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A 40 year old male presents with slowly progressive leg weakness since a decade. He has findings of sensory motor neuropathy with pes cavus and areflexia. There is no history of diabetes or toxin exposure. There is history of similar illness in siblings and maternal uncles but not in maternal cousins. His mother has pes cavus. Nerve conduction study shows median nerve conduction to be 39ms. Abnormality in which gene is likely?
Explanation
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PMP 22 duplicationmutation, mutation in GJB1 (Cx 32), MPZ and MFN2 account for almost 70% of CMT. Median nerve conduction velocity and family history help to suggest diagnosis. In patients with demyelinating neuropathy (median nerve conduction<35msec) PMP duplicationmutation followed by MPZ mutation are likely. In patients with axonal CMT, MFN2 mutation is most likely. Absence of male to male transmission suggests X linked CMT. CMTX1 caused by mutation in GJB1 (Connexin 32) accounts for 7-16% of CMT and is the most likely in this scenario. It is associated with mild demyelination and females are less affected. Hearing loss, CNS white matter involvement, ataxiadysarthria at high altitude have been reported in CMTX1.
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Reference
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Ramchandran S. Charcot-Marie-Tooth Disease and Other Genetic Polyneuropathies. Continuum 201723(5)1360-1377.
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Option 1
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Is Correct (Option 1):
PMP 22 (Peripheral myelin protein 22)
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Option 2
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Is Correct (Option 2):
MPZ (Myelin protein zero)
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Option 3
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Is Correct (Option 3):
MFN 2 (Mitofusin 2)
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Option 4
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Is Correct (Option 4):
GJB 1 (Gap junction protein beta 1)
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