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First Aid > Biochemistry - Genetics > Flashcards

Biochemistry - Genetics Flashcards

1
Q

Codominance

  • Definition
  • Example
A
  • Both alleles contribute to the phenotype of the heterozygote.
  • Blood groups A, B, AB; a1-antitrypsin deficiency.
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2
Q

Variable expressivity

  • Definition
  • Example
A
  • Phenotype varies among individuals with same genotype.
  • 2 patients with neurofibromatosis type 1 (NF1) may have varying disease severity.
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3
Q

Incomplete penetrance

  • Definition
  • Example
A
  • Not all individuals with a mutant genotype show the mutant phenotype.
  • BRCA1 gene mutations do not always result in breast or ovarian cancer.
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4
Q

Pleiotropy

  • Definition
  • Example
A
  • One gene contributes to multiple phenotypic effects.
  • Untreated phenylketonuria (PKU) manifests with light skin, intellectual disability, and musty body odor.
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5
Q

Anticipation

  • Definition
  • Example
A
  • Increased severity or earlier onset of disease in succeeding generations.
  • Trinucleotide repeat diseases (e.g., Huntington disease).
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6
Q

Loss of heterozygosity

  • Definition
  • Example
A
  • If a patient inherits or develops a mutation in a tumor suppressor gene, the complementary allele must be deleted/mutated before cancer develops.
    • This is not true of oncogenes.
  • Retinoblastoma and the “two-hit hypothesis.”
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7
Q

Dominant negative mutation

  • Definition
  • Example
A
  • Exerts a dominant effect.
    • A heterozygote produces a nonfunctional altered protein that also prevents the normal gene product from functioning.
  • Mutation of a transcription factor in its allosteric site.
    • Nonfunctioning mutant can still bind DNA, preventing wild-type transcription factor from binding.
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8
Q

Linkage disequilibrium

  • Definition
A
  • Tendency for certain alleles at 2 linked loci to occur together more often than expected by chance.
  • Measured in a population, not in a family, and often varies in different populations.
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9
Q

Mosaicism

  • Definition
  • Example
A
  • Presence of genetically distinct cell lines in the same individual.
    • Arises from mitotic errors after fertilization.
    • Somatic mosaicism—mutation propagates through multiple tissues or organs.
    • Gonadal mosaicism—mutation only in egg or sperm cells.
  • McCune-Albright syndrome is lethal if the mutation is somatic, but survivable if mosaic.
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10
Q

Locus heterogeneity

  • Definition
  • Example
A
  • Mutations at different loci can produce a similar phenotype.
  • Albinism.
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11
Q

Allelic heterogeneity

  • Definition
  • Example
A
  • Different mutations in the same locus produce the same phenotype.
  • β-thalassemia.
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12
Q

Heteroplasmy

  • Definition
A
  • Presence of both normal and mutated mtDNA, resulting in variable expression in mitochondrial inherited disease.
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13
Q

Uniparental disomy

  • Definition
  • Heterodisomy
  • Isodisomy
A
  • Definition
    • Offspring receives 2 copies of a chromosome from 1 parent and no copies from the other parent.
    • Uniparental is eUploid (correct number of chromosomes), not aneuploid.
    • Most occurrences of UPD Ž–> normal phenotype.
    • Consider UPD in an individual manifesting a recessive disorder when only one parent is a carrier.
  • Heterodisomy (heterozygous)
    • Indicates a meiosis I error.
  • Isodisomy (homozygous)
    • Indicates a meiosis II error or postzygotic chromosomal duplication of one of a pair of chromosomes, and loss of the other of the original pair.
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14
Q

Hardy-Weinberg population genetics

  • Definition
  • Hardy-Weinberg law assumptions
A
  • If a population is in Hardy-Weinberg equilibrium and if p and q are the frequencies of separate alleles, then:
    • p2 + 2pq + q2 = 1
    • p + q = 1
    • p2 = frequency of homozygosity for allele p
    • q2 = frequency of homozygosity for allele q
    • 2pq = frequency of heterozygosity (carrier frequency, if an autosomal recessive disease).
    • The frequency of an X-linked recessive disease in males = q and in females = q2.
  • Hardy-Weinberg law assumptions
    • ƒƒNo mutation occurring at the locus
    • Natural selection is not occurring
    • Completely random mating
    • No net migration
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15
Q

