Molecular Genetics And Gene Expression - USMLE Step 1

1. A 2-week-old male newborn has poor feeding, hypotonia, and failure to thrive. Physical exam shows hypogonadism and weak cry. Family history is unremarkable. Karyotype is 46,XY. Methylation analysis of 15q11-q13 shows only the maternal methylation pattern, consistent with loss of paternally expressed genes. The geneticist explains that imprinting affects transcription by methylation-dependent silencing of one parental allele. Which gene expression pathway alteration explains the phenotype?
   Answer: Loss of paternal allele expression at 15q11-q13 due to imprinting defect This question assesses understanding of molecular genetics and gene expression, focusing on epigenetic regulation in imprinting disorders. Gene expression at 15q11-q13 is controlled by methylation, silencing one allele to allow paternal expression for growth regulation. In this vignette, the newborn's hypotonia, poor feeding, and loss of paternal imprint suggest Prader-Willi syndrome, disrupting appetite genes. The correct answer explains the loss of paternal expression due to imprinting, leading to phenotypic alterations. Choice B is incorrect as it describes FAP, unrelated to the hypogonadism. Encourage students to analyze methylation patterns and review neonatal presentations. Practice correlating epigenetics with clinical outcomes.
2. A 35-year-old man is evaluated for recurrent spontaneous pneumothoraces and multiple lung cysts on CT. He has numerous small, dome-shaped papules on his nose and cheeks. His mother had bilateral renal tumors at age 50. The patient's brother has similar skin lesions. Physical exam is otherwise normal. Karyotype is 46,XY. Targeted molecular analysis reveals a heterozygous pathogenic variant in FLCN consistent with Birt-Hogg-Dubé syndrome. The genetic counselor explains that folliculin acts as a tumor suppressor and influences transcriptional programs by modulating nutrient-sensing pathways, including AMPK–mTOR balance. Which of the following is most likely disrupted in this patient?
   Answer: mTOR pathway restraint that normally limits cell growth under low energy conditions This question assesses understanding of molecular genetics and gene expression, focusing on nutrient-sensing pathways and their role in syndromic disorders. Gene expression is regulated by environmental cues, with pathways like mTOR balancing cell growth in response to energy availability via tumor suppressors. In this vignette, the patient's pneumothoraces, lung cysts, skin papules, and FLCN variant point to Birt-Hogg-Dubé syndrome, disrupting AMPK-mTOR balance. The correct answer highlights the loss of mTOR restraint under low energy, leading to aberrant growth in lungs and kidneys. Choice A is incorrect as it pertains to Lynch syndrome with microsatellite instability, not relevant to the pulmonary and dermatologic features here. Encourage students to map genetic defects to downstream pathways and consider family history for autosomal dominant patterns. Practice differentiating similar syndromes by key clinical and molecular features.
3. A 6-year-old child has retinoblastoma diagnosed after leukocoria is noted on a school vision screen. Family history reveals the father lost an eye in infancy due to the same cancer. MRI shows an intraocular mass without metastasis. Karyotype is 46,XX. Germline testing identifies a heterozygous pathogenic variant in RB1. The oncologist explains that pRb normally binds E2F, repressing transcription of S-phase genes; loss allows E2F-driven transcription and uncontrolled cell cycle entry. Which gene expression pathway alteration explains the phenotype?
   Answer: Loss of E2F repression leading to increased transcription of S-phase entry genes This question assesses understanding of molecular genetics and gene expression, focusing on cell cycle regulators in hereditary cancers. Gene expression for cell cycle progression is repressed by pRb binding E2F, preventing S-phase entry in retinal cells. In this vignette, the child's retinoblastoma and RB1 variant suggest hereditary retinoblastoma, with loss of E2F repression. The correct answer identifies increased transcription of S-phase genes due to absent pRb. Choice D is incorrect as it pertains to BRCA-associated cancers, not matching the ocular tumor. Encourage students to apply two-hit models and discuss enucleation criteria. Practice correlating germline mutations with tumor risks.
4. A 3-year-old girl has normal early development followed by loss of speech and purposeful hand use, with repetitive hand-wringing movements. She develops seizures and gait instability. Family history is negative. Karyotype is 46,XX. Sequencing shows a pathogenic variant in MECP2. The clinician explains that MeCP2 normally represses transcription by binding methylated CpG sites and recruiting histone deacetylases; mutation causes inappropriate gene expression in neurons. How does the identified mutation affect protein function?
