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

AAV-mediated editing of PMP22 rescues Charcot-Marie-Tooth disease type 1A features in patient-derived iPS Schwann cells

BACKGROUND

Charcot-Marie-Tooth disease type 1A (CMT1A) is one of the most common hereditary peripheral neuropathies caused by duplication of 1.5 Mb genome region including PMP22 gene. We aimed to correct the duplication in human CMT1A patient-derived iPS cells (CMT1A-iPSCs) by genome editing and intended to analyze the effect on Schwann cells differentiated from CMT1A-iPSCs.

METHODS

We designed multiple gRNAs targeting a unique sequence present at two sites that sandwich only a single copy of duplicated peripheral myelin protein 22 (PMP22) genes, and selected one of them (gRNA3) from screening their efficiencies by T7E1 mismatch detection assay. AAV2-hSaCas9-gRNAedit was generated by subcloning gRNA3 into pX601-AAV-CMV plasmid, and the genome editing AAV vector was infected to CMT1A-iPSCs or CMT1A-iPSC-derived Schwann cell precursors. The effect of the genome editing AAV vector on myelination was evaluated by co-immunostaining of myelin basic protein (MBP), a marker of mature myelin, and microtubule-associated protein  2(MAP2), a marker of neurites or by electron microscopy.

RESULTS

Here we show that infection of CMT1A-iPS cells (iPSCs) with AAV2-hSaCas9-gRNAedit expressing both hSaCas9 and gRNA targeting the tandem repeat sequence decreased PMP22 gene duplication by 20-40%. Infection of CMT1A-iPSC-derived Schwann cell precursors with AAV2-hSaCas9-gRNAedit normalized PMP22 mRNA and PMP22 protein expression levels, and also ameliorated increased apoptosis and impaired myelination in CMT1A-iPSC-derived Schwann cells.

CONCLUSIONS

In vivo transfer of AAV2-hSaCas9-gRNAedit to peripheral nerves could be a potential therapeutic modality for CMT1A patient after careful examinations of toxicity including off-target mutations.

Genetic mechanisms of peripheral nerve disease

Peripheral neuropathies of genetic etiology are a very diverse group of disorders manifesting either as non-syndromic inherited neuropathies without significant manifestations outside the peripheral nervous system, or as part of a systemic or syndromic genetic disorder. The former and most frequent group is collectively known as Charcot-Marie-Tooth disease (CMT), with prevalence as high as 1:2,500 world-wide, and has proven to be genetically highly heterogeneous. More than 100 different genes have been identified so far to cause various CMT forms, following all possible inheritance patterns. CMT causative genes belong to several common functional pathways that are essential for the integrity of the peripheral nerve. Their discovery has provided insights into the normal biology of axons and myelinating cells, and has highlighted the molecular mechanisms including both loss of function and gain of function effects, leading to peripheral nerve degeneration. Demyelinating neuropathies result from dysfunction of genes primarily affecting myelinating Schwann cells, while axonal neuropathies are caused by genes affecting mostly neurons and their long axons. Furthermore, mutation in genes expressed outside the nervous system, as in the case of inherited amyloid neuropathies, may cause peripheral neuropathy resulting from accumulation of β-structured amyloid fibrils in peripheral nerves in addition to various organs. Increasing insights into the molecular-genetic mechanisms have revealed potential therapeutic targets. These will enable the development of novel therapeutics for genetic neuropathies that remain, in their majority, without effective treatment.

Paternal gender specificity and mild phenotypes in Charcot-Marie-Tooth type 1A patients with de novo 17p12 rearrangements

Background

Charcot–Marie–Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) are developed by duplication and deletion of the 17p12 (PMP22) region, respectively.

Methods

De novo rates were determined in 211 CMT1A or HNPP trio families, and then, analyzed gender‐specific genetic features and clinical phenotypes of the de novo cases.

Results

This study identified 40 de novo cases (19.0%). Paternal origin was highly frequent compared to maternal origin (p = .005). Most de novo CMT1A rearrangements occurred between non‐sister chromatids (p = .003), but it was interesting that three of the four sister chromatids exchange cases were observed in the less frequent maternal origin. Paternal ages at the affected child births were slightly higher in the de novo CMT1A group than in the non‐de novo CMT1A control group (p = .0004). For the disability score of CMTNS, the de novo CMT1A group had a slightly lower value compared to the control group (p = .005). Electrophysiological studies showed no significant differences between the two groups.