Imprinting

  • Definition
  • Examples
A
  • Definition
    • At some loci, only one allele is active
    • The other is inactive (imprinted/inactivated by methylation).
    • With one allele inactivated, deletion of the active allele –> disease.
  • Examples
    • Both Prader-Willi and Angelman syndromes are due to mutation or deletion of genes on chromosome 15.
    • Can also occur as a result of uniparental disomy.
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16
Q

Prader-Willi syndrome

A
  • Maternal imprinting: gene from mom is normally silent and paternal gene is deleted/mutated.
    • Results in hyperphagia, obesity, intellectual disability, hypogonadism, and hypotonia.
  • 25% of cases due to maternal uniparental disomy
    • Two maternally imprinted genes are received
    • No paternal gene received
  • Prader-Willi syndrome –> Paternal gene
  • AngelMan syndrome –> Maternal gene
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17
Q

Angelman syndrome

A
  • Paternal imprinting: gene from dad is normally silent and maternal gene is deleted/mutated.
    • Results in inappropriate laughter (“happy puppet”), seizures, ataxia, and severe intellectual disability.
  • 5% of cases due to paternal uniparental disomy
    • Two paternally imprinted genes are received
    • No maternal gene received
  • Prader-Willi syndrome –> Paternal gene
  • AngelMan syndrome –> Maternal gene
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18
Q

Autosomal dominant mode of inheritance

A
  • Often due to defects in structural genes.
  • Many generations, both male and female, affected.
  • Often pleiotropic.
  • Family history crucial to diagnosis.
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19
Q

Autosomal recessive mode of inheritance

A
  • 25% of offspring from 2 carrier parents are affected.
  • Often due to enzyme deficiencies.
  • Usually seen in only 1 generation.
  • Commonly more severe than dominant disorders
  • Patients often present in childhood.
  • Increased risk in consanguineous families.
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20
Q

X-linked recessive mode of inheritance

A
  • Sons of heterozygous mothers have a 50% chance of being affected.
  • No male-to-male transmission.
  • Commonly more severe in males.
  • Females usually must be homozygous to be affected.
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21
Q

X-linked dominant mode of inheritance

A
  • Transmitted through both parents.
  • Mothers transmit to 50% of daughters and sons.
  • Fathers transmit to all daughters but no sons.
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22
Q

Hypophosphatemic rickets

A
  • X-linked dominant
  • Formerly known as vitamin D–resistant rickets.
  • Inherited disorder resulting in increased phosphate wasting at proximal tubule.
  • Results in rickets-like presentation.
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23
Q

Mitochondrial inheritance

A
  • Transmitted only through the mother.
  • All offspring of affected females may show signs of disease.
  • Variable expression in a population or even within a family due to heteroplasmy.
24
Q

Mitochondrial myopathies

A
  • Rare disorders of mitochondrial inheritance
  • Often present with myopathy, lactic acidosis and CNS disease.
  • 2° to failure in oxidative phosphorylation.
  • Muscle biopsy often shows “ragged red fibers.”
25
Q
A
  • Autosomal dominant
26
Q
A
  • Autosomal recessive
27
Q
A
  • X-linked recessive
28
Q
A
  • X-linked dominant
29
Q
A
  • Mitochondrial inheritance
30
Q

Autosomal dominant polycystic kidney disease (ADPKD)

A
  • Autosomal dominant.
  • Formerly known as adult polycystic kidney disease.
  • Always bilateral, massive enlargement of kidneys due to multiple large cysts.
  • 85% of cases are due to mutation in PKD1.
  • Chromosome 16.
    • 16 letters in “polycystic kidney.”
  • Remainder due to mutation in PKD2 (chromosome 4).
31
Q

Familial adenomatous polyposis

A
  • Autosomal dominant.
  • Colon becomes covered with adenomatous polyps after puberty.
  • Progresses to colon cancer unless colon is resected.
  • Mutations on chromosome 5 (APC gene).
    • 5 letters in “polyp.”
32
Q

Familial hypercholesterolemia

A
  • Autosomal dominant.
  • Elevated LDL due to defective or absent LDL receptor.
  • Leads to severe atherosclerotic disease early in life, and tendon xanthomas (classically in the Achilles tendon).
33
Q

Hereditary hemorrhagic telangiectasia

A
  • Autosomal dominant.
  • Inherited disorder of blood vessels.
  • Findings: telangiectasia, recurrent epistaxis, skin discolorations, arteriovenous malformations (AVMs), GI bleeding, hematuria.
  • Also known as Osler-Weber-Rendu syndrome.
34
Q

