Revision sheet: Genetic Mutations and Syndromes

📋 Course Outline

1. Genetic Mutation Fundamentals
2. Inheritance Patterns and Mutation Types
3. Alagille Syndrome
4. McCune–Albright Syndrome
5. Primary Ciliary Dyskinesia
6. Treacher Collins Syndrome

📖 1. Genetic Mutation Fundamentals

🔑 Key Concepts & Definitions

• Genetic mutation : A permanent alteration of the DNA sequence that can produce a clinically significant syndrome.

★ Must-know

📌 Germ-cell mutations can be inherited, whereas somatic-cell mutations arise in body cells and may result from environmental factors such as radiation.

Further detail

• Genetic mutations can participate in physiologic and pathologic processes, including evolution, immune-system development, and cancer.

• Major complications of genetic mutations include spontaneous abortion, severe organ defects such as tetralogy of Fallot and renal dysplasia, visual or hearing disturbances, growth deficiency, intellectual disability, and cancer.

💡 Memory Hook

DNA alteration → altered protein or gene expression → disease

📖 2. Inheritance Patterns and Mutation Types

📝 Essential Points

• A point mutation substitutes one DNA base and may be a transition between two purines or two pyrimidines, or a transversion between a purine and a pyrimidine.

• Silent mutations encode the same amino acid, missense mutations encode a different amino acid, and nonsense mutations create an early stop codon that can produce a nonfunctional protein.

• Deletions remove DNA bases, insertions add DNA bases, splice-site mutations alter mRNA splicing, and frameshift mutations shift the reading frame.

• Mendelian disorders result from a single gene mutation with large effects, chromosomal disorders involve numerical or structural chromosome abnormalities, and complex multigenic disorders involve genes that increase disease risk without independently causing disease.

• Autosomal dominant inheritance requires one mutated copy, autosomal recessive inheritance requires both copies, X-linked mutations are not transmitted from father to son, Y-linked mutations pass from father to son, codominant alleles both affect the same trait, and mitochondrial mutations show maternal inheritance.

📖 3. Alagille Syndrome

🔑 Key Concepts & Definitions

• Alagille syndrome : An autosomal dominant disorder causing liver, skeletal, ocular, cardiac, and renal defects, also called arteriohepatic dysplasia or Alagille–Watson syndrome.

★ Must-know

• About 95% of Alagille syndrome cases involve the JAG1 gene on chromosome 20p12, while NOTCH2 mutations on chromosome 1p13 are associated with renal malformations.

• Loss of intrahepatic bile ducts in Alagille syndrome causes chronic cholestasis with increased conjugated bilirubin, liver enzymes, and cholesterol and may progress to cirrhosis or liver failure.

Further detail

• Characteristic Alagille features include butterfly-shaped or hemivertebrae, a triangular face with prominent forehead and hypertelorism, short stature, peripheral pulmonary artery stenosis or tetralogy of Fallot, renal dysplasia, and ocular abnormalities such as embryotoxon.

• Treatment of Alagille syndrome includes ursodeoxycholic acid with cholestyramine, rifampin, or naltrexone, nutritional support, biliary diversion or liver transplantation, and surgical repair of significant heart defects.

📖 4. McCune–Albright Syndrome

🔑 Key Concepts & Definitions

• McCune–Albright syndrome : Characterized by fibrous dysplasia, hyperfunctioning endocrinopathy, and unilateral café-au-lait skin spots.

★ Must-know

• A postzygotic mutation in GNAS1 on chromosome 20q13.1–13.2 causes somatic mosaicism and abnormal Gs alpha protein signaling through the G-protein, cyclic AMP, and adenylate cyclase pathway.

• Fibrous dysplasia commonly affects the skull base and proximal femur, with radiographs showing expansile lytic lesions, cortical thinning, a ground-glass appearance, and a possible shepherd’s-crook deformity of the proximal femur.

• Endocrine manifestations include hyperthyroidism, Cushing’s syndrome, growth-hormone excess, renal phosphate wasting, and precocious puberty.

Further detail

• Bisphosphonates can reduce pain in fibrous dysplasia but do not stop disease progression, while endocrine medications treat the associated hormone excesses.

💡 Memory Hook

Fibrous dysplasia → endocrinopathy → café-au-lait spots

📖 5. Primary Ciliary Dyskinesia

🔑 Key Concepts & Definitions

• Primary ciliary dyskinesia : A congenital autosomal recessive disease in which defective ciliary motility impairs mucociliary clearance and decreases fertility.

★ Must-know

• Immotile or abnormally moving cilia cause recurrent respiratory infections, chronic rhinosinusitis, bronchiectasis, and impaired movement of fallopian-tube cilia and sperm flagella.

• Kartagener syndrome is the combination of situs inversus, chronic sinusitis, and bronchiectasis occurring in primary ciliary dyskinesia.

Further detail

• Mutations in DNAH5 and DNAI1 affect axonemal outer dynein arms, while mutations in RSPH4A and RSPH9 affect radial spokes.

• Diagnosis can include reduced or absent nasal nitric oxide, spirometry showing obstruction, genetic testing, high-speed video microscopy, transmission electron microscopy, and confirmation from cultured immotile cilia.

📌 Treatment includes antibiotics, nebulized hypertonic saline, mucolytic agents, nasal saline and intranasal glucocorticoids, daily chest physiotherapy, vaccination, smoking cessation, and polyp removal when required.

💡 Memory Hook

Immotile cilia → poor clearance → recurrent infections

📖 6. Treacher Collins Syndrome

🔑 Key Concepts & Definitions

• Treacher Collins syndrome : A severe genetic disorder of craniofacial development, also called mandibulofacial dysostosis or Franceschetti–Zwahlen–Klein syndrome.

★ Must-know

• A TCOF1 mutation on chromosome 5q32 causes treacle-protein insufficiency, impaired ribosome biogenesis, neuroepithelial apoptosis, reduced neural crest cells, and first- and second-branchial-arch anomalies.

• Treacher Collins syndrome is autosomal dominant with variable penetrance, and approximately 60% of cases are spontaneous.

• Typical findings include mandibular and zygomatic hypoplasia, retrognathia, downslanting eyelids, lower-eyelid coloboma, malformed external and middle ears, cleft or high-arched palate, and dental abnormalities.

Further detail

• Management may require reconstructive surgery, tracheostomy for severe neonatal airway obstruction, glossopexy, correction of cleft palate or choanal atresia, gastrostomy, hearing aids, and speech therapy.

💡 Memory Hook

Treacle deficiency → neural crest loss → craniofacial hypoplasia

📊 Synthesis Tables

Inheritance Pattern Comparison

Pattern	Key genetic feature	Transmission
Autosomal dominant	One mutated copy	Affected parent may transmit to offspring
Autosomal recessive	Both copies mutated	May appear when both parents contribute a mutation
X-linked	Mutation on X chromosome	No father-to-son transmission
Y-linked	Mutation on Y chromosome	Father-to-son transmission
Mitochondrial	Mutation in mitochondrial DNA	Maternal inheritance

Syndrome Defining Features

Syndrome	Primary mechanism	Characteristic manifestations
Alagille syndrome	JAG1 or NOTCH2 mutation	Cholestasis with multisystem defects
McCune–Albright syndrome	Postzygotic GNAS1 mutation	Fibrous dysplasia, endocrinopathy, café-au-lait spots
Primary ciliary dyskinesia	Ciliary structural or motility defect	Recurrent infections, bronchiectasis, infertility
Treacher Collins syndrome	TCOF1, POLR1C, or POLR1D mutation	Craniofacial hypoplasia and conductive hearing loss