Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant neuropathy, most often associated with a deletion of the 17p11.2 region, which is duplicated in 70% of patients with Charcot-Marie-Tooth type 1 (CMT1A). Most de novo CMT1A and HNPP cases have been of paternal origin. A rare case of de novo HNPP of maternal origin was analysed to determine the underlying mechanism. Affected individuals in the family carried a deletion corresponding to the CMT1A/HNPP monomer unit associated with a rearrangement of the CMT1A-REP sequences. Segregation analysis of 17p11-p12 markers in the family indicated that the deletion was not generated by unequal crossing over between homologous 17 chromosomes, as in de novo cases from paternal origin, but rather by an intrachromosomal rearrangement. Two distinct mechanisms can therefore lead to the same 17p11.2 deletion. This result suggests that intrachromosomal rearrangement may be specific to maternal transmissions.
Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant peripheral neuropathy (1 ). HNPP patients present variously located recurrent truncular palsies or sensory loss, precipitated by minor trauma. In most cases, patients recover within days or weeks, but relapses may be frequent, and paresis may last for long periods. Nerve conduction studies reveal a characteristic pattern of decreased motor velocities, prolonged distal latencies and altered sensory nerve action potentials, even in clinically non affected areas or in asymptomatic at risk individuals (2 ). Peripheral nerve biopsies show a characteristic focal sausage-shaped thickening of the myelin sheath (tomacula) in numerous internodes.
The presence of an interstitial deletion of the 17p11.2 region associated with this disorder was demonstrated in most of the HNPP families (3 ,4 ). The same region, designated the CMT1A/HNPP monomer unit, is duplicated in a more frequently diagnosed neuropathy, Charcot-Marie-Tooth type 1A disease (CMT1A) (5 -8 ). Thus, it was hypothesized that an unequal crossing-over would generate both a duplication that could lead to CMT1A or a deletion that could result in HNPP. The identification of two homologous sequences flanking the CMT1A/HNPP monomer unit, the CMT1A-REPs, supported this hypothesis (9 ). Recently, Chance et al. (10 ) proposed that the CMT1A duplication and the HNPP deletion could both arise from recombination events that occur within a limited region of the CMT1A-REP. De novo deletions have been reported in five HNPP patients, four of paternal origin (3 ,11 ,12 ) and one of possible maternal origin (13 ). In CMT1A, however, the de novo duplications, where the parental origin could be determined, were paternal (5 ,14 ,15 ).
In order to analyse the mechanism underlying maternal de novo deletions in HNPP, we performed a molecular study of a family with a HNPP patient presenting a de novo deletion from maternal origin using 17p11-p12 markers and the CMT1A-REP probe pNEA102. The deletion was found to be associated with a rearrangement of the CMT1A-REPs but, unlike previous reports, haplotype reconstruction strongly supported the hypothesis that an intrachromosomal rearrangement generated the deletion.
Eight members of the family SAL-902 were genotyped for eight microsatellites and two RFLP markers from 17p11-12. Segregation analysis with the RM11GT microsatellite at D17S122 locus demonstrated the presence of a 17p11.2 deletion in patient 15 who carried allele 5, but received no allele from his mother (2/3) at this locus, showing that, in this family, the phenotype was associated with a deletion within 17p11.2 as described previously (2 -4 ,11 -13 ). The absence of maternal contribution to the patient 15 at the D17S122 locus, demonstrated the maternal origin of this de novo case of HNPP. All tested at risk individuals (7 ,17 ,20 ) shown to be unaffected by clinical and electrophysiological examination were heterozygous for at least one of the markers within the CMT1A/HNPP monomer unit (Fig. 1 ).
We report a de novo case of HNPP of maternal origin. The deletion detected in affected members is associated with a rearrangement of CMT1A-REP sequences and corresponds to the CMT1A/HNPP monomer unit. However, haplotype reconstruction highly suggests that this deletion was not caused by an unequal crossing-over as described previously (3 ,11 ,12 ). It is highly probable that the de novo deletion observed in family SAL-902 results from an intrachromosomal rearrangement.
Our observation provides the first evidence that rearrangements involving identical autosomal sequences can occur either by unequal crossing-over between homologues or by intra-homologue rearrangement. Interestingly, all documented de novo duplications of paternal origin resulting in CMT1A, have been shown by haplotype reconstruction to result from an unequal recombination between homologue chromosomes 17 (5 ,14 ,15 ). In HNPP, both paternal de novo deletions (3 ,11 ,12 ) but also a possible maternal deletion (13 ) have been reported. The latter received no allele of the pEW401HE 17p11.2 marker (D17S61) from his mother who was shown to be unaffected by clinical and electrophysiological examination. The molecular mechanism underlying this deletion was not analyzed, however.