   Answer: Loss of function reducing methylated-DNA binding and transcriptional repression in neurons This question assesses understanding of molecular genetics and gene expression, focusing on epigenetic modifiers in autism-spectrum disorders. Gene expression is modulated by MeCP2, which represses transcription via methylated CpG binding and deacetylase recruitment in neurons. In this vignette, the girl's regression, hand-wringing, and MECP2 variant indicate Rett syndrome, with loss of repression. The correct answer describes loss-of-function reducing binding and repression. Choice B is incorrect as it pertains to MEN2, unrelated to stereotypies. Encourage students to track developmental milestones and discuss therapies. Practice identifying mutation effects on protein function.
5. The deleted DNA segment in this patient most likely functions as which of the following?
   Answer: An enhancer Enhancers are DNA regulatory elements that can be located far upstream, downstream, or within an intron of a gene they regulate. They bind specific transcription factors to dramatically increase the rate of transcription. The locus control region (LCR) for the β-globin cluster is a powerful enhancer element required for high-level, tissue-specific expression. Its deletion leads to severe thalassemia despite a normal gene sequence.
6. A 24-year-old man presents with recurrent kidney stones, bone pain, and fatigue. Labs show hypercalcemia and elevated PTH. MRI reveals a pituitary microadenoma. His father had "parathyroid surgery" in his 30s. Physical exam is unremarkable. Karyotype is 46,XY. Germline sequencing identifies a heterozygous pathogenic variant in MEN1. The physician explains that menin normally regulates transcription by interacting with histone-modifying complexes and restraining cell cycle progression. Loss of menin increases expression of growth-promoting genes in endocrine tissues. What is the mechanism of the genetic defect in this condition?
   Answer: Autosomal dominant tumor suppressor loss requiring second-hit in target tissues This question assesses understanding of molecular genetics and gene expression, focusing on inheritance mechanisms in tumor suppressor gene disorders. Gene expression is controlled by histone modifications and cell cycle regulators, with tumor suppressors like menin preventing unchecked proliferation in endocrine cells. In this vignette, the patient's hypercalcemia, pituitary adenoma, and MEN1 variant suggest multiple endocrine neoplasia type 1, requiring biallelic inactivation. The correct answer captures the autosomal dominant loss with second-hit, leading to dysregulated growth gene expression. Choice C is incorrect as it describes repeat expansions in conditions like Huntington disease, not fitting the endocrine tumor pattern. Encourage students to apply Knudson's two-hit hypothesis and evaluate family histories for penetrance. Practice distinguishing endocrine syndromes by molecular and clinical profiles.
7. The PML-RARA fusion protein most likely causes transcriptional repression by recruiting which of the following enzyme complexes to the promoters of target genes?
   Answer: Histone deacetylases (HDACs) The pathologic PML-RARA fusion protein binds to retinoic acid response elements in the DNA of genes required for myeloid differentiation. It then aberrantly recruits corepressor complexes that contain histone deacetylases (HDACs). HDACs remove acetyl groups from histones, leading to chromatin condensation and potent transcriptional repression, which causes a block in cell differentiation and contributes to leukemia. All-trans-retinoic acid (ATRA) therapy works by causing the fusion protein to release the HDAC complex.
8. A 26-year-old woman has recurrent epistaxis and visible telangiectasias on the lips and tongue. She reports exertional dyspnea; CT angiography shows pulmonary arteriovenous malformations. Her mother had similar nosebleeds. Karyotype is 46,XX. Genetic testing reveals a heterozygous pathogenic variant in ENG (endoglin), consistent with hereditary hemorrhagic telangiectasia. The physician explains that endoglin is part of the TGF-$\beta$/BMP signaling complex in endothelial cells, influencing SMAD-dependent transcription required for normal vessel development. Which of the following is most likely disrupted in this patient?
