GENETIC MYOPATHIES (INHERITED MUSCLE DISEASES, MUSCULAR DYSTROPHIES)

Muscle histology: Review

• Epimysium envelopes the muscle.
• Perimysium divides the muscle into fascicles.
• Endomysium lies within the muscle fascicle: it comprises a loose areolar connective tissue that maintains the extracellular environment for proper muscle cell functioning.

Muscle fascicle histology: Review

• The muscle cell is covered in endomysium.
• The cell has many nuclei.
• Dot-like myofibrils constitute the muscle cell milieu.

Muscle Cell Physiology: Review/Pathology Introduction

• Proteins stabilize myofibrils to the muscle cell and can be linked to well-defined related myopathies.
• Each Myofibril includes a Z-disk, which transects an I Band, flanked by A Bands.
• Repeating light and dark bands gives muscle fibers a striated appearance.
• Desmin filaments encircle the Z disks.
• Desmin-related myopathy (DRM) is an inherited disease which results in disorganized and weak skeletal muscle fibers. DRM can be fatal, as it also affects cardiac and smooth muscles.
• Plectin links the desmin filaments.
• Alpha-B-crystallin (a heat shock protein) protects desmin from stress-induced damange. Together: desmin, plectin, and alpha-B-crystallin constitute a Z-disk protection network.
• Dystrophin-associated glycoprotein complex (DAGC) comprises:
  -Dystroglycan subcomplex, which links dystrophin to laminin, a key external lamina protein (called laminin-2 in skeletal muscle, which has an associated myopathic syndrome).
  -Sarcoglycan subcomplex, which, when defective can cause sarcoglycanopathies – a similar manifestation of weakness as those from dystrophinopathies – and are a common cause of limb-girdle muscular dystrophy.
  –Dystrophin stabilizes the sarcolemma during muscle contraction. When pathologic, it produces the dystrophinopathies (eg, Duchenne Muscular Dystrophy, Becker Muscular Dystrophy).
• Syntrophins are recruited to the sarcolemma and manage the assembly of other proteins.
• Dystrobrevins link desmin to dystrophin and syntrophin.

MUSCULAR DYSTROPHIES

DYSTROPHINOPATHIES (DUCHENNE, BECKER):

Overview

• X-linked, Recessive form of muscular dystrophy that affects boys and occurs from a genetic mutation that prevents the synthesis of dystrophin.
• Muscle is replaced with fatty and fibrous connective tissue, which presents with pseudohypertrophic muscles: muscles that are enlarged from fat and connect tissue (not muscle).
• Duchenne Muscular Dystrophy is a severe dystrophinopathy wherein children are non-ambulatory at ~ age 13 – pseudohypertrophy of calf muscles is a notable clinical finding.
• Becker Muscular Dystrophy is a less severe dystrophinopathy in that patients aren’t non-ambulatory until ~ age 40.
• As a helpful mnemonic add the treatment adage that the goal of corticosteroids is to: Make Duchenne boys into Becker men.

Duchenne Muscular Dystrophy

• Most severe form
• Manifests in childhood with proximal weakness (especially calf hypertrophy)
• Loss of ambulation ~ age 13

Becker Muscular Dystrophy

• Less severe form
• Manifests in early teens
• Loss of ambulation ~ age 40

Genetic & Diagnostic Characteristics of Duchenne & Becker Muscular Dystrophies

• Genetics
  -X-Linked, Recessive
  -Dystrophin gene mutation that leads to reduction/absence of dystrophin protein with resultant sarcolemma damage
• Diagnosis: Elevated CK (~20,000), Dystrophin Gene Deletion
• Treatment: Corticosteroids

Myotonic Dystrophy, Type 1

• Characterized by myotonia (eg, inability to release a grip) [relaxes with repetition vs. paramyotonia which worsens with repetition]. Responds to Mexilitine.
• Weakness of lower extremities, hands, neck, and face
• Additional systemic features
  -Cataracts
  -Cardiac conduction defects
  -Early Frontal balding

Myotonic Dystrophy, Type 2

• Myotonia
• Weakness of neck, shoulders, elbows, and hips.

Genetic & Diagnostic Characteristics of the Myotonic Dystrophies

• Autosomal Dominant
• Anticpiation in DM-1 with the DMPK gene [CTG Trinucleotide repeats]
• EMG findings of myotonic discharges.

