A 7-year-old girl presents with mild intellectual disability (IQ 72), learning difficulties, and anxiety. Her father has moderate intellectual disability with characteristic long narrow face, large ears, and macroorchidism. Genetic testing reveals a full mutation (>200 CGG repeats) in the FMR1 gene on Xq27.3 with hypermethylation. The inheritance pattern marked **B** in the diagram — X-linked recessive with manifesting female carrier due to skewed Lyonization — best explains her clinical presentation. Which of the following BEST describes the molecular mechanism underlying her intellectual disability despite being heterozygous for the FMR1 mutation?
A. Haploinsufficiency from a single functional copy of the FMR1 gene, similar to autosomal dominant inheritance
B. Random X-inactivation (Lyonization) resulting in approximately 50% of neurons expressing the mutant FMR1 allele and loss of FMRP function in those cells
C. Skewed X-inactivation exclusively toward the normal X chromosome, preventing any expression of the mutant allele
D. Complete inactivation of the normal X chromosome, leaving only the mutant allele expressed in all cells
Explanation
Why Random X-inactivation (Lyonization) resulting in approximately 50% of neurons expressing the mutant FMR1 allele and loss of FMRP function in those cells is right
Female carriers of the full FMR1 mutation are heterozygotes (Xx) with two X chromosomes. According to the Lyon hypothesis, each cell randomly inactivates one X chromosome early in embryogenesis. In this girl, approximately 50% of her neurons express the mutant FMR1 allele (which is hypermethylated and transcriptionally silent), producing no functional FMRP in those cells. The remaining ~50% express the normal allele. This mosaic pattern of FMRP deficiency in roughly half her neurons is sufficient to cause mild-to-moderate intellectual disability, learning difficulties, and anxiety — making her a "manifesting carrier." This mechanism directly explains why she has intellectual disability despite being heterozygous, and why her phenotype is typically milder than affected males (who have the full mutation in 100% of cells). (Nelson Textbook of Pediatrics 22e — Genetic Causes of Intellectual Disability)
Why each distractor is wrong
Complete inactivation of the normal X chromosome, leaving only the mutant allele expressed in all cells: This would represent extreme skewed X-inactivation toward the mutant allele, which would produce a phenotype as severe as an affected male. While skewed inactivation can occur and worsen her phenotype, the question stem indicates she has mild (not severe) intellectual disability, and the typical mechanism in manifesting carriers is random inactivation, not complete skewing.
Haploinsufficiency from a single functional copy of the FMR1 gene, similar to autosomal dominant inheritance: FMR1 is X-linked recessive, not dominant. Haploinsufficiency (reduced gene dosage causing disease) is not the mechanism here; rather, the problem is loss of FMRP in a subset of cells due to random X-inactivation. One functional copy in the other 50% of cells is insufficient to prevent symptoms.
Skewed X-inactivation exclusively toward the normal X chromosome, preventing any expression of the mutant allele: If X-inactivation were completely skewed toward the normal X, she would express the normal FMR1 allele in nearly all cells and would be cognitively normal or minimally affected. This does not match her presentation of intellectual disability and learning difficulties.
High-YieldNEET PG
Manifesting female carriers of Fragile X (full mutation) have intellectual disability due to random X-inactivation creating a mosaic of FMRP-deficient and FMRP-expressing neurons; severity depends on the degree of skewing toward the mutant allele.
Nelson Textbook of Pediatrics 22e — Genetic Causes of Intellectual Disability
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