   Answer: Endothelial SMAD signaling downstream of TGF-$\beta$/BMP required for vascular remodeling This question assesses understanding of molecular genetics and gene expression, focusing on TGF-β signaling in vascular disorders. Gene expression in endothelium is regulated by TGF-β/BMP via SMADs, with endoglin facilitating receptor complexes for vessel maturation. In this vignette, the woman's telangiectasias, AVMs, and ENG variant indicate hereditary hemorrhagic telangiectasia, impairing SMAD signaling. The correct answer explains the disrupted endothelial SMAD pathway, leading to abnormal vascular transcription. Choice B is incorrect as it describes pemphigus vulgaris, unrelated to the bleeding diathesis. Encourage students to screen for visceral AVMs and review family histories. Practice integrating imaging with molecular diagnostics.
9. Which of the following patterns of histone modification is most consistent with the condensed chromatin state and silencing of the MLH1 gene?
   Answer: Decreased acetylation Histone acetylation, mediated by histone acetyltransferases (HATs), neutralizes the positive charge of lysine residues, relaxing the chromatin structure into euchromatin, which is accessible to transcription machinery. Conversely, histone deacetylation, carried out by histone deacetylases (HDACs), restores the positive charge, leading to a more condensed chromatin structure (heterochromatin) and transcriptional repression. Therefore, decreased acetylation is a hallmark of silenced genes.
10. A 9-year-old boy has intellectual disability, long face, large ears, and macroorchidism. He has autistic behaviors and poor eye contact. His maternal uncle has similar features. Karyotype is 46,XY. Molecular testing shows hypermethylation of the FMR1 promoter with >200 CGG repeats, consistent with fragile X syndrome. The clinician notes that promoter methylation silences transcription, reducing FMRP and causing dysregulated synaptic protein translation. What is the mechanism of the genetic defect in this condition?
    Answer: CGG repeat expansion causing promoter hypermethylation and reduced FMR1 transcription This question assesses understanding of molecular genetics and gene expression, focusing on repeat expansions and epigenetic silencing. Gene expression can be silenced by promoter hypermethylation from CGG expansions in FMR1, reducing FMRP for synaptic translation control. In this vignette, the boy's intellectual disability, macroorchidism, and FMR1 repeats indicate fragile X syndrome, with transcriptional silencing. The correct answer describes the CGG expansion causing methylation and reduced transcription. Choice B is incorrect as it pertains to Duchenne dystrophy, not matching the behavioral features. Encourage students to recognize X-linked patterns and test for expansions. Practice integrating physical exam with molecular diagnostics.
11. The mutated gene that fails to regulate the oncogene most likely encodes which of the following?
    Answer: A microRNA (miRNA) MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression post-transcriptionally. They bind to complementary sequences, typically in the 3' UTR of target mRNAs, leading to either mRNA degradation or translational repression. A loss-of-function mutation in a miRNA that targets an oncogene would result in the loss of this repression, leading to overexpression of the oncoprotein.
12. The addition of a poly-A tail to eukaryotic mRNA primarily serves to protect it from degradation by which of the following?
    Answer: 3' to 5' exonucleases The poly-A tail is a long chain of adenine nucleotides added to the 3' end of a pre-mRNA during processing. In the cytoplasm, it serves as a binding site for poly-A-binding protein (PABP), which protects the mRNA from degradation by 3' to 5' exonucleases. The gradual shortening of the tail over time is a key determinant of mRNA half-life.
13. A 6-year-old boy has congenital heart defects, cleft palate, and recurrent infections with low T-cell counts. He has hypocalcemia. Family history is negative. Karyotype is 46,XY. Chromosomal microarray shows a 22q11.2 deletion involving TBX1. The geneticist explains that TBX1 is a transcription factor important for pharyngeal arch development; reduced TBX1 alters downstream developmental gene expression. Which of the following is most likely disrupted in this patient?
    Answer: Transcription factor dosage controlling pharyngeal arch gene expression during embryogenesis This question assesses understanding of molecular genetics and gene expression, focusing on haploinsufficiency in microdeletion syndromes. Gene expression during embryogenesis is controlled by dosage-sensitive transcription factors like TBX1 for pharyngeal development. In this vignette, the boy's heart defects, cleft palate, and 22q11.2 deletion indicate DiGeorge syndrome, reducing TBX1 dosage. The correct answer explains disrupted transcription factor dosage altering developmental genes. Choice E is incorrect as it describes Huntington, unrelated to hypocalcemia. Encourage students to evaluate immune function and use microarray. Practice distinguishing microdeletion phenotypes.