Oculopharyngeal Muscular Dystrophy

• Ptosis and dysphagia
• Onset > age 40
• Genetics
  -Autosomal Dominant
  -PABPN1 gene

Fascioscapulohumeral Muscular Dystrophy

• Asymmetric facial and scapular muscles (scapular winging) and humeral (upper arm) atrophy: difficulty whistling, closing eyes, throwing ball
• Symptoms appear in adolescence
• Genetics
  -Autosomal Dominant
  -D4Z4 contraction on chromosome 4q35 (Majority of genetic cause)

Limb-Girdle Muscular Dystrophies

• Present with proximal weakness at any age, manifesting with waddling gait, scapular winging, possible joint contractures.
• Cardiomyopathy or respiratory compromise are key potential complications
• Diagnosis: Elevated CK, genetic testing, and immunohistochemical muscle biopsy staining
• Genetics
  -Various genetic pathologies involving muscle cell proteins: sarcolemmal, cytosolic, nuclear envelope.
  -Notable forms: -LMNA gene [lamin A/C], CAPN3 gene [calpainopathy], DYSF gene [dysferlinopathy], SGC genes [sarcoglycanopathies]

Emery-Dreifuss Muscular Dystrophy

• Skeletal & cardiac muscle are affected
• Early contractures (joint deformities)
• Upper arm/lower leg wasting
• Genetics
  -X-linked
  -EMD gene (most commonly), which forms emerin: a nuclear envelope protein.
  -LMNA gene (less commonly), which forms lamin A/C

CONGENITAL MYOPATHIES

Central Core Myopathy

• Floppy infants
• Associated skeletal abnormalities: scoliosis, hip dislocation, joint deformities
• Risk of malignant hyperthermia from anesthetics
• Pathology: microscopic cores in the center of muscle fibers
• Genetics
  -Autosomal Dominant
  -RYR1 gene for the Ryanodine Receptor 1, which forms a channel that releases calcium from muscle cells

Nemaline Rod Myopathy

• Infant onset is most common – Hypotonia and poor respiration
• Adults – Proximal weakness and skeletal abnormalities (scoliosis and contractures)
• Abnormal clumps of threaded filaments (hence: “nema” for “thread”) in muscle fibers that can look like rods.
• Genetics
  -Autosomal recessive.
  -Mutations in sarcommeric proteins.

METABOLIC MYOPATHIES

Pompe Disease (Acid Maltase Deficiency)

• Infantile-onset (classic vs non-classic): myopathy, hypotonia, hepatomegaly, congenital heart defects
• Late-onset: Progressive weakness and respiratory failure.
• Genetics
  -Autosomal recessive.
  -GAA gene for acid alpha-glucosidase (acid maltase), which is key for break-down of glycogen to glucose within lysosomes.

McArdle Disease (Glycogen Storage Disease Type V)

• Exercise-induced Pain/Cramps/Fatigue, which alleviates with rest (“Second Wind” phenomenon).
• Severe forms cause rhabdomyolysis with myoglobinuria.
• Genetics
  -PYCM gene for myophosphorylase, which is specific to muscle. It breaks down glycogen to glucose-1-phosphate.
• Forearm ischemic exercise test: lactate [no change], ammonia [normal increase].
• Muscle biopsy: lack of phosphorylase. Subsarcolemmal glycogen deposits.
• Treatment: Enzyme replacement therapy, Avoidance of maximal exercise, High-protein and low carbohydrate diet.

Carnitine Palmitoyltransferase Deficiency 2

• Lipid metabolism disorder that prevents the body from using fat for energy during periods of fasting.
• Carnitine Palmitoyltransferase 2 (CPT2) is involved in inner mitochondrial membrane transport – deficiency comprises mitochondrial fatty oxidation.
• Three forms of the disorder (from most severe to least): lethal neonatal, infantile hepato-cardio-muscle, and myopathic.
  -Myopathic form causes: myalgias and rhabdomyolysis.
• Genetics
  -CPT2 gene mutation involving fatty acid oxidation within mitochondria.
  -Long-chain fatty acids must attach to carnitine to enter mitochondria. Once inside, CPT2 removes carnitine for fatty oxidation – without CPT2, fatty acids can’t be used for energy.
• Treatment: High carbohydrate, Low fat diet.