Conclusion

This study suggests that de novo CMT1A patients tend to have milder symptoms and that the paternal ages at child births in the de novo group are higher than those of the non‐de novo group.

A Review of Copy Number Variants in Inherited Neuropathies

The rapid development in the last 10-15 years of microarray technologies, such as oligonucleotide array Comparative Genomic Hybridization (CGH) and Single Nucleotide Polymorphisms (SNP) genotyping array, has improved the identification of fine chromosomal structural variants, ranging in length from kilobases (kb) to megabases (Mb), as an important cause of genetic differences among healthy individuals and also as disease-susceptibility and/or disease-causing factors. Structural genomic variations due to unbalanced chromosomal rearrangements are known as Copy-Number Variants (CNVs) and these include variably sized deletions, duplications, triplications and translocations. CNVs can significantly contribute to human diseases and rearrangements in several dosage-sensitive genes have been identified as an important causative mechanism in the molecular aetiology of Charcot-Marie-Tooth (CMT) disease and of several CMT-related disorders, a group of inherited neuropathies with a broad range of clinical phenotypes, inheritance patterns and causative genes. Duplications or deletions of the dosage-sensitive gene PMP22 mapped to chromosome 17p12 represent the most frequent causes of CMT type 1A and Hereditary Neuropathy with liability to Pressure Palsies (HNPP), respectively. Additionally, CNVs have been identified in patients with other CMT types (e.g., CMT1X, CMT1B, CMT4D) and different hereditary poly- (e.g., giant axonal neuropathy) and focal- (e.g., hereditary neuralgic amyotrophy) neuropathies, supporting the notion of hereditary peripheral nerve diseases as possible genomic disorders and making crucial the identification of fine chromosomal rearrangements in the molecular assessment of such patients. Notably, the application of advanced computational tools in the analysis of Next-Generation Sequencing (NGS) data has emerged in recent years as a powerful technique for identifying a genome-wide scale complex structural variants (e.g., as the ones resulted from balanced rearrangements) and also smaller pathogenic (intragenic) CNVs that often remain beyond the detection limit of most conventional genomic microarray analyses; in the context of inherited neuropathies where more than 70 disease-causing genes have been identified to date, NGS and particularly Whole-Genome Sequencing (WGS) hold the potential to reduce the number of genomic assays required per patient to reach a diagnosis, analyzing with a single test all the Single Nucleotide Variants (SNVs) and CNVs in the genes possibly implicated in this heterogeneous group of disorders.

Analysis of PMP22 duplication and deletion using a panel of six dinucleotide tandem repeats

BACKGROUND

Charcot-Marie-Tooth type 1A (CMT1A) is the most common type of hereditary motor and sensory neuropathies (HMSN), caused by the duplication of the 17p11.2 region that includes the PMP22 gene. Reciprocal deletion of the same region is the main cause of hereditary neuropathy with liability to pressure palsies (HNPP). CMT1A accounts for approximately 50% of HMSN patients. Diagnostics of CMT1A and HNPP are based on quantitative analysis of the affected region or RFLP detection of breakage points. The aim of this study was to improve the sensitivity and efficiency of CMT1A and HNPP genetic diagnostics by introducing analysis of six STR markers (D17S261-D17S122-D17S839-D17S1358-D17S955-D17S921) spanning the duplicated region.

METHODS

Forty-six CMT1A and seven HNPP patients, all genetically diagnosed by RFLP analysis, were tested for duplication or deletion using six STR markers.

RESULTS

In all CMT1A and HNPP patients, microsatellite analysis comprising six STR markers confirmed the existence of a duplication or deletion. In 89% (41/46) CMT1A patients the confirmation was based on detecting three alleles on at least one locus. In the remaining 11% (5) CMT1A patients, duplication was also confirmed based on two peaks with clear dosage difference for at least two different markers. All HNPP patients (7/7) displayed only one allele for each analyzed locus.

CONCLUSIONS

Microsatellite analysis using six selected STR loci showed a high level of sensitivity and specificity for genetic diagnostics of CMT1A and HNPP. The results here strongly suggest STR marker analysis as a method of choice in PMP22 duplication/deletion testing.