Terminal or interstitial deletions detected by classical cytogenetic techniques are generally caused by the occurrence of one or two breaks respectively, within a chromosome with the loss of the acentric fragment. An intrachromosomal mechanism was also postulated to cause the de novo deletions occurring in males in Duchenne muscular dystrophy or haemophilia A (20 ). It was demonstrated that some deletions in these genes, which are located on the X chromosome can occur in male gametes. Since male gametes bear only one X chromosome, the origin of these rearrangements is mitotic and the corresponding process is certainly intrachromosomic. Intrachromosomal rearrangement has also been invoked for deletions of several Mb on autosomes, in particular in the Angelman/Prader-Willi syndrome (21 ,22 ). For de novo deletions occurring in males in Duchenne muscular dystrophy or in haemophilia A, the excision of a chromatidal loop during replication was proposed (20 ). This mechanism was also postulated for the deletions within the beta-globin gene domain resulting in the hereditary persistence of fetal hemoglobin (HPFH) (23 ). In Angelman/Prader-Willi syndrome, unequal sister chromatid exchange was thought as an alternative to explain the deletion (21 ,22 ). Unfortunately, the molecular tools which would distinguish between these two intrachromosomal mechanisms in our family are not yet available.
Since deletions in HNPP can result both from crossing-over between homologues and intra-chromosomal rearrangement but duplications only by crossing over, HNPP should be more frequent than CMT1A with duplication. HNPP is apparently less prevalent, however, than CMT1A but this is probably because of underdiagnosis (2 ). Since unequal crossing-overs resulting in deletion or duplication were observed during male meioses, the intrachromosomal deletion of this region detected in this study may preferentially occur in females, although additional cases must be analyzed before a generalization can be made.
Index case 15 from family SAL-902, examined at age 41, described three episodes of acute sensory-motor truncular deficits without obvious precipitating factors. Several nerves were affected simultaneously during the third episode. Motor nerve conduction (MNCV) and sensory nerve conduction velocities (SNCV) were abnormal bilaterally in the median, ulnar and peroneal nerves. Tomaculous changes involving 36% of internodes were found in superficial peroneal nerve biopsies. Clinical and electrophysiological examinations were performed in all first degree relatives. Individual 32, the son of the index case, presented three episodes of truncular sensory-motor palsies and his brother, 31, described paresthesias with a right ulnary distribution lasting a few hours. Patients 31 and 32 both presented diffuse MNCV and SNCV abnormalities. Parents and siblings never complained of motor or sensory abnormalities. The mother (6 ) of index case 15 was a 73 year old farmer, who presented with unilateral electrophysiological carpal tunnel syndrome but had no other abnormal electrophysiological features. The father of the index case was not available for examination. All siblings had normal MCV and SCV. Individuals 6 and 15 were genotyped twice with two different blood samples in order to eliminate sampling or typing errors.
The genomic probes EW401 (D17S61) detects two MspI alleles with sizes of 5.5 and 4.4 kb, and VAW409R3a (D17S122) three MspI alleles with sizes of 2.8, 2.7 and 1.9 kb. Probe pNEA102, which detects two EcoRI fragments of 6.0 and 7.8 kb corresponding respectively, to the proximal and distal CMT1A-REP which flank the CMT1A/HNPP monomer unit, was also used. The probes were hybridized to Southern blots after preannealing with placental DNA. The CMT1A-REP deletion was assessed by visual comparison of the hybridization signals of the two EcoRI fragments hybridized with probe pNEA102.
The following (CA)n microsatellites, which span 13 cM on chromosome 17p11-p12, were used: D17S122 (RM11GT)(6 ), D17S842 (afm240xe5), D17S783 (afm026vh7), D17S953 (afm304xh5), D17S839 (afm200yb12), D17S955 (afm317yg1), D17S921 (afm191xh12) and D17S922 (afm197xh6). Genetic distances between the Généthon markers were as follows: (D17S842-D17S783), 6 cM, D17S953, 5 cM, (D17S839- D17S955), 1 cM, D17S921, 1 cM, D17S922 (24 ). Genotypes were determined with the PCR/blotting technique (25 ).
The authors thank Dr J. R. Lupski who kindly provided the pNEA102 probe. They are grateful to Christiane Penet, Yolaine Pothin and Isabelle Lagroua for technical and medical assistance, and would like to thank Drs Merle Ruberg and Josué Feingold for critical reading of the manuscript. This study was supported by the Association Française contre les Myopathies (AFM), the Assistance Publique des Hôpitaux de Paris (AP-HP) and the Association pour le Développement de la Recherche sur les Maladies Génétiques Neurologiques et Psychiatriques (ADRMGNP).
jnl.info{at}oup.co.uk
Human Molecular Genetics
Pages
Introduction
Results
Discussion
Materials And Methods
Clinical data
RFLP markers
Microsatellite markers
Acknowledgements
References
REFERENCES
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