Hereditary spherocytosis

A
  • Autosomal dominant.
  • Spheroid erythrocytes due to spectrin or ankyrin defect.
  • Hemolytic anemia.
  • Increased MCHC.
  • Treatment: splenectomy.
35
Q

Huntington disease

A
  • Autosomal dominant.
  • Findings: depression, progressive dementia, choreiform movements, caudate atrophy, and decreased levels of GABA and ACh in the brain.
  • Gene on chromosome 4.
    • “Hunting 4 food.”
  • Trinucleotide repeat disorder: (CAG)n.
  • Increased repeats –> decreased age of onset.
36
Q

Marfan syndrome

A
  • Autosomal dominant.
  • Fibrillin-1 gene mutation –> connective tissue disorder affecting skeleton, heart, and eyes.
  • Findings
    • Tall with long extremities, pectus excavatum, hypermobile joints, and long, tapering fingers and toes (arachnodactyly).
    • Cystic medial necrosis of aorta –> aortic incompetence and dissecting aortic aneurysms.
    • Floppy mitral valve.
  • Subluxation of lenses, typically upward and temporally.
37
Q

Multiple endocrine neoplasias (MEN)

A
  • Autosomal dominant.
  • Several distinct syndromes (1, 2A, 2B) characterized by familial tumors of endocrine glands, including those of the pancreas, parathyroid, pituitary, thyroid, and adrenal medulla.
  • MEN 2A and 2B are associated with ret gene.
38
Q

Neurofibromatosis type 1 (von Recklinghausen disease)

A
  • Autosomal dominant.
  • Neurocutaneous disorder characterized by café-au-lait spots and cutaneous neurofibromas.
  • Autosomal dominant, 100% penetrance, variable expression.
  • Caused by mutations in the NF1 gene on chromosome 17.
    • 17 letters in “von Recklinghausen.”
39
Q

Neurofibromatosis type 2

A
  • Autosomal dominant.
  • Findings: bilateral acoustic schwannomas, juvenile cataracts, meningiomas, and ependymomas.
  • NF2 gene on chromosome 22.
    • Type 2 = 22.
40
Q

Tuberous sclerosis

A
  • Autosomal dominant.
  • Neurocutaneous disorder with multi-organ system involvement, characterized by numerous benign hamartomas.
  • Incomplete penetrance, variable expression.
41
Q

von Hippel-Lindau disease

A
  • Autosomal dominant.
  • Disorder characterized by development of numerous tumors, both benign and malignant.
  • Associated with deletion of VHL gene (tumor suppressor) on chromosome 3 (3p).
    • Von Hippel-Lindau = 3 words for chromosome 3.
42
Q

Autosomal recessive diseases

A
  • Kids Go TWASH CAMPS
  • Kartagener syndrome
  • Glycogen storage diseases
  • Thalassemias
  • Wilson disease
  • ARPKD (formerly known as infantile polycystic kidney disease)
  • Sickle cell anemia
  • Hemochromatosis
  • Cystic fibrosis
  • Albinism
  • Mucopolysaccharidoses (except Hunter syndrome)
  • Phenylketonuria
  • Sphingolipidoses (except Fabry disease)
43
Q

Cystic fibrosis

  • Genetics
  • Pathophysiology
A
  • Genetics
    • Autosomal recessive
    • Defect in CFTR gene on chromosome 7
    • Commonly a deletion of Phe508.
    • Most common lethal genetic disease in Caucasian population.
  • Pathophysiology
    • CFTR encodes an ATP-gated Cl- channel that secretes Cl- in lungs and GI tract, and reabsorbs Cl- in sweat glands.
    • Mutations –> misfolded protein –> protein retained in RER and not transported to cell membrane, causing decreased Cl- (and H2O) secretion.
    • Increased intracellular Cl- results in compensatory increased Na+ reabsorption via epithelial Na+ channels –> increased H2O reabsorption –> abnormally thick mucus secreted into lungs and GI tract. 
    • Increased Na+ reabsorption also causes more negative transepithelial potential difference.
44
Q