CHANNELOPATHIES

Myotonia Congenita (Thomsen & Becker Disease)

• Abnormal muscle excitability that is NON-dystrophic, exacerbated in cold.
• Muscle stiffness. Myotonia (eg, inability to release a grip) [relaxes with repetition vs. paramyotonia which worsens with repetition]. Responds to Mexilitine.
• Genetics
  • CLCN1 gene – chloride channelopathy, SCN4A – sodium channelopathy
    -Thomsen – Autosomal Dominant. Becker – Autosomal Recessive.

Familial Periodic Paralysis

• Flaccid weakness in the setting of hypokalemia or hyperkalemia that can last hours to days.
• Triggered by intense exercise, large carbohydrate meal, viral infection, or medications.
• Genetics
  -Autosomal Dominant
  -CACNA1S gene – calcium channelopathy, SCN4A – sodium channelopathy
• Don’t forget other potential causes of episodic weakness: Myasthenia Gravis, Lambert-Eaton, Thryotoxicosis, and Metabolic Derangement – calcium, phosphorous, magnesium, sodium.

MITOCHONDRIAL MYOPATHIES

• Maternally-inherited
• Ragged red fibers / Subsarcolemmal accumulation of abnormal mitochondria

Skull Malformations

• Macrocephaly, Megalencephaly, and Microcephaly.

Craniosynostosis

Premature closure of the cranial sutures.

• Disorders of a single suture: Trigonocephaly, Plagiocephaly, Scaphocephaly, and Brachycephaly.
• Disorders that involve multiple suture synostoses.

SKULL MALFORMATIONS

Macrocephaly

• Cranial enlargement (to > 98% of normal range). Although it can be due to enlargement of any of the 3 brain compartments: brain tissue, CSF, or blood, it’s most commonly due to obstructive hydrocephalus – enlargement of the CSF ventricles. It typically necessitates ventriculostomy or shunting.

Megalencephaly

• Generalized cranial enlargement of the gray and white matter of the brain due to either anatomic abnormalities (eg, neurocutaneous disorders) or metabolic abnormalities (eg, lysosomal storage disorders or leukodystrophies).
• Megalencephaly is technically a form of macrocephaly, since, indeed it involves enlargement of the cranium – but, again, the majority of cases of macrocephaly are NOT generalized brain matter enlargement but rather due to hydrocephalus.

Microcephaly

• A generalized abnormally small cranium (to < 98% of normal range), either due to a primary genetic cause (ie, a chromosomal or metabolic abnormality (eg, phenylketonuria) or an acquired condition (eg, perinatal infection).

NORMAL SKULL ANATOMY AND DEVELOPMENT

Neurocranium

Divides into the…

• Cranial vault, which provides a roof for the brain.
• Skull base, which provides a floor for the brain.

Viscerocranium

• Comprises the facial bones.

SKULL DEVELOPMENT

• The cranial vault and viscerocranium develop via intramembranous ossification (again, which has no intermediate cartilaginous model).
• The skull base develops via endochondral ossification, which develops via a cartilaginous matrix.

The cranial vault comprises:

• Frontal bone, Parietal bone, upper portion of the Occipital bone, and Squamous portion of the Temporal bone – all of these develop via intramembranous ossification.

The key bones of the the skull base:

• The lower portion of the Occipital bone, the Petrous portion of the Temporal bone, and the Sphenoid bone – all of these bones develop via endochondral ossification.
• Key viscerocranial bones:
  The Zygomatic bone and Maxilla and Mandible – they develop via intramembranous ossification.
• The bones of the face derive from embryonic cells from the pharyngeal arches (from neural crest cells (other than the laryngeal cartilages, which derive from mesoderm)).

SKULL SUTURES

• At birth, the skull has openings (sutures) to accommodate brain growth, because the cranial vault ossifies early via intramembranous ossification. These sutures allow the fetal skull to ossify quickly prior to delivery [via intramembranous ossification] (so the brain doesn’t get squashed) and yet still accommodate skull distortion during birth (called, molding) and permit rapid brain growth during the first two years of life when the brain quadruples in size to 75 percent of its adult volume!
• Metopic – forms between midline aspects of the left/right aspects of the frontal bone.
• Coronal – lies between the frontal and parietal bones.
• Sagittal – lies between the bilateral parietal bones.
• Lambdoid – lies between the parietal bones and occipital bone.
• There are three additional minor sutures: frontonasal, temporosquamosal, and frontosphenoidal.

