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

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

DUCHENE MUSCULAR DYSTROPHY

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

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

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

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

Metabolic pathways review Glcolysis Glycogenolysis Beta oxidation of fatty acids

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.

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

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.

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

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

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

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.

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

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.

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)?

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

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.

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

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

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