Cystic fibrosis

  • Diagnosis
  • Complications
  • Treatment
A
  • Diagnosis
    • Increased Cl- concentration (>60 mEq/L) in sweat is diagnostic.
    • Can present with contraction alkalosis and hypokalemia (ECF effects analogous to a patient taking a loop diuretic) because of ECF H2O/Na+ losses and concomitant renal K+/H+ wasting.
  • Complications
    • Recurrent pulmonary infections (e.g., Pseudomonas), chronic bronchitis and bronchiectasis –> reticulonodular pattern on CXR, pancreatic insufficiency, malabsorption and steatorrhea, nasal polyps, and meconium ileus in newborns.
    • Infertility in males (absence of vas deferens, absent sperm).
    • Fat-soluble vitamin deficiencies (A, D, E, K).
  • Treatment
    • N-acetylcysteine to loosen mucus plugs (cleaves disulfide bonds within mucus glycoproteins)
    • Dornase alfa (DNAse) to clear leukocytic debris.
45
Q

X-linked recessive disorders

  • Disorders
  • Female carriers
A
  • Oblivious Female Will Obviously Give Her Boys Her x-Linked Disorders
    • Ocular albinism
    • Fabry disease
    • Wiskott-Aldrich syndrome
    • Ornithine transcarbamylase deficiency
    • G6PD deficiency
    • Hunter syndrome
    • Bruton agammaglobulinemia
    • Hemophilia A & B
    • Lesch-Nyhan syndrome
    • Duchenne (and Becker) muscular dystrophy
  • Female carriers can be variably affected depending on the percentage inactivation of the X chromosome carrying the mutant vs. normal gene.
46
Q

Duchenne muscular dystrophy

  • Mutation
  • Symptoms
  • Dystrophin
  • Diagnosis
A
  • Mutation
    • X-linked frameshift mutation –>Ž truncated dystrophin protein Ž–> accelerated muscle breakdown.
    • Duchenne = deleted dystrophin.
  • Symptoms
    • Weakness begins in pelvic girdle muscles and progresses superiorly.
    • Pseudohypertrophy of calf muscles due to fibrofatty replacement of muscle [A].
    • Gower maneuver—patients use upper extremity to help them stand up.
    • Onset before 5 years of age.
    • Dilated cardiomyopathy is common cause of death.
  • Dystrophin
    • Dystrophin gene (DMD) has the longest coding region of any human gene –> increased chance of spontaneous mutation.
    • Dystrophin helps anchor muscle fibers, primarily in skeletal and cardiac muscle.
    • It connects the intracellular cytoskeleton (actin) to the transmembrane proteins α- and β-dystroglycan, which are connected to the extracellular matrix (ECM).
    • Loss of dystrophin results in myonecrosis. 
  • Diagnosis
    • Increased CPK and aldolase are seen
    • Western blot and muscle biopsy confirm diagnosis.
47
Q

Muscular dystrophies

  • Becker
  • Myotonic type 1
A
  • Becker
    • Usually, X-linked point mutation in dystrophin gene (no frameshift).
    • Less severe than Duchenne.
    • Onset in adolescence or early adulthood.
    • Deletions can cause both Duchenne and Becker.
  • Myotonic type 1
    • CTG trinucleotide repeat expansion in the DMPK gene
    • Ž–> abnormal expression of myotonin protein kinase
    • –>Ž myotonia, muscle wasting, frontal balding, cataracts, testicular atrophy, and arrhythmia.
48
Q

Fragile X syndrome

  • Definition
  • Findings
A
  • Definition
    • X-linked defect affecting the methylation and expression of the FMR1 gene.
    • The 2nd most common cause of genetic intellectual disability (after Down syndrome).
    • Trinucleotide repeat disorder (CGG)n.
  • Findings
    • Postpubertal macroorchidism (enlarged testes), long face with a large jaw, large everted ears, autism, mitral valve prolapse.
    • ​Fragile X = eXtra large testes, jaw, ears.
49
Q

Trinucleotide repeat expansion diseases

A
  • Try (trinucleotide) hunting for my fried eggs (X).
    • Huntington disease, myotonic dystrophy, Friedreich ataxia, fragile X syndrome.
  • _X-G_irlfriend’s First Aid Helped Ace My Test.
    • Fragile X syndrome = (CGG)n.
    • Friedreich ataxia = (GAA)n.
    • Huntington disease = (CAG)n.
    • Myotonic dystrophy = (CTG)n.
  • May show genetic anticipation (disease severity increases and age of onset decreases in successive generations).
50
Q

