A 18-month-old boy born to non-consanguineous Indian parents presents with severe growth retardation, microcephaly, and recurrent seizures beginning at 14 months of age. On examination, he has a distinctive facial appearance with a high forehead, prominent glabella, hypertelorism, and a broad nose continuous with the forehead (Greek warrior helmet facies). His developmental milestones are significantly delayed. Chromosomal microarray reveals a deletion at 4p16.3 involving the region marked **A** in the diagram. Which of the following best explains the pathophysiology of seizures in this patient?
A. Haploinsufficiency of LETM1 leading to impaired mitochondrial calcium homeostasis
B. Gain-of-function mutation in FGFR3 causing abnormal fibroblast growth signaling
C. Loss of histone methyltransferase activity due to WHSC1/NSD2 deletion affecting chromatin remodeling and neuronal development
D. Microdeletion of the SNRPN gene resulting in genomic imprinting defects
Explanation
Why "Haploinsufficiency of LETM1 leading to impaired mitochondrial calcium homeostasis" is right
The deletion at 4p16.3 marked A in Wolf-Hirschhorn syndrome involves the WHSCR-1 and WHSCR-2 regions, with LETM1 (leucine zipper and EF-hand motifs-containing transmembrane protein 1) being a critical contributor to the seizure phenotype. LETM1 is essential for mitochondrial calcium homeostasis and neuronal excitability regulation. Haploinsufficiency of LETM1 directly impairs mitochondrial calcium handling, leading to neuronal hyperexcitability and the characteristic seizures (90–100% of WHS patients, typically beginning in the first 2–3 years) that are often atypical absence or generalized tonic-clonic. This mechanism is well-established in Nelson Textbook of Pediatrics and GeneReviews.
Why each distractor is wrong
Gain-of-function mutation in FGFR3 causing abnormal fibroblast growth signaling: Although FGFR3 is located at 4p16.3, it is NOT involved in Wolf-Hirschhorn syndrome. FGFR3 gain-of-function mutations cause achondroplasia (an autosomal dominant skeletal dysplasia), not deletion-based Wolf-Hirschhorn. This is a classic exam distractor designed to test understanding of the distinction between WHSC1/NSD2 (deletion, Wolf-Hirschhorn) and FGFR3 (mutation, achondroplasia) at the same locus.
Loss of histone methyltransferase activity due to WHSC1/NSD2 deletion affecting chromatin remodeling and neuronal development: While WHSC1/NSD2 is indeed a histone methyltransferase and is one of the most important contributors to the overall Wolf-Hirschhorn phenotype (affecting growth, intellectual disability, and facial dysmorphism), the seizure phenotype is specifically and predominantly attributable to LETM1 haploinsufficiency, not WHSC1/NSD2 loss.
Microdeletion of the SNRPN gene resulting in genomic imprinting defects: SNRPN is located on chromosome 15q11-q13 and is involved in Prader-Willi syndrome, not Wolf-Hirschhorn syndrome. This distractor tests whether the student confuses different chromosomal deletion syndromes.
High-YieldNEET PG
Wolf-Hirschhorn = 4p16.3 deletion (WHSC1/NSD2 + LETM1); seizures are due to LETM1 haploinsufficiency → impaired mitochondrial Ca²⁺ homeostasis. Do NOT confuse with FGFR3 achondroplasia (same locus, opposite mechanism—gain-of-function mutation, not deletion).
Nelson Textbook of Pediatrics 22e; GeneReviews — Wolf-Hirschhorn Syndrome
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