Severe phenotypes in a Charcot-Marie-Tooth 1A patient with PMP22 triplication

Charcot-Marie-Tooth disease (CMT) is a genetically and clinically heterogeneous hereditary motor and sensory neuropathy signified by a distal symmetric polyneuropathy. The most frequent subtype is type 1A (CMT1A) caused by duplication in chromosome 17p12 that includes PMP22. This study reports a woman with a family history of CMT1A due to PMP22 duplication. However, she presented with a more severe phenotype than her sibling or ancestors and was found to have a PMP22 triplication instead of the duplication. This was caused by de novo mutation on her affected mother's duplication chromosome. Her lower limb magnetic resonance imaging revealed severe diffused atrophy and fatty replacement. However, her affected sister with typical PMP22 duplication showed almost intact lower limb. Triplication patient's median motor nerve conduction velocity was far lower compared with her sister. Her onset age was faster (8 years) than her sister (42 years). CMT1A triplication might be generated by a female-specific chromosomal rearrangement mechanism that is different from the frequent paternal-originated CMT1A duplication. It also suggests that the wide phenotypic variation of CMT1A might be partly caused by unstable genomic rearrangement, including PMP22 triplication.

PMP22 messenger RNA levels in skin biopsies: testing the effectiveness of a Charcot-Marie-Tooth 1A biomarker

Charcot-Marie-Tooth disease type 1A (CMT1A) is associated with increased gene dosage for PMP22. Therapeutic approaches are currently aiming at correcting PMP22 over-expression. It is unknown whether PMP22 can be used as a biological marker of disease progression and therapy efficacy. We performed quantitative real-time polymerase chain reaction on skin biopsies of 45 patients with CMT1A, obtained at study entry and after 24-months of treatment either with ascorbic acid or placebo. Data of a subgroup of patients were also compared with matched healthy subjects. Finally, we analysed PMP22 messenger RNA levels in sural nerve biopsies. We did not find significant differences in the levels of any known PMP22 transcripts in treated or untreated patients with CMT1A, thus confirming that ascorbic acid does not impact on the molecular features of CMT1A. Most importantly, we did not observe any correlation between PMP22 messenger RNA levels and the different clinical and electrophysiological outcome measures, underscoring the weakness of PMP22 to mirror the phenotypic variability of patients with CMT1A. We did not find increased PMP22 messenger RNA levels in skin and sural nerve biopsies of patients with CMT1A compared with relative controls. In conclusion, this study shows that ascorbic acid does not impact on PMP22 transcriptional regulation and PMP22 is not a suitable biomarker for CMT1A.

Genetics of Charcot-Marie-Tooth (CMT) Disease within the Frame of the Human Genome Project Success

Charcot-Marie-Tooth (CMT) neuropathies comprise a group of monogenic disorders affecting the peripheral nervous system. CMT is characterized by a clinically and genetically heterogeneous group of neuropathies, involving all types of Mendelian inheritance patterns. Over 1,000 different mutations have been discovered in 80 disease-associated genes. Genetic research of CMT has pioneered the discovery of genomic disorders and aided in understanding the effects of copy number variation and the mechanisms of genomic rearrangements. CMT genetic study also unraveled common pathomechanisms for peripheral nerve degeneration, elucidated gene networks, and initiated the development of therapeutic approaches. The reference genome, which became available thanks to the Human Genome Project, and the development of next generation sequencing tools, considerably accelerated gene and mutation discoveries. In fact, the first clinical whole genome sequence was reported in a patient with CMT. Here we review the history of CMT gene discoveries, starting with technologies from the early days in human genetics through the high-throughput application of modern DNA analyses. We highlight the most relevant examples of CMT genes and mutation mechanisms, some of which provide promising treatment strategies. Finally, we propose future initiatives to accelerate diagnosis of CMT patients through new ways of sharing large datasets and genetic variants, and at ever diminishing costs.

Molecular genetics of autosomal-dominant demyelinating Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous group of disorders and is the most common inherited neuromuscular disorder, with an estimated overall prevalence of 17-40/10,000. Although there has been major advances in the understanding of the genetic basis of CMT in recent years, the most useful classification is still a neurophysiological classification that divides CMT into type 1 (demyelinating; median motor conduction velocity < 38 m/s) and type 2 (axonal; median motor conduction velocity > 38 m/s). An intermediate type is also increasingly being described. Inheritance can be autosomal-dominant (AD), X-linked, or autosomal-recessive (AR). AD CMT1 is the most common type of CMT and was the first form of CMT in which a causative gene was described. This review provides an up-to-date overview of AD CMT1 concentrating on the molecular genetics as the clinical, neurophysiological, and pathological features have been covered elsewhere. Four genes (PMP22, MPZ, LITAF, and EGR2) have been described in the last 15 yr associated with AD CMTI and a further gene (NEFL), originally described as causing AD CMT2 can also cause AD CMT1 (by neurophysiological criteria). Studies have shown many of these genes, when mutated, can cause a wide range of CMT phenotypes from the relatively mild CMT1 to the more severe Dejerine-Sottas disease and congenital hypomyelinating neuropathy, and even in some cases axonal CMT2. This review discusses what is known about these genes and in particular how they cause a peripheral neuropathy, when mutated.

