is hypocalcemia due to?

A

Lack of responsiveness of target tissues to PTH

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14
Q

What mutations is hypocalcemia due to?

A

Mutations in G proteins important for PTH signaling

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15
Q

What is vitamin D deficiency due to?

A

Lack of sunlight
Malabsorption of vitamin D

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16
Q

What does lack of vitamin D inhibit?

A

Ca and Pi uptake in gut (due to
downregulation of calcium and phosphate transport proteins,
Calbindins, TRPV6, NaPi-IIb)

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17
Q

What does lack of vitamin D lead to in growing children?

A

Rickets

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18
Q

What is rickets?

A
  • Impaired bone mineralization/outward curvature of long bones (bowing)
  • Insufficiently mineralized vertebrae/curved spine
  • Disorganized growth plate/growth retardation
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19
Q

What does lack of vitamin D and phosphate lead to in adults?

A

Osteomalacia

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20
Q

What is failure of osteoid to fully calcify - soft bones?

A

Osteomalacia

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21
Q

What is VDDR type I (vit d-dependent rickets) due to a defect in?

A

Renal 25-OH-vit D-1-alpha-hydroxylase

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22
Q

What is VDDR type II due to a defect in?

A

Vit D receptor (VDR)

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23
Q

Which VDDR has very low 1,25(OH)2D3?

A

VDDR Type I

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24
Q

Which VDDR has high 1,25(OH)2D3?

A

VDDR type II

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25
Q

What are the diseases of hypocalcemia?

A

Pseudohypoparathyroidism
Hypoparathyroidism
Vitamin D Deficient Rickets
Vitamin D Dependant Rickets

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26
Q

What are levels of hypercalcemia?

A

High serum calcium
total calcium >10.5mg/dL
(2.6mM) or ionized calcium
>5.4mg/dL (1.3mM)

27
Q

What are causes of hypercalcemia?

A
  1. Elevated PTH levels
  2. Elevated 1,25(OH)2D3 levels
  3. Miscellaneous
28
Q

How does hypercalcemia occur in relation to PTH?

A

PTH hypersecretion not adequately inhibited by normal negative feedback response to elevated Ca2+

29
Q

What are inherited forms of familial primary hyperparathyroidism?

A

Multiple endocrine neoplasia (MEN)
MEN1
MEN2A

30
Q

What is due to inactivation of tumor suppressor gene Menin (gene name MEN, One copy of gene is mutated, but “second hit” needed for tumor formation?

31
Q

What is due to gain of function mutation in RET protooncogene?

32
Q

What do heterozygotes for inactive CaSR have?

A

Familial hypocalciuric hypercalcemia (FHH)

33
Q

What do homozygoyes for inactive CaSR have?

A

neonatal severe hyperparathyroidism (NSHPT)

34
Q

How do inactivating mutations of CaSR occur?

A

CaSR doesn’t suppress PTH production even though Ca 2+
levels are high (i.e. parathyroid misreads Ca2+ levels as
being low) therefore PTH is inappropriately elevated

35
Q

How can hypercalcemia of malignancy occur?

A

Due to tumors secreting factors that stimulate bone resorption

36
Q

What are diseases of hypercalcemia?

A

Primary Hyperparathyroidism
Familial Primary Hyperparathyroidism
Hypercalcemia of Malignancy

37
Q

What is oversecretion of PTH in response to conditions of
hypocalcemia and/or decreased 1,25(OH)2D3?

A

Secondary hyperparathyroidism

38
Q

What is the common cause of secondary hyperparathyroidism?

A

Chronic renal failure

39
Q

What are phosphate levels of hypophosphatemia?

A

Low serum phosphate
<2.5-4.5 mg/dL

40
Q

What are causes of hypophosphatemia?

A
  1. Decreased intestinal absorption of phosphate
  2. Increased urinary excretion
  3. Redistribution from extracellular fluid into cells/tissues
41
Q

What are the main diseases associated with hypophosphatemia?

A

Rickets/osteomalacia

42
Q

What is x-linked hypophosphatemic rickets (XLH)?

A

Due to mutations in PHEX gene on X-chromosome

43
Q

What is PHEX produced by?

A

Osteoblasts, osteocytes, odontoblasts

44
Q

What does PHEX inhibit?

A

FGF23 production

45
Q

What do XLH mutations of PHEX lead to?

A

Inappropriately elevated FGF23 production, even though serum phosphate is low (mainly by osteocytes)

46
Q

What does FGF23 reduce?

A

renal reabsorption of phosphate (leads to
renal phosphate wasting - i.e. more excreted in urine)

47
Q

When FGF23 reduces renal absorption what does this cause?

A

Low serum Pi
Phosphaturia
Low 1,25(OH)2D3

48
Q

What is a rare form of inherited hypophosphatemic rickets?

A

ADHR (Autosomal Dominant Hypophosphatemic Rickets)

49
Q

What is ADHR due to mutations in?

50
Q

What happens in mutations in FGF23?

A

Alter cleavage site so it cannot be proteolytically inactivated – FGF23 stays active longer

51
Q

What is recessively inherited hypophosphatemic rickets?

A

ARHR (Autosomal Recessive
Hypophosphatemic Rickets)

52
Q

What is ARHR due to mutations in?

53
Q

What is Dmp1 expresed by?

A

osteocytes and negatively regulates FGF23

54
Q

What does a mutation in Dmp1 lead to?

A

Overproduction of FGF23

55
Q

What is hereditary hypophosphatemic rickets with hypercalciuria (HHRH) due to?

A

heterozygous or homozygous loss of function mutations in type II sodium phosphate cotransporter NaPiIIc

56
Q

What three genetic conditions result in elevated FGF23?

A

XLH
ADHR
ARHR1

57
Q

What genetic condition results in reduced function of Na+ dependent phosphate transporter?

58
Q

Acquired syndrome of renal phosphate wasting

A

Tumor Induced Osteomalacia (TIO)

59
Q

How are TIO levels associated with FGF23?

A

levels go down with surgical
resection of tumor – may require supplementation with phosphate and calcitriol

60
Q

What does acute hyperphosphatemia suppress?

A

1 α-hydroxylase activity in kidney, lowering 1,25D 3 levels, which further exacerbates hypocalcemia by reducing Ca2+ uptake in gut/renal reabsorption

61
Q

What are causes of Hyperphosphatemia?

A

Acute phosphate load
Decreased urinary excretion
Redistribution to extracellular space
Genetic causes of hyperphosphatemia

62
Q

What dental abnormalities do vit d-dependent rickets cause?

A

thin enamel, hypomineralization→
microscopic cracks harbor bacteria → frequent cavities

63
Q

Patients with hypercalcemia of malignancy often treated
with

A

High dose bisphosphonates

D1 Fall - Biochem (29 decks)

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