By Adam Medeiros, M.S.
Athena Diagnostics, Inc.
A recent study by Athena Diagnostics examined molecular evaluation for Charcot-Marie-Tooth disease (CMT).1 CMT describes a clinically diverse group of peripheral neuropathies that have been associated with 80 disease-causing genes.2 Affecting nearly 1 in 2,500 individuals, CMT is one of the most prevalent neurological disorders in the United States.3 Onset of disease typically occurs by age 20 and symptoms include impaired tendon reflexes, distal muscle weakness, sensory loss, and deformities of feet.4 Genetic screening for CMT traditionally followed a tiered, reflexive approach based on family history and electrodiagnostic testing as described in the 2009 American Academy of Neurology Practice Parameter for distal symmetric polyneuropathy.5
The Athena study, published in Molecular Genetics and Genomic Medicine on August 21, 2014, reviewed the frequency of mutation in 14 genes most commonly associated with CMT for 17,880 individuals referred to a clinical laboratory and is one of the largest studies of its kind. Using a combination of Sanger sequencing, next-generation sequencing, and multiplex ligation-dependent probe amplification results, the authors found that 94.9% of causative genetic findings occurred in four genes: PMP22, GJB1, MPZ, and MFN2. These findings were corroborated by an earlier study demonstrating that these four genes account for more than 90% of molecular findings for CMT.6 The most commonly observed variant types included missense (70.6%), nonsense (14.3%), and frameshift (8.7%) mutations while a single variant (MFN2:c.2219G>C; p.W740S) accounted for 5.4% of all positive results.
Based on these findings, the authors proposed an initial CMT screen of PMP22, GJB1, MPZ, and MFN2 followed by evaluation of rare disease genes if the initial assessment is negative. The 2009 AAN Practice Parameter did not provide guidance for cases of ambiguous or unknown family history, therefore performing an initial screen based on clinical phenotype could prevent painful and expensive nerve conduction velocity and electromyography studies. A 2005 study found that nearly 20% of CMT1A cases were sporadic, demonstrating the value of molecular testing for cases of unknown genetic etiology.7
In addition to proposing a new course of molecular diagnostic testing for CMT, the authors describe 87 previously unpublished predicted pathogenic variants across 11 genes. Pathogenicity assessment for each variant utilized Athena Insight, a standardized, evidence-based pathogenicity scoring system. Frameshift mutations accounted for 42.5% of all novel variants, followed by nonsense (28.7%) and splice site (20.7%) mutations. These findings contribute to understanding the role of variants of unknown clinical significance, and highlight the involvement of these rare variants, in the molecular diagnosis of neuropathy.
While next-generation sequencing unlocks the potential to screen hundreds of genes simultaneously, the authors predict that analyzing more genes will lead to more variants of unknown clinical significance and complicate result interpretation. Therefore a four-gene initial assessment for CMT, established through empirical data from a cohort of unprecedented size, would produce interpretable results in 18.5% of CMT patients without the need for electrodiagnostic screening.
Medical Director’s Comment
A genetic evaluation for PMP22, GJB1, MPZ, and MFN2 early in the course of a polyneuropathy will lead to prompt identification and better treatment of CMT. Practice guidelines should include the testing for these four genes as part of the routine laboratory evaluation of these patients.
1. Elzinga CD, Medeiros AC, DiVincenzo C, et al. The allelic spectrum of Charcot-Marie-Tooth disease in over 17,000 individuals with neuropathy. Neurology. 2014; 82:P2.058.
2. Timmerman V, Strickland AV, Zuchner S. Genetics of Charcot-Marie-Tooth (CMT) Disease within the frame of the human genome project success. Genes. 2014; 5:13–32.
3. Wiszniewski W, Szigeti K, Lupski JR. Chapter 126 – hereditary motor and sensory neuropathies. Pp. 1–24 in L. R. David, E. P. Reed, and K. Bruce, eds. Emery and Rimoin’s principles and practice of medical genetics. 6th ed. 2013. Academic Press, Oxford, UK.
4. Saporta AS, Sottile SL, Miller LJ, et al. Charcot-Marie-Tooth disease subtypes and genetic testing strategies. Ann. Neurol. 2013; 69:22–33.
5. England JD, Gronseth GS, Franklin G, et al. Practice parameter: evaluation of distal symmetric polyneuropathy: role of autonomic testing, nerve biopsy, and skin biopsy (an evidence-based review). Report of the American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine, and American Academy of Physical Medicine and Rehabilitation. Neurology. 2009b; 72: 177–184.
6. Murphy SM, Laura M, Fawcett K, et al. Charcot-Marie-Tooth disease: frequency of genetic subtypes and guidelines for genetic testing. Journal of neurology, neurosurgery, and psychiatry 2012; 83:706-10.7.
7. Marques Jr. W, Freitas MR, Nascimento OJ, et al. 17p duplicated Charcot-Marie-Tooth 1A: characteristics of a new population. J. Neurol. 2005; 252:972–979.