Murine therapeutic models for Charcot-Marie-Tooth (CMT) disease

INTRODUCTION OR BACKGROUND

Charcot-Marie-Tooth (CMT) disease represents a broad group of inherited motor and sensory neuropathies which can originate from various genetic aberrations, e.g. mutations, deletions and duplications.

SOURCES OF DATA

We performed a literature review on murine animal models of CMT disease with regard to experimental therapeutic approaches. Hereby, we focussed on the demyelinating subforms of CMT (CMT1). PubMed items were CMT, animal model, demyelination and therapy.

AREAS OF AGREEMENT

Patients affected by CMT suffer from slowly progressive, distally pronounced muscle atrophy caused by an axonal loss. The disease severity is highly variable and impairments may result in wheelchair boundness. No therapy is available yet.

AREAS OF CONTROVERSY

Numerous rodent models for the various CMT subtypes are available today. The selection of the correct animal model for the specific CMT subtype provides an important prerequisite for the successful translation of experimental findings in patients.

GROWING POINTS

Despite more than 20 years of remarkable progress in CMT research, the disease is still left untreatable. There is a growing number of experimental therapeutic strategies that may be translated into future clinical trials in patients with CMT.

AREAS TIMELY FOR DEVELOPING RESEARCH

The slow disease progression and insensitive outcome measures hamper clinical therapy trials in CMT. Biomarkers may provide powerful tools to monitor therapeutic efficacy. Recently, we have shown that transcriptional profiling can be utilized to assess and predict the disease severity in a transgenic rat model and in affected humans.

Inheritance of Charcot-Marie-Tooth disease 1A with rare nonrecurrent genomic rearrangement

Rare copy number variations by the nonrecurrent rearrangements involving PMP22 have been recently suggested to be associated with CMT1A peripheral neuropathy. As a mechanism of the nonrecurrent rearrangement, replication-based fork stalling template switching (FoSTeS) by microhomology-mediated break-induced replication (MMBIR) has been proposed. We found three Korean CMT1A families with putative nonrecurrent duplication. The duplications were identified by microsatellite typing and applying a CGH microarray. The breakpoint sequences in two families suggested an Alu-Alu-mediated rearrangement with the FoSTeS by the MMBIR, and a two-step rearrangement of the replication-based FoSTeS/MMBIR and meiosis-based recombination. The two-step mechanism has still not been reported. Segregation analysis of 17p12 microsatellite markers and breakpoint junction analysis suggested that the nonrecurrent rearrangements are stably inherited without alteration of junction sequence; however, they may allow some alteration of the genomic contents in duplication across generations by recombination event. It might be the first study on the pedigree analysis of the large CMT1A families with nonrecurrent rearrangements. It seems that the exact mechanism of the nonrecurrent rearrangements in the CMT1A may have a far more complex process than has been expected.

Mechanisms for nonrecurrent genomic rearrangements associated with CMT1A or HNPP: rare CNVs as a cause for missing heritability

Genomic rearrangements involving the peripheral myelin protein gene (PMP22) in human chromosome 17p12 are associated with neuropathy: duplications cause Charcot-Marie-Tooth disease type 1A (CMT1A), whereas deletions lead to hereditary neuropathy with liability to pressure palsies (HNPP). Our previous studies showed that >99% of these rearrangements are recurrent and mediated by nonallelic homologous recombination (NAHR). Rare copy number variations (CNVs) generated by nonrecurrent rearrangements also exist in 17p12, but their underlying mechanisms are not well understood. We investigated 21 subjects with rare CNVs associated with CMT1A or HNPP by oligonucleotide-based comparative genomic hybridization microarrays and breakpoint sequence analyses, and we identified 17 unique CNVs, including two genomic deletions, ten genomic duplications, two complex rearrangements, and three small exonic deletions. Each of these CNVs includes either the entire PMP22 gene, or exon(s) only, or ultraconserved potential regulatory sequences upstream of PMP22, further supporting the contention that PMP22 is the critical gene mediating the neuropathy phenotypes associated with 17p12 rearrangements. Breakpoint sequence analysis reveals that, different from the predominant NAHR mechanism in recurrent rearrangement, various molecular mechanisms, including nonhomologous end joining, Alu-Alu-mediated recombination, and replication-based mechanisms (e.g., FoSTeS and/or MMBIR), can generate nonrecurrent 17p12 rearrangements associated with neuropathy. We document a multitude of ways in which gene function can be altered by CNVs. Given the characteristics, including small size, structural complexity, and location outside of coding regions, of selected rare CNVs, their identification remains a challenge for genome analysis. Rare CNVs may potentially represent an important portion of "missing heritability" for human diseases.

