This page copies a pamphlet produced by Athena Diagnostics. Last updated: 4/21/1995.

Athena Diagnostics

CMT Type1A DNA Test

Guide to Test Results and Interpretation

Athena Diagnostics, formerly Genica Pharmaceuticals Corporation, is located at Four Biotech Park, 377 Plantation Street, Worchester, MA 01605. They can be reached by calling 1-800-394-4493.

Table of Contents

Guide to test results and interpretation

Introduction

Charcot-Marie-Tooth (CMT) polyneuropathy syndrome, or hereditary motor and sensory neuropathies (HMSN I, II, and III) are a genetically and clinically heterogeneous group of disorders of the peripheral nerves characterized by an insidious onset and slowly progressive weakness of the distal muscles with mild sensory impairment. CMT is the most common inherited peripheral neuropathy, and with an estimated prevalence of 1:2500, it is one of the most common genetic disorders.[1,2,3] Figure 1 describes the inheritance patterns of CMT.

Figure 1

In the overwhelming majority of pedigrees, the mode of inheritance is autosomal dominant, however there are pedigrees which exhibit X-linked inheritance and rarer ones exhibiting autosomal recessive inheritance. Electrophysiological studies distinguish two major forms: CMT type 1 (CMT1), which accounts for 76% of CMT, is a demyelinating neuropathy characterized by moderate to severely decreased motor nerve conduction velocities (NCV), and type 2 (CMT2), the neuronal form, which is characterized by normal or mildly reduced NCV.[1,2] In the majority of autosomal dominant CMT1 families, the disease locus is linked to markers on the proximal short arm of chromosome 17.[4,5] Patients demonstrating mutations in this region are classified as CMT type 1A. In addition to familial cases, frequent de novo CMT1A mutations have also been described, 90% of which are due to the CMT1A duplication.[6]

The CMT1A duplication

CMT1A is associated with a 1.5 million base pair tandem DNA duplication in 17p11.2 (figure 2).[4-10] The CMT1A duplication has been identified in multiple unrelated families as well as in sporadic (de novo ) CMT1 patients.[6-7] The PMP22 gene, which encodes a peripheral nerve myelin protein, has recently been mapped within the CMT1A duplication.[11-13] The overexpression of this gene is therefore thought to be the cause of the clinical and electrophysiological phenotype of CMT type 1. A DNA deletion associated with Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) has also been mapped to chromosome 17p11.2 and includes all markers that are known to map within the CMT1A duplication.[14]

Figure 2

In several studies, 70-90% of CMT1A patients had the chromosome 17 duplication.[4-8] It is a valuable biological marker for CMT1A in many pedigrees and useful in the clinical evaluation of patients because it is detectable in the majority of CMT1 patients. It distinguishes CMT from chronic inflammatory demyelinating polyneuropathy (CIDP), which can closely resemble the CMT1 phenotype.

The CMT1A DNA test

Athena Diagnostic's test for CMT1A is a molecular test which identifies the chromosome 17 duplication using the most informative DNA detection method currently available. The test was developed at Baylor College of Medicine by Dr. James Lupski, who first reported the association of CMT1 with a chromosome 17 duplication.[4] Patients' white blood cells are separated and carefully embedded in agarose prior to processing the DNA. This process minimizes the shear stress of DNA extraction techniques. Pulsed-field gel electrophoresis is utilized to separate large segments of DNA containing CMT1A duplication-specific junction fragments. These junction fragments are then detected by hybridization with a CMT1A duplication specific probe (CMT1A-REP) using standard Southern blotting techniques.[10] Figure 3 illustrates an idiogram and corresponding autoradiograph from a CMT1A duplication positive patient compared with a normal patient.

Figure 3

Test result and interpretation

The presence of the 500kb CMT1A duplication specific junction fragment indicates a positive result and is diagnostic for Charcot-Marie-Tooth type 1A (refer to figure 3). A positive result has important applications in genetic counselling of patients and families.

