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Autosomal recessive diseases (10)
2. ARPKD - autosomal recessive polycystic kidney disease (formerly known as infantile polycystic kidney disease)
3. Cystic fibrosis
4. Glycogen storage diseases
6. Mucopolysaccharidoses (except Hunter's)
8. Sickle cell anemias
9. Sphingolipidoses (except Fabry's)
Which gene is defective? On which chromosome? Which deletion?
Autosomal-recessive defect in CFTR gene on chromosome 7, commonly deletion of Phe 508.
What does the CFTR channel normally do, and what happens with the defective channel? What diseases/conditions result (5)? What happens to the mutated channel?
Normal CFTR channel actively secretes Cl⁻ in lungs and GI tract and actively reabsorbs Cl⁻ from sweat.
Defective Cl⁻ channel → secretion of abnormally thick mucus that plugs lungs, pancreas, and liver →
1. Recurrent pulmonary infections (Pseudomonas spp and S. aureus)
2. Chronic bronchitis
4. Pancreatic insufficiency (malabsorption and steatorrhea)
5. Meconium ileus in newborns.
Mutation causes abnormal protein folding, resulting in degradation of channel before reaching cell surface.
What else is CF associated with (4)?
Infertility in males due to bilateral abscence of vas deferens.
Fat-soluble vitamin deficiencies (A, D, E, K).
Can present as failure to thrive in infancy.
Most common lethal genetic disease of Caucasians
CF: diagnosis and treatment
↑ concentration of Cl⁻ ions in sweat test is diagnostic.
Treatment: N-acetylcysteine to loosen mucous plugs (cleaves disulfide bonds within mucous glycoproteins).
X-linked recessive disorders (8)
"Be Wise, Fool's GOLD Heeds Silly Hope"
1. Bruton's agammaglobulinemia
2. Wiskott-Aldrich syndrome
3. Fabry's disease
4. G6PD deficiency
5. Ocular albinism
6. Lesch-Nyhan syndrome
6. Duchenne's (and Becker's) muscular dystrophy
7. Hunter's Syndrome
8. Hemophilia A and B.
Female carriers are rarely affected due to random inactivation of an X chromosome in each cell.
"Be Wise, Fool's GOLD Heeds Silly Hope"
Duchenne's: which gene is affected? What is the normal function of this gene?
Duchenne's = Deleted Dystrophin
Dystrophin gene (DMD) is the longest known human gene → ↑ rate of spontaneous mutation.
Dystrophin helps anchor muscle fibers, primarily in skeletal and cardiac muscle
X-linked frame-shift mutation → deletion of dystrophin gene → accelerated muscle breakdown.
Weakness begins in pelvic girdle muscles and progresses superiorly.
Duchenne's: clinical presentation (4)
1. Pseudohypertrophy of calf muscles due to fibrofatty replacement of muscle.
2. Cardiac myopathy.
3. Use of Gower's maneuver, requiring assistance of the upper extremities to stand up, is characteristic.
4. Onset before 5 years of age.
Becker's: genetic defect, prognosis, age of onset
X-linked mutated dystrophin gene.
Less severe than Duchenne's.
Onset in adolescence or early adulthood
Fragile X syndrome: genetic defect
Trinucleotide repeat disorder (CGG)n.
X-linked defect affecting the methylation and expression of the FMRI gene (required for normal neural development).
2nd most common cause of genetic mental retardation (after Down syndrome)
Fragile X syndrome: findings (5)
1. Macro-orchidism (enlarged testes)
2. Long face with a large jaw
3. Large everted ears
5. Mitral valve prolapse
"Fragile X = eXtra-large testes, jaw, ears"
Trinucleotide repeat expansion diseases (4)
HUNTINGton's disease, MYotonic dystrophy, FRIEDreich's ataxia, fragile X syndrome.