Genomic rearrangements in inherited disease and cancer

Genomic rearrangements in inherited disease and cancer involve gross alterations of chromosomes or large chromosomal regions and can take the form of deletions, duplications, insertions, inversions or translocations. The characterization of a considerable number of rearrangement breakpoints has now been accomplished at the nucleotide sequence level, thereby providing an invaluable resource for the detailed study of the mutational mechanisms which underlie genomic recombination events. A better understanding of these mutational mechanisms is vital for improving the design of mutation detection strategies. At least five categories of mutational mechanism are known to give rise to genomic rearrangements: (i) homologous recombination including non-allelic homologous recombination (NAHR), gene conversion, single strand annealing (SSA) and break-induced replication (BIR), (ii) non-homologous end joining (NHEJ), (iii) microhomology-mediated replication-dependent recombination (MMRDR), (iv) long interspersed element-1 (LINE-1 or L1)-mediated retrotransposition and (v) telomere healing. Focussing on the first three of these general mechanisms, we compare and contrast their hallmark characteristics, and discuss the role of various local DNA sequence features (e.g. recombination-promoting motifs, repetitive sequences and sequences capable of non-B DNA formation) in mediating the recombination events that underlie gross genomic rearrangements. Finally, we explore how studies both at the level of the gene (using the neurofibromatosis type-1 gene as an example) and the whole genome (using data derived from cancer genome sequencing studies) are shaping our understanding of the impact of genomic rearrangements as a cause of human genetic disease.

Mechanisms for human genomic rearrangements

Genomic rearrangements describe gross DNA changes of the size ranging from a couple of hundred base pairs, the size of an average exon, to megabases (Mb). When greater than 3 to 5 Mb, such changes are usually visible microscopically by chromosome studies. Human diseases that result from genomic rearrangements have been called genomic disorders. Three major mechanisms have been proposed for genomic rearrangements in the human genome. Non-allelic homologous recombination (NAHR) is mostly mediated by low-copy repeats (LCRs) with recombination hotspots, gene conversion and apparent minimal efficient processing segments. NAHR accounts for most of the recurrent rearrangements: those that share a common size, show clustering of breakpoints, and recur in multiple individuals. Non-recurrent rearrangements are of different sizes in each patient, but may share a smallest region of overlap whose change in copy number may result in shared clinical features among different patients. LCRs do not mediate, but may stimulate non-recurrent events. Some rare NAHRs can also be mediated by highly homologous repetitive sequences (for example, Alu, LINE); these NAHRs account for some of the non-recurrent rearrangements. Other non-recurrent rearrangements can be explained by non-homologous end-joining (NHEJ) and the Fork Stalling and Template Switching (FoSTeS) models. These mechanisms occur both in germ cells, where the rearrangements can be associated with genomic disorders, and in somatic cells in which such genomic rearrangements can cause disorders such as cancer. NAHR, NHEJ and FoSTeS probably account for the majority of genomic rearrangements in our genome and the frequency distribution of the three at a given locus may partially reflect the genomic architecture in proximity to that locus. We provide a review of the current understanding of these three models.

A Family Harboring CMT1A Duplication and HNPP Deletion

Charcot-Marie-Tooth disease type 1A (CMT1A) is associated with duplication of chromosome 17p11.2-p12, whereas hereditary neuropathy with liability to pressure palsies (HNPP), which is an autosomal dominant neuropathy showing characteristics of recurrent pressure palsies, is associated with 17p11.2-p12 deletion. An altered gene dosage of PMP22 is believed to the main cause underlying the CMT1A and HNPP phenotypes. Although CMT1A and HNPP are associated with the same locus, there has been no report of these two mutations within a single family. We report a rare family harboring CMT1A duplication and HNPP deletion.

