Fragile X syndrome vs Other intellectual disability syndromes
How to tell Fragile X syndrome and Other intellectual disability syndromes apart when a board question makes them look alike.
Fragile X is CGG-repeat FMR1 with long face, large ears, macroorchidism, and autism-like behavior; other ID syndromes need a different signature clue.
Side by side
Fragile X causes intellectual disability, long face, large ears, macroorchidism after puberty, autism-like features, and full-mutation CGG expansion over 200 repeats with FMR1 hypermethylation.
Other intellectual disability syndromes may be chromosomal, metabolic, teratogenic, or single-gene disorders and require their own dysmorphic or biochemical clues.
Full-mutation FMR1 CGG expansion over 200 repeats causes hypermethylation/silencing and loss of FMRP; premutation 55-200 repeats can cause FXTAS or primary ovarian insufficiency.
Other syndromes impair neurodevelopment through different mechanisms such as aneuploidy, imprinting, metabolic toxicity, or structural malformation.
Macroorchidism, long face, large ears, family history through carrier females, and trinucleotide repeat support fragile X.
Specific features such as congenital anomalies, metabolic crises, or alternate karyotype point away from fragile X.
The trap
Intellectual disability alone is nonspecific; look for macroorchidism and trinucleotide repeat inheritance before choosing fragile X.
The repair move
For intellectual disability syndromes, demand the signature dysmorphic, gonadal, inheritance, or metabolic clue.
Try a free question
CoreA 12-year-old boy has moderate intellectual disability, a long narrow face, large protruding ears, prominent jaw, post-pubertal macroorchidism, and autism-like behaviors with hand-flapping. His maternal uncle had similar features. Genetic testing reveals a CGG trinucleotide repeat expansion (>200 repeats) in FMR1 with hypermethylation. Which condition is most likely?
Show answer and explanation
Answer: A. Fragile X syndrome. Fragile X syndrome is the most common inherited cause of intellectual disability. Full mutation (>200 CGG repeats in 5' UTR of FMR1) leads to hypermethylation and silencing of FMR1, producing loss of FMRP. Features: long face, large ears, prominent jaw, macroorchidism (post-pubertal), autism features, intellectual disability. X-linked dominant inheritance (premutation 55-200 repeats can cause FXTAS in older men and POI in women).
Why the other choices are tempting
- B. Down syndrome (trisomy 21) features hypotonia, single palmar crease, Brushfield spots, and AV septal defects, not macroorchidism with long face.
- C. Klinefelter syndrome (47,XXY) features tall stature, hypogonadism, and gynecomastia with low testosterone, not macroorchidism with elongated facial features.
- D. Williams syndrome (7q11.23 deletion) features elfin facies, supravalvular aortic stenosis, and hypercalcemia.
- E. Rett syndrome features regression in girls with MECP2 mutation, stereotyped hand-wringing, and microcephaly, not the fragile X pattern.
Flashcards
Tap a card to flip it.
What clue supports Fragile X syndrome?
Choose Fragile X syndrome when the stem gives this pattern: Fragile X causes intellectual disability, long face, large ears, macroorchidism after puberty, autism-like features, and CGG repeat expansion with anticipation in FMR1.
What clue supports Other intellectual disability syndromes?
Choose Other intellectual disability syndromes when the stem gives this contrasting pattern: Other intellectual disability syndromes may be chromosomal, metabolic, teratogenic, or single-gene disorders and require their own dysmorphic or biochemical clues.
Quick answers
What is the fastest way to tell Fragile X syndrome from Other intellectual disability syndromes?
Fragile X is CGG-repeat FMR1 with long face, large ears, macroorchidism, and autism-like behavior; other ID syndromes need a different signature clue.
What is the key clue for Fragile X syndrome?
Fragile X causes intellectual disability, long face, large ears, macroorchidism after puberty, autism-like features, and full-mutation CGG expansion over 200 repeats with FMR1 hypermethylation.
What is the key clue for Other intellectual disability syndromes?
Other intellectual disability syndromes may be chromosomal, metabolic, teratogenic, or single-gene disorders and require their own dysmorphic or biochemical clues.
What is the common trap with Fragile X syndrome vs Other intellectual disability syndromes?
Intellectual disability alone is nonspecific; look for macroorchidism and trinucleotide repeat inheritance before choosing fragile X.