Mitochondrial And Non-Mendelian Inheritance - USMLE Step 1

1. This tissue-specific vulnerability is primarily due to the dependence of these cells on which of the following processes?
   Answer: Oxidative phosphorylation Mitochondria are the primary sites of oxidative phosphorylation (the electron transport chain and ATP synthase), the most efficient process for generating ATP. Tissues like the brain, heart, and skeletal muscle have extremely high energy (ATP) demands to maintain ion gradients, contract, and perform their functions. Mutations in mitochondrial DNA often impair the components of the electron transport chain, leading to a severe deficit in ATP production. This energy failure disproportionately affects high-demand tissues, explaining the characteristic clinical manifestations of mitochondrial diseases.
2. Which of the following molecular modifications is the primary mechanism responsible for establishing and maintaining the parent-specific imprinting of these genes?
   Answer: DNA methylation Genomic imprinting is an epigenetic process that involves the differential marking of genes based on their parental origin. The primary molecular mechanism for establishing and maintaining these imprints is DNA methylation. Specific CpG islands within imprinting control regions (ICRs) are methylated in a sex-specific manner during gametogenesis. This methylation pattern is then maintained in somatic cells after fertilization, leading to the silencing of either the maternal or paternal allele.
3. The silencing of the maternal genes in this chromosomal region is an example of which of the following mechanisms?
   Answer: Genomic imprinting This patient has Prader-Willi syndrome, which results from loss of paternally expressed genes in the 15q11-q13 region. The maternal copies of these genes are normally silenced through genomic imprinting, an epigenetic mechanism where gene expression depends on the parent of origin. In this region, paternal genes are expressed while maternal genes are imprinted (silenced), so loss of the paternal contribution results in no gene expression from this region.
4. This condition is often associated with dysregulation of imprinted genes on chromosome 11. This illustrates that non-Mendelian inheritance can result from which of the following?
   Answer: Parent-of-origin specific gene expression Beckwith-Wiedemann syndrome is an imprinting disorder, typically involving genes on the short arm of chromosome 11 (11p15), such as IGF2 and CDKN1C. The core pathogenetic mechanism is the dysregulation of parent-of-origin specific gene expression. For example, loss of maternal imprinting (silencing) of the IGF2 gene (a growth promoter) can lead to biallelic expression and overgrowth. This highlights how genomic imprinting, a form of non-Mendelian inheritance, dictates that the effect of a gene depends on whether it was inherited from the mother or the father.
5. The variable severity of the disease among the affected family members is best explained by which of the following concepts?
   Answer: Heteroplasmy The patient's symptoms (myoclonus, seizures, myopathy) and the presence of 'ragged-red fibers' on muscle biopsy are characteristic of Myoclonic Epilepsy with Ragged-Red Fibers (MERRF), a mitochondrial disorder. Mitochondrial diseases exhibit variable expressivity due to heteroplasmy. Heteroplasmy is the presence of a mixed population of normal and mutant mitochondrial DNA (mtDNA) within a single cell. The severity of the disease in an individual depends on the proportion of mutant to wild-type mtDNA in different tissues, which can vary significantly among family members due to random segregation of mitochondria during oogenesis and cell division.
6. Which of the following genetic events best describes the cause of this patient's condition?
   Answer: Paternal uniparental disomy Angelman syndrome results from the lack of expression of the maternal UBE3A gene. In this case, the child has inherited both copies of chromosome 15 from her father and none from her mother. This is known as paternal uniparental disomy (UPD). Because the paternal copy of UBE3A is normally imprinted (silenced) in the brain, the child has no functional UBE3A protein, leading to the Angelman syndrome phenotype. This is the second most common cause of AS after maternal deletion.
7. These characteristic microscopic findings are a direct result of the proliferation of which of the following?
   Answer: Abnormal mitochondria Ragged-red fibers are the pathognomonic finding in mitochondrial myopathies. They are caused by the compensatory proliferation of abnormal, dysfunctional mitochondria, especially beneath the sarcolemma (cell membrane) of the muscle fiber. These aggregates of mitochondria contain paracrystalline inclusions and stain intensely red with the Gomori trichrome stain, giving the fiber a 'ragged' appearance. This proliferation is an attempt by the cell to overcome the energy deficit caused by defective oxidative phosphorylation.
8. This patient's condition is caused by the loss of a gene that is normally expressed only from the maternal allele. This phenomenon is best described as which of the following?
   Answer: Genomic imprinting The patient's clinical features (developmental delay, inappropriate laughter, ataxia, seizures) are characteristic of Angelman syndrome. This condition results from the loss of function of the UBE3A gene, which is subject to genomic imprinting. In neurons, UBE3A is expressed almost exclusively from the maternal allele. A deletion on the maternal chromosome 15, as seen in this patient, leads to a loss of functional UBE3A protein in the brain, causing the syndrome. Genomic imprinting is the process of parent-of-origin-specific gene expression.
9. If this boy grows up and has children with an unaffected woman, what is the likelihood he will pass the mitochondrial DNA deletion to them?
   Answer: 0% Mitochondrial DNA is inherited exclusively from the mother via the cytoplasm of the egg cell. A male's mitochondria, located in the midpiece of the sperm, do not enter the ovum during fertilization or are actively degraded. Consequently, a male cannot transmit his mitochondrial DNA to any of his offspring. The risk of transmission from an affected father is 0%.
10. This condition, characterized by undergrowth, is an example of an imprinting disorder that is functionally opposite to Beckwith-Wiedemann syndrome. What is the most likely diagnosis?