Myelin disorders: Causes and perspectives of Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth (CMT) disease is a common hereditary neuropathy that causes progressive distally pronounced muscle weakness and can lead to life-long disability in patients. In most cases, the disorder has been associated with a partial duplication of human chromosome 17 (CMT1A), causing 1.5-fold overexpression of the peripheral myelin protein 22 kDa (PMP22). Increased PMP22 gene dosage results in demyelination, secondary axonal loss, and neurogenic muscle atrophy. Experimental therapeutic approaches based on the role of progesterone and ascorbic acid in myelin formation recently have reached preclinical proof-of-principle trials in rodents. It was shown that progesterone receptor antagonists can reduce PMP22 overexpression and clinical severity in a CMT1A rat model. Furthermore, ascorbic acid treatment reduced premature death and demyelination in a CMT1A mouse model. Thus, basic research has opened up new vistas for the understanding and treatment of hereditary neuropathies.

Identification of Alu elements mediating a partial PMP22 deletion

Hereditary neuropathy with liability to pressure palsies (HNPP) is most frequently caused by deletion of a 1.4-Mb region in chromosome 17p11.2-12 including the peripheral myelin protein 22 (PMP22) gene. Smaller deletions partially affecting the PMP22 gene are less frequently observed. We identified in a HNPP patient a deletion of the 5' region of PMP22 including non-coding exon 1, coding exons 2 and 3, whereas, exons 4 and 5 were present. PMP22 exon 3- and 4-specific qPCR resulted in a deletion of one exon 3 allele but in the presence of 2 exon 4 alleles. SNP analysis revealed the presence of heterozygosity for PMP22 coding exons 4 and 5. Finally, MLPA specific for the CMT1A region defined this deletion for the entire 5' region of PMP22 (exons 1, 2 and 3). These partial HNPP deletions may be missed by other techniques, e.g., STR marker analysis. Alu elements have been reported to mediate non-allelic recombination events. Bioinformatic analysis revealed 12 Alu elements flanking in close neighbourhood the estimated 40-kb deletion region as candidates for recombination events. PCR primers were designed to identify a breakpoint-spanning product including the respective Alu elements. PCR-driven identification of a junction fragment was successful with AluJo-AluSq and AluYb9-AluSq specific primer pairs comprising the same intronic region of PMP22. Sequence analysis of these breakpoint-overlapping PCR fragments revealed a 29-bp motif including a chi-like sequence (GCTGG) present both in the AluYb9 and the AluSq element. These data confirm that low-copy repeats (LCRs) mediate non-allelic homologous recombinations (NAHR).

Chromosomal phenotypes and submicroscopic abnormalities

The finding, during the last decade, that several common, clinically delineated syndromes are caused by submicroscopic deletions or, more rarely, by duplications, has provided a powerful tool in the annotation of the human genome. Since most microdeletion/microduplication syndromes are defined by a common deleted/duplicated region, abnormal dosage of genes located within these regions can explain the phenotypic similarities among individuals with a specific syndrome. As such, they provide a unique resource towards the genetic dissection of complex phenotypes such as congenital heart defects, mental and growth retardation and abnormal behaviour. In addition, the study of phenotypic differences in individuals with the same microdeletion syndrome may also become a treasury for the identification of modifying factors for complex phenotypes. The molecular analysis of these chromosomal anomalies has led to a growing understanding of their mechanisms of origin. Novel tools to uncover additional submicroscopic chromosomal anomalies at a higher resolution and higher speed, as well as the novel tools at hand for deciphering the modifying factors and epistatic interactors, are 'on the doorstep' and will, besides their obvious diagnostic role, play a pivotal role in the genetic dissection of complex phenotypes.

Close relationship between motor impairments and loss of functional motoneurons in a Charcot-Marie-Tooth type 1A model

Charcot-Marie-Tooth disease type 1A is the most frequent hereditary neuropathy affecting the peripheral nervous system. A partial duplication of chromosome 17 (17p11.2) involving the PMP22 gene is responsible for dysmyelination-demyelination processes leading to motor and sensory impairments. Murine models of this disease are now widely used to investigate the mechanisms occurring at the behavioural and physiological levels. In this study, adult transgenic mice (6 months old) having integrated 7 copies of the human PMP22 gene were used to compare the motor performance, evaluated by using a complex locomotor test (the rotarod test), with both the number of functional motoneurons innervating the soleus muscle and the level of myelination in the sciatic nerve. Two levels of motor deficits were detected and led us to divide the population into two subgroups. In both impaired groups, the level of motor deficit was strongly correlated with the number of functional motoneurons evaluated by retrograde labeling from the muscle, but not with the number of myelinated fibers or the thickness of the myelin sheath (g-ratio). It therefore appears that the number of motor units may be a key element in motor impairments observed in Charcot-Marie-Tooth disease type 1A disease. These findings may have implications for therapeutic procedures, which should focus on the survival of the motoneuronal pool and/or the maintenance of functional neuro-muscular connexions to reduce motor impairments in humans.

