PMP22 (Usually a Deletion)
HNPP is the acronym for Hereditary Neuropathy with liability to Pressure Palsies. HNPP is caused by a deletion of one copy of the PMP22 gene. A CMTer who has HNPP has only one copy of the PMP22 gene instead of the normal two. This copy of the gene is deleted because it lives at a segment of chromosome 17 that is deleted (missing)—17p11.2-p12. This mutation is inherited in an autosomal dominant pattern.
HNPP symptoms usually occur by the time the CMTer is 30 years old. Symptoms such as numbness, tingling, and muscle weakness are episodic and can be localized to a certain area of the body. Activities that are easy and harmless for normal people can severely impact a CMTer who has HNPP. Recovery from an episode can take days to months.
HNPP is sometimes referred to as the genetic opposite of CMT1A. Where HNPP is caused by a deletion of one copy of the PMP22 gene, CMT1A is caused by a duplication, or an extra copy of the same gene. Albeit very rare, HNPP can be caused by point mutations in the PMP22 gene.