Musculoskeletal Geneticsx

Musculoskeletal Genetics Skeletal Dysplasia • Achondroplasia: 1 in 25000 births • Osteogenesis imperfecta: 1 in 20000 births • Morquio syndrome (short trunk): 1 in 200 000 – 500 000 births Achondroplasia • Autosomal dominant • 75-80% mutations are de novo • Homozygous achondroplasia usually fatal • Heterozygous: long-life expectancy • Caused by mutations in Fibroblast Growth Factor Receptor (FGFR3)  Gene on chr.4p16.3  FGFR3 function: Cell growth and division regulation o Cell type determination o Angiogenesis o Wound healing o Embryonic development o FGFRs in Bone Growth • FGFR1: Hypertrophic chondrocyte and perichondrium • FGFR2: Perichondrium, periosteum, primary spongiosa • FGFR3: proliferating chondrocyte • FGFRs are protein tyrosine kinase receptors o EC domains contains 3 Ig-like domains, a single peptide and an acid box. There’s a single transmembrane domain o Ig-like domain contains tyrosine kinase phosphorylation acceptor domains o Function: when FGF binds to FGFR, FGFR monomers dimerise  the tyrosine kinase domains autophosphorylate another  intracellular signalling pathway  modification of transcription (promotes differentiation of cartilage into bone) o FGFR3 Mutations • 99% of cases are gain-of-function mutations at G380R • Receptor becomes constituently active • Premature conversion of growth plate into bone Reduced cellular hypertrophy    1 Reduced cartilage matrix production Early differentiation of cartilage and termination of bone growth Diagnosis Steps • History: birth length, foetal size-gestation discrepancy, age-growth failure, family history, parental height • Physical exam: height, weight, head circumference, ratio of upper segment to lower segment, armspan:height ratio • Imaging: skeletal radiographs  determines which part of bone is involved • Molecular diagnostics Findings • Overall height below average • Upper segment close to average • Lower segment below average • Head circumference above average Prognosis • Complications o Cervicomedullary compression o Spinal stenosis o Tibial bowing o Otitis media o Restrictive or obstructive lung disease • Risk factors o Advanced paternal age • Normal intelligence and life expectancy Osteogenesis Imperfecta • Brittle bone disease • 8 types: 4 show mutations in type I collagen gene COL1A1/COL1A2 • Mostly autosomal dominant: type 1 less severe than type 2 • High levels of de novo mutations • High frequency of bone fractures, loose joints, muscle weakness, brittle teeth, abnormal spine curvature, respiratory issues, blue tinted sclera • Type 1: near normal stature and life expectancy • Type 2: small stature, short life expectancy Bone Matrix    2 • Generally affects 17q21.33 (COL1A1) and 7q21.3 (COL1A2) • Reduced functional collagen  low matrix stability Osteoporosis-like changes: low cortical thickness, high intertrabecular space Mild OI: no expression Severe OI: abnormal expression  mutant chains Muscular Dystrophy • Duchenne: most common and severe form • Becker: less severe mutation of genes as Duchenne • Males present between 3 and 5 years • Slow progressive muscle weakness • Gower’s sign: how the child stands from a sitting position but walking themselves upright with their hands • Mild-moderate intellectual impairment • Most need wheelchair by 11 years, survive to in their 30s Duchenne’s Muscular Dystrophy (DMD) Genetics • Mutations of dystrophin gene on chr.Xp21 • X-linked recessive: 30% de novo mutations • Mutation  semi-functional proteins  BMD • Mutation  non-functional proteins  DMD • 2/3 are deletion mutations • 79 exons  many differentially expressed isoforms o Dp427: skeletal muscle o Dp260: retina o Dp71: brain Dystrophin • Part of group that strengthen muscle fibres and protect them from injury as they contract and relax • Act as anchor, connecting cytoskeleton with EC matrix • May play role in cell signalling DMD Degeneration Pathophysiology • Lack of connection between cytoskeleton and ECM  dystrophin associated protein complex destabilisation o  progressive muscle fibre damage o  increased membrane permeability    3 • CPK (creatine phosphokinase) can leak out, Ca can leak in, macrophage infiltration • Muscle is replaced with fat and CT  hypertrophic calves • < 3% dystrophin in biopsies indicate DMD Multiple Ligation-Dependent Probe Amplification Exon amplification Detects missing exons (deleted) Nowadays this is done fluorescently    4 Microarray Fluorescence • PCR fragments containing exons from dystrophic gene Non-Progressive Cognitive Impairment • Common feature of DMD: 30% have intellectual disability, frequent neurocognitive impairment, higher incidence of neuropsychiatric disorders • Language disorders, global delay • Verbal disability, short term memory, disabilities in learning to read, write, calculate, verbal expression • Intact visuospatial cognitive abilities • Common symptoms with ASD may suggest a genetic link Muscle Regeneration • Adults don’t usually regenerate muscle fibres • Existing muscle fibres can resume growth is needed due to cells serving as myoblasts being retained • These satellite cells can be activated to proliferate in damage muscle • Their progeny can fuse to repair damaged muscle • In DMD, satellite cell repair process is limited because regeneration can’t keep up with degradation • CT eventually replaces muscle cells blocking further regeneration • In females, enough dystrophin is produced by the other Xchromosome so that satellite recovery is sufficient • Stem cell therapy autologous/allogenic stem cell transfer    5