Preferential paternal origin of microdeletions caused by prezygotic chromosome or chromatid rearrangements in Sotos syndrome

Sotos syndrome (SoS) is characterized by pre- and postnatal overgrowth with advanced bone age; a dysmorphic face with macrocephaly and pointed chin; large hands and feet; mental retardation; and possible susceptibility to tumors. It has been shown that the major cause of SoS is haploinsufficiency of the NSD1 gene at 5q35, because the majority of patients had either a common microdeletion including NSD1 or a truncated type of point mutation in NSD1. In the present study, we traced the parental origin of the microdeletions in 26 patients with SoS by the use of 16 microsatellite markers at or flanking the commonly deleted region. Deletions in 18 of the 20 informative cases occurred in the paternally derived chromosome 5, whereas those in the maternally derived chromosome were found in only two cases. Haplotyping analysis of the marker loci revealed that the paternal deletion in five of seven informative cases and the maternal deletion in one case arose through an intrachromosomal rearrangement, and two other cases of the paternal deletion involved an interchromosomal event, suggesting that the common microdeletion observed in SoS did not occur through a uniform mechanism but preferentially arose prezygotically.

Genome architecture catalyzes nonrecurrent chromosomal rearrangements

To investigate the potential involvement of genome architecture in nonrecurrent chromosome rearrangements, we analyzed the breakpoints of eight translocations and 18 unusual-sized deletions involving human proximal 17p. Surprisingly, we found that many deletion breakpoints occurred in low-copy repeats (LCRs); 13 were associated with novel large LCR17p structures, and 2 mapped within an LCR sequence (middle SMS-REP) within the Smith-Magenis syndrome (SMS) common deletion. Three translocation breakpoints involving 17p11 were found to be located within the centromeric alpha-satellite sequence D17Z1, three within a pericentromeric segment, and one at the distal SMS-REP. Remarkably, our analysis reveals that LCRs constitute >23% of the analyzed genome sequence in proximal 17p--an experimental observation two- to fourfold higher than predictions based on virtual analysis of the genome. Our data demonstrate that higher-order genomic architecture involving LCRs plays a significant role not only in recurrent chromosome rearrangements but also in translocations and unusual-sized deletions involving 17p.

Rapid detection of 17p11.2 rearrangements by FISH without cell culture (direct FISH, DFISH): a prospective study of 130 patients with inherited peripheral neuropathies

Charcot-Marie-Tooth (CMT) disease and hereditary neuropathy with pressure palsies (HNPP) are two frequent hereditary motor and sensory neuropathies. CMT is characterized by slowly progressive weakness and atrophy, primarily in peroneal and distal leg muscles. The most frequent form, CMT1A, is due, in most cases, to the duplication of a 1.5 Mb region on chromosome 17p11.2 containing the peripheral myelin protein 22 gene (PMP22). The phenotype seems to result from dosage of the PMP22 gene. This hypothesis is reinforced by the existence of HNPP, which is clinically characterized by various recurrent truncular palsies or sensory loss precipitated by minor trauma, which is caused by deletion of the same 1.5 Mb region in 17p11.2. In clinical practice, the detection of the duplication or the deletion in 17p11.2, which permits a positive diagnosis, is still performed by time consuming methods (Southern blot or various combinations of molecular tools). We developed a method for the rapid detection of 17p11.2 rearrangements, using "direct FISH" and PRINS analyses, which does not require cell culture. In a prospective study of 92 patients with CMT and 38 with suspected HNPP, we compared this new technique to classical strategies like Southern blot. The results demonstrate the high sensitivity and specificity of the new FISH technique for the diagnosis of CMT1A and HNPP. Moreover, because of its simplicity and rapidity, this technique provides a useful alternative to the molecular approaches that have been used to diagnose segmental aneusomies, especially in the case of duplications that often go undetected.