14. This patient's mutation most likely leads to a non-functional protein through which of the following mechanisms?
    Answer: Inclusion of an intronic sequence into the mature mRNA The creation of a new, or 'cryptic,' splice site within an intron can cause the spliceosome to incorrectly process the pre-mRNA. This often leads to the inclusion of a portion of the intron in the mature mRNA. This insertion typically causes a frameshift, leading to a premature stop codon and the production of a truncated, non-functional protein.
15. Activation of the TSH receptor ultimately leads to increased thyroid hormone synthesis primarily through stimulation of which of the following cellular processes?
    Answer: Gene transcription of thyroid hormone synthesis enzymes TSH receptor activation leads to increased cAMP levels and PKA activation, which phosphorylates and activates transcription factors such as CREB. These transcription factors bind to cAMP response elements in the promoters of genes encoding thyroid hormone synthesis enzymes, including thyroid peroxidase (TPO) and thyroglobulin. This results in increased transcription of these essential enzymes, leading to enhanced thyroid hormone production.
16. The mutation most likely disrupts which of the following regulatory sequences?
    Answer: A splice acceptor site Splicing requires conserved consensus sequences at the boundaries of introns. The 3' end of an intron, at the junction with the next exon, contains the splice acceptor site (typically an AG dinucleotide). A mutation in this critical site can prevent the spliceosome from recognizing the exon, leading to 'exon skipping,' where the exon is treated as part of the intron and is spliced out along with it.
17. A 7-year-old boy has multiple café-au-lait macules, axillary freckling, and several soft cutaneous nodules. He has learning difficulties. His mother has similar skin findings. Ophthalmology notes Lisch nodules. Karyotype is 46,XY. Genetic testing identifies a heterozygous pathogenic variant in NF1. The clinician explains that neurofibromin normally inactivates Ras by enhancing GTP hydrolysis; loss increases MAPK signaling and transcription of growth genes. Which gene expression pathway alteration explains the phenotype?
    Answer: Constitutive Ras-MAPK signaling due to loss of Ras GTPase-activating protein activity This question assesses understanding of molecular genetics and gene expression, focusing on Ras signaling in neurocutaneous syndromes. Gene expression for cell growth is regulated by Ras GTPases, with neurofibromin enhancing hydrolysis to inactivate Ras-MAPK pathways. In this vignette, the boy's café-au-lait spots, neurofibromas, and NF1 variant indicate neurofibromatosis type 1, with constitutive Ras signaling. The correct answer explains the increased MAPK activity from lost GTPase function, promoting tumor gene expression. Choice B is incorrect as it describes Ehlers-Danlos syndrome, unrelated to the pigmented lesions. Encourage students to use diagnostic criteria and monitor for complications. Practice differentiating neurocutaneous disorders by molecular mechanisms.
18. The therapeutic effect of this drug is most likely mediated by the inhibition of which of the following enzymes?
    Answer: Histone deacetylase (HDAC) Histone deacetylation removes acetyl groups from lysine residues on histone tails, increasing their positive charge. This enhances the interaction between histones and negatively charged DNA, leading to condensed chromatin (heterochromatin) and transcriptional repression. Inhibiting histone deacetylases (HDACs) preserves histone acetylation, resulting in a relaxed chromatin state (euchromatin) that allows for gene expression.
19. The nucleotide change in the SMN2 gene most directly affects which of the following processes?
    Answer: mRNA splicing regulation Exonic splicing enhancers (ESEs) are short DNA sequences within exons that bind specific serine/arginine-rich (SR) proteins. This binding helps the spliceosome correctly identify and include the exon in the mature mRNA. A mutation that disrupts an ESE, as seen in the SMN2 gene, weakens the recognition of that exon, leading to its frequent exclusion (skipping) during splicing.
20. The glucocorticoid receptor is best classified as which of the following types of protein?
    Answer: A ligand-activated transcription factor Steroid hormone receptors, including the glucocorticoid receptor, are a class of intracellular receptors that function as ligand-activated transcription factors. After binding their specific hormone ligand, they typically dimerize, translocate to the nucleus, and bind to specific DNA sequences (hormone response elements) in the regulatory regions of target genes to either activate or repress transcription.

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