A negative result is indicated by the absence of the CMT1A duplication specific junction fragment (refer to figure 3).[4] The CMT1A duplication has been identified in 70% to 90% of patients with clinical CMT1.[4-8] Point mutations in the peripheral nerve myelin protein gene, PMP22, have recently been identified in some CMT1A patients who did not exhibit the duplication.[15] A negative result therefore does not rule out a diagnosis of Charcot-Marie-Tooth disease.

References

  1. Lupski JR, Garcia CA, Parry G, Patel Pl: Charcot-Marie-Tooth polyneuropathy syndrome: Clinical electrophysiological and genetic aspects. In Appel S (ed.) Current Neurology . Vol 11. Chicago, Mosby Yearbook 1991; pp.1-25.
  2. Dyck PJ, Chance PF, Lebo RV, Carney JA: Hereditary motor and sensory neuropathies. In Peripheral Neuropathy, Third Edition, PJ Dyck, PK Thomas, JW Griffin, PA Low and JF Podulso, (eds.) Philadelphia: W.B. Saunders 1992; pp.1094-1136.
  3. Skre H: Genetic and clinical aspects of Charcot-Marie-Tooth disease. Clin Genet 1974 3; 6:98-118.
  4. Lupski JR, Montes de Oca-Luna R, Slaugenhaupt S, et al: DNA duplication associated with Charcot-Marie-Tooth disease type 1A. Cell 1991; 66:219-232.
  5. Raeymaekers P, Timmerman V, Nelis E, et al: Duplication in chromosome 17p11.2 in Charcot-Marie-Tooth neuropathy type 1 A (CMT1A). Neuromusc Dis l991; 1:93-97.
  6. Hoogendijk JE, Hensel GW, Gabreels-Festen AAWM, et al: De Novo mutation in hereditary motor and sensory neuropathy type 1. Lancet 1992; 339:1081-1082.
  7. Wise CA, Garcia CA, Davis S, et al: Molecular analyses of unrelated Charcot-Marie-Tooth disease patients suggest a high frequency of the CMT1A duplication. Amer J Hum Genetics 1993; 53:853-863.
  8. Lupski JR: An inherited DNA rearrangement and gene dosage effect are responsible for the most common autosomal dominant peripheral neuropathy: Charcot-Marie-Tooth disease type 1 A. Clin Res 1992; 40:645-652.
  9. Lupski JR, Liu P, Williams LL, Patel Pl: Stable inheritance of the CMT1A DNA duplication in two patients with CMT1 and NF1. Am J Med Genet 1993; 45:92-96.
  10. Pentao L, Wise CA, Chinault AC, Patel Pl, Lupski JR: Charcot-Marie-Tooth type 1 A duplication appears to arise from recombination at repeat sequences flanking the 1.5 Mb monomer unit. Nature Genetics 1992; 2:292300.
  11. Patel Pl, Roa BB, Welcher AA, et al: The gene for the peripheral myelin protein PMP-22 is a candidate for Charcot-Marie-Tooth disease type 1A. Nature Genetics 1992; 1:159-165.
  12. Valentijn LJ, Bolhuis PA, Zorn 1, et al: The peripheral myelin gene PMP-22/GAS-3 is duplicated in Charcot-Marie-Tooth disease type 1 A. Nature Genetics 1992; 1 :166-170.
  13. Timmerman V, Nelis E, Van Hul W, et al: The peripheral myelin protein gene PMP-22 is contained within the Charcot-Marie-Tooth disease type 1A duplication. Nature Genetics 1992; 1:171-175.
  14. Chance PF, Alderson MK, Leppig KA, et al: DNA deletion associated with hereditary neuropathy with liability to pressure palsies. Cell , 72,143-151,1993.
  15. Roa BB, Garcia CA, Suter U, et al: Charcot-Marie-Tooth Disease type 1A associated with de novo point mutation in the PMP22 gene. N Engl J Med ,1992; 329, 96-101.