THE FONTANELLES

The large openings that exist in the newborn calvarium.

• The frontal bone covers the majority of the anterior frontal lobes.
• The parietal bones cover the remainder and the parietal lobes.
• The occipital bone covers the occiput.

Anterior fontanelle

• Forms at the junction of the sagittal, coronal, and metopic sutures at the anterior of the skull – it’s palpable in midline, just behind the forehead; it closes at 1.5 to 2 years of age.

Posterior fontanelle

• Forms from the intersection of the sagittal and lambdoid sutures; it closes at 3 – 6 months of age.

CRANIOSYNOSTOSES

• Premature cranial suture closure (synostosis).

Scaphocephaly

• The most common type of synostosis; accounts for half of the incidences of synostosis each year.
• There is synostosis of the sagittal suture – the skull is shaped like the narrow hull of a boat (the derivation of its name).
• In accordance with Virchow’s law, the interruption of brain growth is in perpendicular to the plane of the synostosis – thus the abnormal brain growth is in parallel to the synostosis. The skull elongates (in parallel to the synotic suture) – this results in an elongated, narrow skull.
• The term dolichocephaly (elongated head) is either used synonymously scaphocephaly or as a broader catch-all for elongated head.

Brachycephaly

• Results from bicoronal synostosis.
• The skull cannot develop normally along the sagittal plane and we show, instead, that it manifests with a wide, short skull – again, in accordance with Virchow’s law, the skull develops in parallel to the plane of the synostosis.
• In unilateral coronal synostosis (as opposed to bilateral), also results in plagiocephaly because, as we can imagine, it results in a twisting/oblique appearance (but here of the frontal calvarium).

Trigonocephaly

• Secondary to metopic synostosis, which results in a failure of frontal outward development.
• It manifests with a pointed forehead… the eyebrows may appear “pinched.”
• To help link the name to the shape, we show that trigonocephaly results in a triangular shaped head, when viewed from above.

Lambdoid synostosis

• Posterior plagiocephaly (a twisted skull) due to an inability of a side of the occiput to grow outward, thus there is an oblique oblique deformity of the posterior calvarial vault.

DESCRIPTIVE TERMINOLOGY FOR MULTIPLE SUTURE SYNOSTOSES

Cloverleaf Deformity (aka Kleeblattschädel)

• Multiple sutures fuse prematurely – unfortunately, the brain actually can grow through the anterior fontanelle, can be a finding of certain genetic syndromes as described below…

SYNDROMES:

FGFR2 GENE (FIBROBLAST ABNORMALITY)

Apert Syndrome

• Multiple suture synostoses w/possible cognitive delays
• Syndactly vs Polydactly
• Possible hearing loss, hyperhidrosis, spine fusion, oro-palatal malformations
• Genetics: Autosomal dominant, FGFR2 gene abnormality, which is important in fibroblast embryonic development of bone.

Crouzon Syndrome

• Multiple suture synostoses w/o cognitive delays
• Possible hearing loss and oro-palatal malformations
• Genetics: Autosomal dominant, FGFR2 gene abnormality, which is important in fibroblast embryonic development of bone.

Pfeiffer Syndrome

• Multiple suture synostoses w/anywhere from no neurologic complications to significant complications.
• Syndactly vs Brachydactly
• Possible Anykylosis (bone fusion at the joints)
• Genetics: Autosomal dominant, FGFR2 gene abnormality, which is important in fibroblast embryonic development of bone.

RAB23 OR MEGF8 GENE (VESICLE TRANSPORT)

Carpenter syndrome

• Multiple suture synostoses w/possible cognitive delays
• Brachydactly vs Polydactly vs Syndactly
• Cryptorchidism
• Kyphoscoliosis
• Genetics: Autosomal recessive, RAB23 or MEGF8 gene mutations, which is important in vesicle transport.

Hormonal regulation of extracellular calcium and phosphate concentrations by the parathyroid glands.

