Deletion in the CMT1A locus on chromosome 17p11.2 in hereditary neuropathy with liability to pressure palsies

Hereditary neuropathy with liability to pressure palsies (NHPP) is an autosomal dominant disease of peripheral nerves, characterized by recurrent focal neuropathies often with an underlying asymptomatic polyneuropathy. We report the clinical, electrophysiological, and histopathological findings in three families with HNPP and confirm the presence of a deletion on chromosome 17p11.2, including all the markers known to be duplicated in Charcot-Marie-Tooth disease type 1A. This deletion appears to be the underlying molecular deficit in this disease and provides additional evidence for the importance of this locus for peripheral nerve function.

Charcot-Marie-Tooth disease in northern Sweden: pedigree analysis and the presence of the duplication in chromosome 17p11.2

Sixty-seven patients in 29 families with the diagnosis of Charcot-Marie-Tooth disease or hereditary motor and sensory neuropathy in northern Sweden were examined by pedigree and DNA analysis for the CMT1a duplication within chromosome 17p11.2. There were 39 patients in nine families with Charcot-Marie-Tooth type 1 and autosomal dominant inheritance and in all these cases the duplication was seen. In six patients in three families with Charcot-Marie-Tooth type 1 the pedigrees strongly suggested autosomal recessive inheritance. In two patients DNA analysis was not informative but in the others no duplication was shown. There were also 11 "sporadic" patients and one pair of sibs classified as Charcot-Marie-Tooth type 1, but there was no duplication shown although in four patients DNA analysis was not informative. In nine patients with Charcot-Marie-Tooth type 2 from five families and in 13 unaffected relatives of Charcot-Marie-Tooth patients the CMT1a duplication was not found.

Detection of tandem duplications and implications for linkage analysis

The first demonstration of an autosomal dominant human disease caused by segmental trisomy came in 1991 for Charcot-Marie-Tooth disease type 1A (CMT1A). For this disorder, the segmental trisomy is due to a large tandem duplication of 1.5 Mb of DNA located on chromosome 17p11.2-p12. The search for the CMT1A disease gene was misdirected and impeded because some chromosome 17 genetic markers that are linked to CMT1A lie within this duplication. To better understand how such a duplication might affect genetic analyses in the context of disease gene mapping, we studied the effects of marker duplication on transmission probabilities of marker alleles, on linkage analysis of an autosomal dominant disease, and on tests of linkage homogeneity. We demonstrate that the undetected presence of a duplication distorts transmission ratios, hampers fine localization of the disease gene, and increases false evidence of linkage heterogeneity. In addition, we devised a likelihood-based method for detecting the presence of a tandemly duplicated marker when one is suspected. We tested our methods through computer simulations and on CMT1A pedigrees genotyped at several chromosome 17 markers. On the simulated data, our method detected 96% of duplicated markers (with a false-positive rate of 5%). On the CMT1A data our method successfully identified two of three loci that are duplicated (with no false positives). This method could be used to identify duplicated markers in other regions of the genome and could be used to delineate the extent of duplications similar to that involved in CMT1A.

Origin of the de novo duplication in Charcot-Marie-Tooth disease type 1A: unequal nonsister chromatid exchange during spermatogenesis

A 1.5 Mb duplication within 17p11.2 is the major mutation causing both autosomal dominant and sporadic Charcot-Marie-Tooth disease type 1A (CMT1A). An independent origin for the mutation in each family has been postulated. The proposed genetic mechanism causing the CMT1A duplication is unequal nonsister chromatid exchange at meiosis (unequal crossing-over). We studied the parental origin of the duplication in nine genetically sporadic CMT1A patients and demonstrated that in all cases the mutation was the product of an unequal nonsister chromatid exchange during spermatogenesis. The fact that only paternal de novo duplications were observed in the sporadic CMT1A patients suggests that male specific factors may be operating during spermatogenesis that either help forming the duplication and/or stabilize the duplicated chromosome.