"Try (trinucleotide) HUNTING for MY FRIED eggs (X)"
Huntington's disease = (CAG)n
MyoTonic dystrophy = (CTG)n
FraGile X syndrome = (CGG)n
Friedreich's ataxia = (GAA)n
May show genetic anticipation (disease severity ↑ and age of onset ↓ in successive generations; germline expansion in females)
Down syndrome: chromosome, prevalence, causes
Trisomy 21, 1:700
"Drinking age (21)"
95% due to meiotic nondisjunction of homologous chromosomes (associated with advanced maternal age; from 1:1500 in women < 20 to 1:25 in women > 45).
4% of cases due to Robertsonian translocation.
1% of cases due to Down mosaicism (no maternal association)
Down syndrome: Findings (8)
1. Mental retardation
2. FLAT FACIES
3. EPICANTHAL FOLDS (skin fold of upper eye covering inner corner of eye)
4. SIMIAN CREASE (single crease across palm)
5. Gap between 1st 2 toes
6. Duodenal atresia
7. Congenital heart disease (most commonly septum primum-type ASD)
8. Associated with ↑ risk of ALL and Alzheimer's disease (> 35 years of age)
Most common ____ disorder and most common cause of congenital ____ _____.
Most common chromosomal disorder and most common cause of congenital mental retardation.
Down syndrome: diagnosis
Results of pregnancy quad screen:
↓ α-fetoprotein, ↑ β-hCG, ↓ estriol, ↑ inhibin A.
Ultrasound shows ↑ nuchal translucency
Edwards' syndrome: which chromosome? Prevalence?
Trisomy 18, 1:8000
"Election age (18)"
Most common trisomy resulting in live birth after Down syndrome
Edwards' syndrome: findings (8)
1. Severe mental retardation
2. Rocker-bottom feet (prominent calcaneous, convex rounded bottom to foot)
3. MICROGNATHIA (small jaw)
4. Low-set ears
5. CLENCHED HANDS
6. Prominent occiput (posterior portion of head)
7. Congenital heart disease
8. Death usually occurs within 1 year of birth
Patau's syndrome: chromosome location, findings (9)
Trisomy 13, 1:15,000
1. Severe mental retardation
2. Rocker-bottom feet
3. Microphthalmia (small eye)
5. CLEFT LIP/PALATE
8. Congenital heart disease
9. Death usually occurs within 1 year of birth
Robertsonian translocation: what is it?
NONRECIPROCAL chromosomal translocation that commonly involves chromosome pairs 13, 14, 15, 21, 22. One of the most common types of translocation.
Occurs when the long arms of 2 acrocentric chromosomes (chromosomes with centromere near their ends) fuse at the centromere and the 2 short arms are lost.
Balanced translocation normally do not cause any abnormal phenotype.
Unbalanced translocations can result in miscarriage, still birth, and chromosomal imbalance (e.g. Down syndrome, Patau's syndrome)
Cri-du-chat syndrome: genetic defect, findings (5)
Congenital microdeletion of short arm of chromosome 5 (46,XX or XY, 5p-)
2. Moderate to severe mental retardation
3. High-pitched crying/mewing
4. Epicanthal folds
5. Cardiac abnormalities
"Cri du chat = cry of the cat"
Williams syndrome: genetic defect, findings (6)
Congenital microdeletion of long arm of chromosome 7 (deleted region includes elastin gene)
1. Distinctive "elfin" facies
2. Mental retardation
3. Hypercalcemia (↑ sensitivity to vitamin D)
4. Well-developed verbal skills
5. Extreme friendliness with strangers
6. Cardiovascular problems
22q11 deletion syndromes: variable presentation (5), 2 syndromes
Variable presentation, including:
1. Cleft palate
2. Abnormal facies
3. Thymic aplasia → T cell deficiency
4. Cardiac defects
5. Hypocalcemia 2° to parathyroid aplasia
Due to microdeletion at chromosome 22q11.
DiGeorge syndrome - thymic, parathyroid, and cardiac defects
Velocardiofacial syndrome - palate, facial, and cardiac defects
Due to aberrant development of 3rd and 4th branchial pouches.
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