Key Principles

Free calcium participates in various cellular processes, including:

• Skeletal, cardiac, and smooth muscle contraction
• Nerve conduction
• Blood clotting
• Bone and tooth formation
• Enzyme activation and deactivation

Phosphate is a part of ATP

• Participates in cellular metabolism
• Plays a role in enzyme activation and deactivation

Storage and Release:

Calcium and phosphate are stored within hydroxyapatite crystals of bone

• When bone is resorbed, calcium and phosphate are released into the extracellular fluid
• Calcium and phosphate levels are regulated by the same hormones:
  • Parathyroid hormone, which is secreted by chief cells of the parathyroid glands
  • Vitamin D (in activated form)

(The physiologic role of calcitonin, a hormone released by the thyroid gland in response to increased calcium concentrations, is as of yet uncertain, and, therefore, omitted in this tutorial.)

PARATHYROID HORMONE PATHWAYS:

In response to lowered extracellular calcium concentration, the parathyroid glands secrete parathyroid hormone (PTH).

Bone:

• In bone, episodic, transient binding of parathyroid hormone causes an increase in new bone synthesis
• Prolonged exposure to parathyroid hormone promotes resorption of old bone, and, therefore, the release of calcium and phosphate into extracellular fluid

Clinical consequences of these dichotomous effects:

• Osteoporosis, which is characterized by loss of bone density, can be treated with intermittent PTH administration
• Continuous release of PTH in individuals with hyperPARAthyroidism causes excessive bone resorption

Kidneys:

• Increased calcium reabsorption in the distal convoluted tubule of the nephrons
• Decreased phosphate reabsorption in the proximal convoluted tubule, which leads to phosphaturia, an increase in phosphate in the urine
  – This action is important because, otherwise, reabsorbed phosphate would complex with the reabsorbed calcium, which would negate its physiologic effects in the body.

PTH STIMULATES RENAL ACTIVATION OF VITAMIN D

Kidney

• Vitamin D acts increases renal reabsorption of both calcium and phosphate

Small Intestine

• Vitamin D increases calcium and phosphate reabsorption

Bones

• Vitamin D works with parathyroid hormone to facilitate skeletal remodeling, which requires both synthesis and resorption of bone.

Clinical correlation:

Vitamin D deficiency in children causes rickets, in which skeletal development is impaired, the bones are weak, and, consequently, growth is often stunted.

System-wide consequences of calcium imbalances:

Hypocalcemic individuals experience hyperreflexia, muscle twitching and cramping, numbness and tingling

• Trousseau’s sign, characterized by involuntary hand and feet spasms, carpopedal spasms, which can be provoked by the examiner by inflating a blood pressure cuff to cause prolonged brachial artery occlusion.
• The Chvostek sign, characterized by hyper excitable facial muscle twitching in response to tapping the facial nerve.

Hypercalcemic individuals experience hyporeflexia, muscle weakness, lethargy, and, polyuria.

BONE REMODELING

HOMEOSTATIC PROCESS OF BONE REMODELING

Key Functions

• Regulates calcium blood levels
• Repairs worn-out bone
• Responds to bone stress

Actions

• Osteoblasts form bone from calcium in blood and that osteoclasts break down bone and push calcium into blood.
• LOW blood (plasma) calcium levels stimulate osteoclast activity and inhibit osteoblast activity.
• HIGH blood (plasma) calcium levels inhibit osteoclast activity and stimulate osteoblast activity.
• Reabsorbed bone releases calcium into blood and PTH (parathyroid hormone) is a key physiologic mediator for bone homeostasis.

Clinical Correlation: In Osteoporosis, bone resorption exceeds deposition.

THE BIOLOGICAL PROCESS OF BONE REMODELING

OSTEOBLAST GENERATION

Osteoblasts are the primary mediators of bone formation.

Osteoprogenitor cells

• Spindle-shaped osteoblast precursors.
• They are funneled into ossification centers for linear bone growth.
• They line both the periosteum and the endosteum for appositional bone growth.