The peripheral myelin protein gene PMP-22 is contained within the Charcot-Marie-Tooth disease type 1A duplication

Charcot-Marie-Tooth disease (CMT1) is the most common form of inherited peripheral neuropathy. Although the disease is genetically heterogeneous, it has been demonstrated that the gene defect is the most frequent type (CMT1A) is the result of a partial duplication of band 17p11.2. Recent studies suggested that the peripheral hypomyelination syndrome in the trembler (Tr) mouse, a possible animal model for CMT1 disease, is associated with a point mutation in the peripheral myelin protein-22 gene (pmp-22). Expression of pmp-22 is particularly high in Schwann cells, and the protein is found in peripheral myelin. We now report that the human PMP-22 gene is contained within the CMT1A duplication. We therefore, suggest that increased dosage of the PMP-22 gene may be the cause of CMT1A neuropathy.

Linkage analysis of distal hereditary motor neuropathy type II (distal HMN II) in a single pedigree

We describe a six generation family affected with the autosomal dominant form of distal hereditary motor neuropathy type II (distal HMN II). The distal HMN shows similarities with the hereditary motor and sensory neuropathies type I and II (HMSN I and HMSN II) or Charcot-Marie-Tooth disease type 1 and 2 (CMT 1 and CMT 2) and with some proximal HMN or spinal muscular atrophies (SMA). Gene loci have been assigned to chromosomes 1q, 17p, and 19q for CMT 1 and to chromosome 5q for recessive SMA. In this study we excluded all four regions for the presence of distal HMN II, indicating that this neuropathy is genetically different from CMT 1 and recessive SMA.

Estimation of the size of the chromosome 17p11.2 duplication in Charcot-Marie-Tooth neuropathy type 1a (CMT1a). HMSN Collaborative Research Group

We have previously shown a duplication in 17p11.2 with probe pVAW409R3 (D17S122) in 12 families with hereditary motor and sensory neuropathy type I (HMSN I) or Charcot-Marie-Tooth disease type 1 (CMT1). In this study we aimed to estimate the size of the duplication using additional polymorphic DNA markers located in 17p11.2-p12. Two other 17p11.2 markers, pVAW412R3 (D17S125) and pEW401 (D17S61), were found to be duplicated in all HMSN I patients tested. Furthermore, all HMSN I patients showed the same duplication junction fragment with probe pVAW409R3. On the genetic map the duplicated markers span a minimal distance of 10 cM while on the physical map they are present in the same NotI restriction fragment of 1150 kb. The discrepancy between the genetic and physical map distances suggests that the 17p11.2 region is extremely prone to recombinational events. The high recombination rate may be a contributing factor to the genetic instability of this chromosomal region.

Duplication in chromosome 17p11.2 in Charcot-Marie-Tooth neuropathy type 1a (CMT 1a). The HMSN Collaborative Research Group

Hereditary motor and sensory neuropathy type I (HMSN I) or Charcot-Marie-Tooth disease type 1 (CMT 1) is an autosomal dominant disorder of the peripheral nervous system characterized by progressive weakness and atrophy of distal limb muscles. In the majority of HMSN I families, linkage studies localized the gene (CMT 1a) to the pericentromeric region of chromosome 17. We have detected with probe pVAW409R3 (D17S122) localized in 17p11.2 a duplication, co-segregating with the disease in 12 HMSN I families. In these families the duplication was present in all 128 patients but absent in the 84 unaffected and 44 married-in individuals (lod score of 58.44 at zero recombination). Further, on one HMSN I family the disease newly appeared simultaneously with a de novo duplication originating from an unequal crossing-over event at meiosis. Since different allelic combinations were found segregating with the duplication in different families linkage disequilibrium was not a significant factor. These findings led us to propose that the duplication in 17p11.2 itself is the disease causing mutation in all the HMSN I families analyzed.