Down syndrome

  • Definition
  • Findings
  • Due to…
  • First-trimester ultrasound commonly shows…
  • Second-trimester quad screen shows…
A
  • Definition
    • Autosomal trisomy: trisomy 21, 1:700
      • Drinking age (21).
    • Most common viable chromosomal disorder and most common cause of genetic intellectual disability
  • Findings
    • Intellectual disability, flat facies, prominent epicanthal folds, single palmar crease, gap between 1st 2 toes, duodenal atresia, Hirschsprung disease, congenital heart disease (most commonly ostium primum-type atrial septal defect [ASD]), Brushfield spots.
    • Associated with increased risk of ALL, AML, and Alzheimer disease (> 35 years old).
  • Due to…
    • 95% of cases due to meiotic nondisjunction of homologous chromosomes
      • Associated with advanced maternal age; from 1:1500 in women < 20 to 1:25 in women > 45 years old.
    • 4% of cases due to Robertsonian translocation.
    • 1% of cases due to mosaicism
      • No maternal association
      • Post-fertilization mitotic error).
  • First-trimester ultrasound commonly shows…
    • Increased nuchal translucency and hypoplastic nasal bone
    • Serum PAPP-A is decreased, free β-hCG is increased.
  • Second-trimester quad screen shows…
    • Decreased α-fetoprotein, increased β-hCG, decreased estriol, increased inhibin A.
51
Q

Edwards syndrome

  • Definition
  • Findings
  • First-trimester ultrasound shows…
  • Quad screen shows…
A
  • Definition
    • Autosomal trisomy: trisomy 18, 1:8000
      • Election age (18).
    • Most common trisomy resulting in live birth after Down syndrome.
    • Death usually occurs within 1 year of birth.
  • Findings
    • Severe intellectual disability, rockerbottom feet, micrognathia (small jaw), low-set Ears, clenched hands, prominent occiput, congenital heart disease.
  • First-trimester ultrasound shows…
    • PAPP-A and free β-hCG are decreased.
  • Quad screen shows…
    • Ddecreased α-fetoprotein, decreased β-hCG, decreased estriol, decreased or normal inhibin A.
52
Q

Patau syndrome

  • Definition
  • Findings
  • First-trimester pregnancy screen shows…
A
  • Definition
    • Autosomal trisomy: trisomy 13, 1:15,000
      • Puberty (13).
    • Death usually occurs within 1 year of birth.
  • Findings
    • Severe intellectual disability, rockerbottom feet, microphthalmia, microcephaly, cleft liP/Palate, holoProsencephaly, Polydactyly, congenital heart disease.
  • First-trimester pregnancy screen shows…
    • Decreased free b-hCG, decreased PAPP-A, and increased nuchal translucency.
53
Q

Meiotic nondisjunction

A
54
Q

Robertsonian translocation

  • Definition
  • Balanced vs. unbalanced
A
  • Definition
    • Nonreciprocal chromosomal translocation that commonly involves chromosome pairs 13, 14, 15, 21, and 22.
    • One of the most common types of translocation.
    • Occurs when the long arms of 2 acrocentric chromosomes (chromosomes with centromeres near their ends) fuse at the centromere and the 2 short arms are lost.
  • Balanced vs. unbalanced
    • Balanced translocations normally do not cause any abnormal phenotype.
    • Unbalanced translocations can result in miscarriage, stillbirth, and chromosomal imbalance (e.g., Down syndrome, Patau syndrome).
55
Q

Cri-du-chat syndrome

  • Definition
  • Findings
A
  • Definition
    • Congenital microdeletion of short arm of chromosome 5 (46,XX or XY, 5p-).
  • Findings
    • Microcephaly, moderate to severe intellectual disability, high-pitched crying/mewing, epicanthal folds, cardiac abnormalities (VSD).
    • Cri du chat = cry of the cat.
56
Q

Williams syndrome

  • Definition
  • Findings
A
  • Definition
    • Congenital microdeletion of long arm of chromosome 7 (deleted region includes elastin gene).
  • Findings
    • Distinctive “elfin” facies, intellectual disability, hypercalcemia (increased sensitivity to vitamin D), well-developed verbal skills, extreme friendliness with strangers, cardiovascular problems.
57
Q

22q11 deletion syndromes

  • Due to…
  • Findings
  • DiGeorge syndrome
  • Velocardiofacial syndrome
A
  • Due to…
    • Aberrant development of 3rd and 4th branchial pouches.
  • Findings (CATCH-22)
    • Variable presentation, including Cleft palate, Abnormal facies, Thymic aplasia Ž–> T-cell deficiency, Cardiac defects, 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.

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