    Answer: Russell-Silver syndrome Russell-Silver syndrome (RSS) is an imprinting disorder characterized by prenatal and postnatal growth retardation. It is often considered the molecular opposite of the overgrowth disorder Beckwith-Wiedemann syndrome (BWS). One of the primary causes of RSS is hypomethylation of the imprinting control region 1 (ICR1) on the paternal chromosome 11, leading to decreased expression of the growth promoter IGF2. This contrasts with BWS, which can be caused by hypermethylation of the same region, leading to increased IGF2 expression.
11. This scenario, where an autosomal recessive condition is expressed due to inheriting two chromosomes from a single parental source, is an example of which mechanism?
    Answer: Uniparental isodisomy This is an example of uniparental disomy (UPD) leading to an autosomal recessive disease. Specifically, it is uniparental isodisomy, where the individual inherits two identical copies of one of the parent's homologous chromosomes (in this case, the mother's chromosome 7 carrying the CFTR mutation). The mother is a carrier, but because the child inherited two copies of her mutant-carrying chromosome and no copy from the father, the child is effectively homozygous for the mutation and expresses the disease.
12. This patient's constellation of findings is most consistent with a mutation in which of the following types of genetic material?
    Answer: Mitochondrial DNA This patient's presentation with stroke-like episodes, lactic acidosis, myopathy, and other systemic signs (short stature, hearing loss) is highly suggestive of Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes (MELAS). The maternal history of related symptoms (migraines, diabetes) further supports a maternal inheritance pattern. These disorders are caused by mutations in the mitochondrial DNA (mtDNA), which impair oxidative phosphorylation and cellular energy production, particularly affecting high-energy-demand tissues like the brain and muscles.
13. Which of the following correctly pairs the syndrome with the parental origin of the defective chromosome?
    Answer: Prader-Willi syndrome: loss of paternal contribution; Angelman syndrome: loss of maternal contribution. This question tests the core concept of imprinting for these two classic syndromes. Prader-Willi syndrome (PWS) is caused by the loss of gene expression from the paternally inherited chromosome 15. Angelman syndrome (AS) is caused by the loss of gene expression from the maternally inherited chromosome 15. Therefore, the distinct phenotypes arise from the same chromosomal locus but depend entirely on which parent's genetic contribution is missing or silenced.
14. Compared to nuclear DNA, mitochondrial DNA has which of the following characteristics?
    Answer: A higher mutation rate due to less efficient repair mechanisms Mitochondrial DNA has a significantly higher mutation rate than nuclear DNA. This is attributed to two main factors: 1) Proximity to the electron transport chain, which produces damaging reactive oxygen species, and 2) A less robust and efficient DNA repair system compared to the nucleus. mtDNA is a circular molecule, lacks introns, is not packaged with histones, and is inherited maternally.
15. This pattern of inheritance is most characteristic of a mutation located in which of the following?
    Answer: Mitochondrial genome The described inheritance pattern is classic for a mitochondrial disorder. Key features are: 1) Maternal inheritance: The trait is passed from a mother to ALL of her offspring because mitochondria are inherited via the egg's cytoplasm. 2) Lack of paternal inheritance: An affected father does not pass the trait to his children. 3) Variable expressivity (variable severity): This is due to heteroplasmy, the random segregation of mitochondria during cell division.
16. Assuming this deletion is a de novo event in the father's germline, what is the recurrence risk for this couple's future pregnancies?
    Answer: <1% The majority of deletions causing Prader-Willi syndrome are sporadic, de novo (new) events that occur during gametogenesis in the parent. If the deletion is de novo and the father does not have a chromosomal rearrangement that predisposes to it (which is the most common scenario), the recurrence risk is very low, generally quoted as less than 1%. This small risk accounts for the rare possibility of germline mosaicism in the father.
17. Given the absence of a deletion, which of the following is the most likely underlying genetic mechanism?
    Answer: The patient inherited two copies of chromosome 15 from his mother. Prader-Willi syndrome (PWS) is caused by the absence of paternally expressed genes in the 15q11-q13 region. While the most common cause is a paternal deletion, the second most frequent cause is maternal uniparental disomy (UPD) for chromosome 15. In maternal UPD, the individual inherits two copies of chromosome 15 from the mother and none from the father. Since the PWS-critical genes are normally silenced (imprinted) on the maternal chromosome, inheriting two maternal copies results in no expression of these genes, leading to the PWS phenotype.
18. Which of the following statements most accurately describes the inheritance pattern and risk for her offspring?
    Answer: All of her offspring will inherit the mutation, but their disease severity is unpredictable. Mitochondrial DNA is inherited maternally, so an affected mother will pass the mutation to all of her offspring (both sons and daughters). However, the clinical severity of mitochondrial disorders is highly variable due to heteroplasmy—the presence of both mutant and wild-type mtDNA. The proportion of mutant mtDNA inherited by each child is random and unpredictable, leading to a wide range of clinical phenotypes, as seen in her own family. Thus, all her children will inherit the mutation, but their clinical severity cannot be precisely predicted.
19. What is the term for the co-existence of two or more mitochondrial DNA populations within a single individual?
    Answer: Heteroplasmy Heteroplasmy is the term used to describe the presence of a mixture of more than one type of mitochondrial DNA (mtDNA) within a cell or individual. In the context of mitochondrial disease, this usually refers to a mix of wild-type (normal) and mutant mtDNA. The ratio of mutant to normal mtDNA can vary among tissues and determines the severity of the disease, a key feature of mitochondrial inheritance.
20. What is the probability that their son will be affected by this disorder?
    Answer: 0% Mitochondrial disorders are transmitted exclusively through the maternal line. This is because the zygote's mitochondria are derived entirely from the ovum; the sperm's mitochondria are typically destroyed after fertilization. Therefore, an affected male cannot pass a mitochondrial DNA mutation to any of his children, regardless of their sex. The probability of his son or daughter inheriting the disorder from him is 0%.

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