1. BIOCHEMICAL BASIS OF BONE AND MUSCLE DISEASES
BIOCHEMISTRY
MSK,2015
BIOCHEMICAL BASIS OF BONE AND MUSCLE DISEASES
NABIL BASHIR

2. Outlines Know and understand the molecular basis of :
Duchene Muscular Dystrophy.
Glycogen storage diseases of muscle
Muscle Mitochondrial diseases.
Osteogenesis imperfecta
Osteoporosis .
Paget disease
Ehlar Danlos syndromes

3. DUCHENE MUSCULAR DYSTROPHY

4. Mutations in the dystrophin gene
Becker and Duchenne muscular dystrophy
BMD is a less-severe disease (patients are still walking after 16 yrs)
DMD is a more-severe disease (patients are not walking at 12 yrs)
both can be caused by massive deletions in the dystrophin gene (as well
as other types of mutations)
the severity is not necessarily correlated with the size of the deletion
dystrophin cDNA (coding region)
The size of a deletion is not necessarily correlated with the severity of a disease. As shown in this slide, dystrophin gene mutations can give rise to less-severe Becker's muscular dystrophy or more-severe Duchenne's muscular dystrophy. Apparently, the most important regions of the dystrophin protein are the N- and C-terminal globular domains. Loss of either of these results in a major disruption of dystrophin function (see next slide). These regions are connected by a very long series of spectrin-like repeats. 5’ 3’
dystrophin protein
spectrin-like repeat domain

5. truncated but functional protein with intact N- and C-termini
mutations causing BMD can be very large in-frame deletions 5’ 3’
truncated but functional protein with intact N- and C-termini
partially functional dystrophin protein
mutations causing DMD can be small out-of-frame deletions
An in-frame deletion of a large region of the spectrin-like repeats, as in one form of BMD (shown by the large white rectangle), shortens the molecule but retains the important N- and C-terminal domains. In contrast, in a case of DMD, a smaller, out-of-frame deletion (the deletion is shown as a small white rectangle and the out-of-frame sequence is shown as a cross-hatched rectangle) results in a protein lacking the dystrophin C-terminal domain. 5’ 3’
C-terminal truncated protein (with out-of-frame translation product)
non-functional dystrophin protein

6. Metabolic myopathies
inadequate production of cellular energy in the muscle.
Mitochondrial myopathies are caused by mutations, or changes, in genes

7. Muscle Energy Metabolism
Under normal circumstances, energy for skeletal muscle function (ATP) is derived from:
Muscle glycogen,
Blood glucose, and
Free fatty acids.
Defects in any one of the steps involved in this complex metabolic pathway can lead to an insufficient supply of ATP and an inability to sustain normal muscle function

8. Metabolic pathways review
Glcolysis
Glycogenolysis
Beta oxidation of fatty acids

9. Muscle glycogen storage diseases
Type V (McArdle Disease ): Deficiency of phosphorylase:
an elevated CK (8,404 U/L; normal, 30–220 U/L),
mild elevations of aspartate aminotransferase (75 U/L; normal, 15–41 U/L), and
alanine aminotransferase (82 U/L; normal, 17–63 U/L).
Urinalysis is negative for myoglobin.

10. Muscle glycogen storage diseases……
Type VII: Deficiency of phosphofructokinase→
hemolytic anemia
myogenic hyperuricemia.
accumulation of normal glycogen in muscle, .

11. Fatty Acid Oxidation Disorders
FAO (b-oxidation of fatty acids) is the major source of energy during periods of sustained, low-intensity exercise or prolonged fasting.
exercise intolerance and myoglobinuria are the most common presenting features.

12. 1) Carnitine palmitoyltransferase II (CPT II) deficiency,
The major disorders of lipid metabolism that present with isolated myopathy include:
1) Carnitine palmitoyltransferase II (CPT II) deficiency,
2) Long-chain acyl-CoA dehydrogenase (LCHAD),
3) Trifunctional protein including LCHAD, and
4) Very long-chain acyl-CoA dehydrogenase (VLCAD

13. CPT II
Missense mutations : production of some partially functional enzyme activity →milde myopathic form.
protein truncating mutations produce the more severe neonatal and infantile phenotypes.
Serum CK levels are usually normal

16. Therapeutic interventions
avoid prolonged fasting (i.e., longer than 10 hours).
Avoide intensive exercise .
Carbohydrate loading prior to and during exercise may prevent attacks.
supplementation with medium-chain triglycerides, which provides an alternative substrate for fatty acid oxidation involving long-chain fatty acids

17. Osteogenesis imperfecta (OI)
Type 1 collagen (the major protein in bone and skin) mutation .
structure of type 1 collagen molecules,(quality)
number of collagen molecules made (quantity).
Either of these changes results in weak bones that fracture easily.

18. Osteoporosis
The hallmark of osteoporosis is a reduction in skeletal mass.
caused by an imbalance between bone resorption and bone formation.

19. Estrogen Deficiency……
Estrogen deficiency leads to increased expression of RANKL by osteoblasts and
decreased release of OPG;
increased RANKL results in recruitment of higher numbers of preosteoclasts as well as increased activity and lifespan of mature osteoclasts.

20. Paget disease Paget disease of bone (PDB) :
increased bone turnover within discrete lesions throughout the skeleton.
mutations in SQSTM1 gene (sequestosome 1) that encodes the p62 protein often found in PDB patients.
The most common mutation replaces proline with leucine at position 392 (written as Pro392Leu or P392L).
SQSTM1 gene mutations overactivates the pathway that promotes osteoclast formation. (RANK-NF-kappaB)?

21. Biochemical findings in paget disease……
↑ serum alkaline phosphatase
normal calcium, phosphate.
↑ serum and urinary hydroxyproline .

22. Ehlers–Danlos syndrome (EDS)
Ehlers–Danlos syndrome (EDS) is an inherited connective tissue disorder with heterogeneous presentations .
EDS is caused by a defect in the synthesis of collagen, specifically mutations in the COL5A and COL3A genes.

23. Individual with EDS displaying hypermobile joints
the signs are ultimately due to faulty or reduced amounts of collagen.

24. Individual with EDS displaying skin hyperelasticity
the signs are ultimately due to faulty or reduced amounts of collagen.

25. Molecular Diagnosis
Both DNA and biochemical studies can be used to help identify affected individuals. Diagnostic tests include:
Collagen gene mutation testing,
Lysyl hydroxylase or oxidase activity