Achondroplasia
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Achondroplasia
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Genetic disorders
Achondroplasia
Alagille syndrome (NORD)
Familial adenomatous polyposis
Familial hypercholesterolemia
Hereditary spherocytosis
Huntington disease
Li-Fraumeni syndrome
Marfan syndrome
Multiple endocrine neoplasia
Myotonic dystrophy
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von Hippel-Lindau disease
Albinism
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Gaucher disease (NORD)
Glycogen storage disease type I
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Glycogen storage disease type IV
Glycogen storage disease type V
Hemochromatosis
Krabbe disease
Leukodystrophy
Mucopolysaccharide storage disease type 1 (Hurler syndrome) (NORD)
Niemann-Pick disease type C
Niemann-Pick disease types A and B (NORD)
Phenylketonuria (NORD)
Polycystic kidney disease
Primary ciliary dyskinesia
Sickle cell disease (NORD)
Tay-Sachs disease (NORD)
Wilson disease
Cri du chat syndrome
Williams syndrome
Angelman syndrome
Prader-Willi syndrome
Beckwith-Wiedemann syndrome
Mitochondrial myopathy
Klinefelter syndrome
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Friedreich ataxia
Huntington disease
Myotonic dystrophy
Down syndrome (Trisomy 21)
Edwards syndrome (Trisomy 18)
Patau syndrome (Trisomy 13)
Alport syndrome
Fragile X syndrome
Fabry disease (NORD)
Glucose-6-phosphate dehydrogenase (G6PD) deficiency
Hemophilia
Lesch-Nyhan syndrome
Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)
Muscular dystrophy
Ornithine transcarbamylase deficiency
Wiskott-Aldrich syndrome
X-linked agammaglobulinemia
Autosomal trisomies: Pathology review
Miscellaneous genetic disorders: Pathology review
Muscular dystrophies and mitochondrial myopathies: Pathology review
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Achondroplasia
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In HBO’s adaptation of Game of Thrones, the character Tyrion Lannister is treated poorly by his father and siblings because he is born with dwarfism.
In a classic scene in the show, he says [“I’m guilty of being a dwarf! [father says: You’re not on trial for being a dwarf] Oh! Yes I am, I’ve been on trial for that my entire life].
Both Tyrion and his real-life counterpart—Peter Dinklage—have achondroplasia, an autosomal dominant genetic condition which is the most common cause of dwarfism and results from a heterozygous mutation in a gene called FGFR3, or fibroblast growth factor receptor 3, on chromosome 4, which codes for FGFR3 protein.
When FGFR3 protein binds fibroblast growth factors, or FGFs, it slows down the growth of certain bones.
The mutation causing achondroplasia is almost always the 380th amino acid, which is glycine, getting swapped out for arginine in the FGFR3 protein, and this swap causes the FGFR3 receptor to be constitutively active, which means constantly, active.
In other words, the mutation makes the receptor behave as though it’s binding an FGF even when it’s not, which sends a strong signal to inhibit bone growth.
More specifically, FGFR3 that is “always on” causes chondrocytes at the growth plate to proliferate slowly and become disorganized.
So, because of this it mostly affects endochondral bone formation, which is the process of bone forming right on previously-laid-down cartilage matrix, which causes the bone to elongate.
With the mutation though, this elongation is inhibited, which means long bones like the humerus and phalanges are affected.
Alright so the mutation affects endochondral bone formation, but bones that are products of intramembranous bone formation are way less affected.
This is where bone grows without an existing cartilage matrix.
This includes flat bones like the skull and ribs. Also an intramembranous process is appositional growth, which is the process of widening of long bones, so that happens pretty normally too.
Sources
- "Robbins Basic Pathology" Elsevier (2017)
- "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
- "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
- "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
- "Achondroplasia" The Lancet (2007)
- "Distraction Osteogenesis of the Lower Extremity in Patients With Achondroplasia/Hypochondroplasia Treated With Transplantation of Culture-Expanded Bone Marrow Cells and Platelet-Rich Plasma" Journal of Pediatric Orthopaedics (2007)
- "Achondroplasia: pathogenesis and implications for future treatment" Current Opinion in Pediatrics (2010)
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