Osteoblasts

• Lie along bone matrix. Bone matrix comprises an inorganic component: hydroxyapatite and an organic component: osteoid.
• Osteoblasts are critical to bone formation, they:
  -Secrete osteoid (the organic (unmineralized) portion of bone – ie, the type 1 collagen fibers and ground substance).
  -Mineralize hydroxyapatite (the hydroxylated calcium and phosphate component of bone) via osteocalcin and osteonectin
  -Mediate osteoclastogenesis (the formation of osteoclasts) via M-CSF (macrophage colony stimulating factor) and RANKL with inhibition by osteoprotogerin.

• Stimulation of osteoprogenitor cell differentiation:
  -Members of the (bone morphogenetic protein) BMP family
  -Transforming Growth Factor Beta

Medulla

• Accounts for approximately 20% of the total adrenal tissue.
• Is neuroectodermal origin
• Secretes the catecholamines:
  Epinephrine
  Norepinephrine

Cortex

• Accounts for the other 80% of the adrenal gland.
• It is of mesodermal origin.
• Secretes:
  Mineralocorticoids.
  Glucocorticoids.
  Androgens.

Cortical Hormones

• Hormonal secretion is triggered by ACTH stimulation (from anterior lobe of pituitary)
• Secretes steroid hormones; cholesterol is common precusor.
• Enzyme availability accounts for differential hormonal production in the cortical layers.

Zona reticularis

• Primarily produces androgens
  Includes DHEA and androstenendione, which are precursor hormones that can be converted to testosterone and estradiol; thus, they are a non-gonadal source of “sex-steroids.”

Zona fasciculata

• Primarily produces glucocorticoids, including cortisol, which has multiple effects throughout the body: increases gluconeogenesis and glycogen storage, suppresses the inflammatory response, and maintains vascular response to catecholamines. Thus, it is secreted in response to mental or physical stressors.

Zona glomerulosa

• Produces mineralocorticoids
• Specifically, angiotensin II (which we discuss in renal physiology), drives the production of aldosterone, which increases sodium reabsorption and potassium secretion in the late distal tubules and collecting ducts of the nephron;
  Thus, it is secreted in response to decreased extracellular fluid volume to conserve body water.

Clinical Correlations:

Adrenal cortex malfunction has deleterious effects; these effects are predictable based on the source of the deficiency.

Primary adrenocortical insufficiency, aka, Addison’s disease

• The destruction of the adrenal cortex inhibits hormone production.
• A distinguishing characteristic of this disorder is that ACTH levels are high, which causes hyperpigmentation. In healthy individuals, circulating cortical hormones inhibit secretion of ACTH from the anterior pituitary via negative feedback; in the absence of cortical hormone production, ACTH secretion goes unchecked.

Secondary adrenocortical insufficiency

• ACTH secretion is insufficient to stimulate conversion of cholesterol for production of the cortical hormones.
  Incidentally, aldosterone levels may be normal, because its production does not depend on increased levels of ACTH.

Cushing’s Syndrome

• Refers to collection of signs and symptoms caused by excess cortisol production:
  Hypertension, hyperglycemia, muscle wasting, “moon face,” and central obesity (fat deposits disproportionally at the core).
• Excessive cortisol production can be caused by a variety of factors, including glucocorticoid drugs or pituitary tumors (Cushing’s disease).
• Drugs that reduce cortisol secretion can be used to counteract some of these effects.

SODIUM/POTASSIUM PUMP

• Found in the membrane of all animal cells
• Active transport – uses ATP (around 30% of a cell’s total ATP usage) breaking it down to ADP and phosphate
• Helps maintain membrane voltage (thought to contribute about 10% of total voltage)
• Maintains sodium and potassium concentration gradients
• Helps maintain cellular volute by regulating a cell’s osmolarity
• Transports 3 sodium ions out of the cell and 2 potassium ions into the cell

The sodium and potassium ion gradients set up by the pump are required for numerous functions such as:

• Nerve cell action potentials
• Muscle contractions
• Glucose absorption by intestinal cells

SODIUM/POTASSIUM PUMP CYCLE

1. Intracellular sodium ions bind the protein

2. Protein becomes phosphorylated (phosphate added)

3. Conformational change in the protein due to the phosphorylation ejects the sodium ions to the now accessible extracellular space

4. Extracellular potassium binds to the protein

5. Protein is dephosphorylated (phosphate is removed)

6. Due to dephosphorylation, protein returns to original conformation and ejects the potassium ions intracellularly. The pump is now ready to start the cycle again at step 1.