Molecular Genetics And Gene Expression
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USMLE Step 1 › Molecular Genetics And Gene Expression
A 12-year-old girl has short stature, webbed neck, and widely spaced nipples. She has primary amenorrhea. Echocardiogram shows coarctation of the aorta. Karyotype analysis reveals 45,X. The clinician explains that haploinsufficiency of genes that escape X-inactivation, such as SHOX, alters transcriptional programs for skeletal growth. Which of the following is most likely disrupted in this patient?
RET loss-of-function causing Hirschsprung disease via neural crest migration failure
APC loss causing increased Wnt target gene transcription in colon epithelium
Methylation defect at 15q11-q13 causing loss of maternal UBE3A expression
Trisomy 21 causing increased APP gene dosage and early Alzheimer disease
Reduced dosage of X-linked genes escaping inactivation, impairing growth-related transcription
Explanation
This question assesses understanding of molecular genetics and gene expression, focusing on dosage effects in sex chromosome disorders. Gene expression from X-linked genes escaping inactivation, like SHOX, influences skeletal growth when dosage is reduced. In this vignette, the girl's short stature, webbed neck, and 45,X karyotype indicate Turner syndrome, with haploinsufficiency altering growth transcription. The correct answer identifies reduced dosage of escaping genes, impairing skeletal programs. Choice B is incorrect as it describes Down syndrome, unrelated to the cardiac coarctation. Encourage students to interpret karyotypes and monitor for comorbidities. Practice distinguishing aneuploidy phenotypes by molecular mechanisms.
A 2-year-old boy has profound speech delay, poor eye contact, and repetitive behaviors. He also has macrocephaly. His father had thyroid cancer in his 30s. Karyotype is 46,XY. Sequencing identifies a heterozygous pathogenic variant in PTEN. The clinician explains that PTEN normally dephosphorylates PIP3, limiting PI3K-AKT signaling and downstream transcriptional programs promoting growth and survival. Which gene expression pathway alteration explains the phenotype?
Reduced SMAD transcription due to decreased active TGF-$\beta$
Increased PI3K-AKT signaling leading to enhanced growth and survival gene transcription
Increased mismatch repair activity causing reduced mutation rate
Decreased Hedgehog signaling due to Smoothened inhibition
Reduced Ras-MAPK signaling due to neurofibromin overactivity
Explanation
This question assesses understanding of molecular genetics and gene expression, focusing on phosphatase regulation in overgrowth syndromes. Gene expression for growth is inhibited by PTEN dephosphorylating PIP3 to limit PI3K-AKT signaling. In this vignette, the boy's macrocephaly, autism, and PTEN variant suggest PTEN hamartoma syndrome, with increased signaling. The correct answer identifies enhanced PI3K-AKT transcription promoting growth. Choice D is incorrect as it describes Marfan, not matching macrocephaly. Encourage students to screen for cancers and review signaling pathways. Practice correlating behavioral features with genetics.
A 42-year-old man presents with episodic headaches, diaphoresis, and hypertension. Plasma metanephrines are elevated. CT shows an adrenal mass consistent with pheochromocytoma. Family history reveals his mother had medullary thyroid carcinoma. Exam shows a thyroid nodule. Karyotype is 46,XY. Germline testing identifies a heterozygous activating mutation in RET consistent with MEN2. The clinician explains that constitutive receptor tyrosine kinase signaling increases MAPK pathway activity and transcription of proliferation genes. How does the identified mutation affect protein function?
Loss of function causing reduced cAMP production in endocrine tissues
Dominant negative effect on microtubule polymerization and axonal transport
Frameshift causing nonsense-mediated decay and absent receptor expression
Gain of function causing ligand-independent receptor tyrosine kinase signaling
Splice-site defect causing truncated collagen and brittle bones
Explanation
This question assesses understanding of molecular genetics and gene expression, focusing on proto-oncogene mutations in hereditary cancer syndromes. Gene expression is influenced by receptor tyrosine kinases like RET, which activate MAPK pathways for cell proliferation when mutated. In this vignette, the patient's pheochromocytoma, thyroid nodule, and RET mutation suggest MEN2, with constitutive signaling. The correct answer describes the gain-of-function effect on tyrosine kinase activity, increasing transcription of growth genes. Choice D is incorrect as it pertains to osteogenesis imperfecta, unrelated to the endocrine tumors here. Encourage students to correlate mutation types with functional impacts and screen for associated cancers. Practice integrating biochemical tests with genetic findings for diagnosis.
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?
Splice variant increasing CFTR channel opening in airway epithelium
Dominant negative effect blocking collagen triple-helix assembly
Loss of function reducing methylated-DNA binding and transcriptional repression in neurons
Gain of function increasing ligand-independent receptor tyrosine kinase signaling
Frameshift increasing $\beta$-globin production and causing polycythemia
Explanation
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.
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?
Mismatch repair deficiency causing microsatellite instability
Genomic imprinting at 15q11-q13 causing loss of paternal gene expression
Transcription factor dosage controlling pharyngeal arch gene expression during embryogenesis
Trinucleotide repeat expansion in HTT causing striatal neurodegeneration
Mitochondrial tRNA mutation causing lactic acidosis and stroke-like episodes
Explanation
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.
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?
Reduced SMN protein causing anterior horn degeneration
Increased cGMP in photoreceptors due to PDE6 loss
Constitutive Ras-MAPK signaling due to loss of Ras GTPase-activating protein activity
Defective collagen type III synthesis causing arterial rupture
Impaired lysosomal $\alpha$-galactosidase A causing globotriaosylceramide storage
Explanation
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.
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?
Heme synthesis due to porphobilinogen deaminase deficiency
mTOR pathway restraint that normally limits cell growth under low energy conditions
Lysosomal enzyme targeting via mannose-6-phosphate
Mismatch repair leading to microsatellite instability in colon epithelium
Chloride channel gating in airway epithelium from CFTR dysfunction
Explanation
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.
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?
Keratinocyte desmosome adhesion due to desmoglein antibodies
Phagolysosome formation due to NADPH oxidase deficiency
Endothelial SMAD signaling downstream of TGF-$\beta$/BMP required for vascular remodeling
Voltage-gated sodium channel inactivation in skeletal muscle
Thymidylate synthase inhibition causing impaired DNA synthesis
Explanation
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.
A 1-month-old infant has severe hypotonia, feeding difficulties, and respiratory insufficiency. EMG suggests denervation. Family history reveals a prior sibling death in infancy. Karyotype is 46,XX. Molecular testing shows homozygous deletion of SMN1. The neurologist notes that SMN protein is required for snRNP assembly and proper pre-mRNA splicing; loss alters expression of motor neuron survival genes. Which of the following is most likely disrupted in this patient?
Translation initiation via eIF2 phosphorylation during stress response
snRNP assembly required for pre-mRNA splicing in motor neurons
DNA methylation of CpG islands silencing one parental allele
Microtubule depolymerization preventing mitotic spindle formation
Ras GTPase activation causing increased MAPK transcriptional output
Explanation
This question assesses understanding of molecular genetics and gene expression, focusing on RNA processing in neuromuscular disorders. Gene expression requires proper pre-mRNA splicing via snRNPs assembled by SMN protein in motor neurons. In this vignette, the infant's hypotonia, denervation, and SMN1 deletion indicate spinal muscular atrophy, disrupting splicing. The correct answer explains impaired snRNP assembly altering survival genes. Choice B is incorrect as it relates to imprinting, unrelated to EMG findings. Encourage students to use deletion testing and discuss therapies. Practice differentiating motor neuron diseases by molecular defects.
A 33-year-old woman has numerous colorectal adenomas on screening colonoscopy. She also has osteomas of the jaw and multiple epidermoid cysts. Her father died of colon cancer at age 41. Karyotype is 46,XX. Molecular testing identifies a heterozygous truncating mutation in APC, consistent with familial adenomatous polyposis. The physician explains that APC normally promotes $\beta$-catenin degradation; loss leads to nuclear $\beta$-catenin and increased transcription of proliferation genes. Which of the following is most likely disrupted in this patient?
Proteasomal degradation of $\beta$-catenin that normally limits Wnt target gene transcription
Spindle microtubule attachment due to defective kinetochore proteins
Glycogen breakdown due to myophosphorylase deficiency
Nucleotide excision repair of thymine dimers after UV exposure
Mitochondrial oxidative phosphorylation due to mtDNA deletion
Explanation
This question assesses understanding of molecular genetics and gene expression, focusing on Wnt signaling in hereditary colorectal cancer. Gene expression in the intestine is controlled by Wnt pathways, where APC degrades β-catenin to prevent proliferation gene transcription. In this vignette, the patient's adenomas, osteomas, and APC mutation suggest familial adenomatous polyposis, with accumulated β-catenin. The correct answer identifies the disrupted β-catenin degradation, leading to increased Wnt target genes. Choice B is incorrect as it relates to xeroderma pigmentosum, not matching the colonic polyps. Encourage students to recognize extracolonic manifestations and recommend surveillance. Practice interpreting colonoscopy results with genetic data.