Molecular mechanisms for genomic disorders

Genomic rearrangements play a major role in the pathogenesis of human genetic diseases. Nonallelic homologous recombination (NAHR) between low-copy repeats (LCRs) that flank unique genomic segments results in changes of genome organization and can cause a loss or gain of genomic segments. These LCRs appear to have arisen recently during primate speciation via paralogous segmental duplication, thus making the human species particularly susceptible to genomic rearrangements. Genomic disorders are defined as a group of diseases that result from genomic rearrangements, mostly mediated by NAHR. Molecular investigations of genomic disorders have revealed genome architectural features associated with susceptibility to rearrangements and the recombination mechanisms responsible for such rearrangements. The human genome sequence project reveals that LCRs may account for 5% of the genome, suggesting that many novel genomic disorders might still remain to be recognized.

Genetic proof of unequal meiotic crossovers in reciprocal deletion and duplication of 17p11.2

A number of common contiguous gene syndromes have been shown to result from nonallelic homologous recombination (NAHR) within region-specific low-copy repeats (LCRs). The reciprocal duplications are predicted to occur at the same frequency; however, probably because of ascertainment bias and milder phenotypes, reciprocal events have been identified in only a few cases to date. We previously described seven patients with dup(17)(p11.2p11.2), the reciprocal of the Smith-Magenis syndrome (SMS) deletion, del(17)(p11.2p11.2). In >90% of patients with SMS, identical approximately 3.7-Mb deletions in 17p11.2 have been identified. These deletions are flanked by large (approximately 200 kb), highly homologous, directly oriented LCRs (i.e., proximal and distal SMS repeats [SMS-REPs]). The third (middle) SMS-REP is inverted with respect to them and maps inside the commonly deleted genomic region. To investigate the parental origin and to determine whether the common deletion and duplication arise by unequal crossovers mediated through NAHR between the proximal and distal SMS-REPs, we analyzed the haplotypes of 14 families with SMS and six families with dup(17)(p11.2p11.2), using microsatellite markers directly flanking the SMS common deletion breakpoints. Our data indicate that reciprocal deletion and duplication of 17p11.2 result from unequal meiotic crossovers. These rearrangements occur via both interchromosomal and intrachromosomal exchange events between the proximal and distal SMS-REPs, and there appears to be no parental-origin bias associated with common SMS deletions and the reciprocal duplications.

Hereditary neuropathy with liability to pressure palsies with a small deletion interrupting the PMP22 gene

Hereditary neuropathy with liability to pressure palsies is associated with a deficiency in the Peripheral Myelin Protein 22 (PMP22). Most hereditary neuropathy with liability to pressure palsies cases are caused by a deletion of a 1.5 Mb region on chromosome 17p11.2-12 encompassing the PMP22 gene. We describe a hereditary neuropathy with liability to pressure palsies family that lacks the common deletion, but carries a small deletion spanning the 3' region of the PMP22 gene, causing only a partial deletion of one copy of the gene.

Molecular-evolutionary mechanisms for genomic disorders

Molecular studies of unstable regions in the human genome have identified region-specific low-copy repeats (LCRs). Unlike highly repetitive sequences (e.g. Alus and LINEs), LCRs are usually of 10-400 kb in size and exhibit > or = 95-97% similarity. According to computer analyses of available sequencing data, LCRs may constitute >5% of the human genome. Through the process of non-allelic homologous recombination using paralogous genomic segments as substrates, LCRs have been shown to facilitate meiotic DNA rearrangements associated with disease traits, referred to as genomic disorders. In addition, this LCR-based complex genome architecture appears to play a major role in both primate karyotype evolution and human tumorigenesis.

Genes in a refined Smith-Magenis syndrome critical deletion interval on chromosome 17p11.2 and the syntenic region of the mouse

Smith-Magenis syndrome (SMS) is a multiple congenital anomaly/mental retardation syndrome associated with behavioral abnormalities and sleep disturbance. Most patients have the same approximately 4 Mb interstitial genomic deletion within chromosome 17p11.2. To investigate the molecular bases of the SMS phenotype, we constructed BAC/PAC contigs covering the SMS common deletion interval and its syntenic region on mouse chromosome 11. Comparative genome analysis reveals the absence of all three approximately 200-kb SMS-REP low-copy repeats in the mouse and indicates that the evolution of SMS-REPs was accompanied by transposition of adjacent genes. Physical and genetic map comparisons in humans reveal reduced recombination in both sexes. Moreover, by examining the deleted regions in SMS patients with unusual-sized deletions, we refined the minimal Smith-Magenis critical region (SMCR) to an approximately 1.1-Mb genomic interval that is syntenic to an approxiamtely 1.0-Mb region in the mouse. Genes within the SMCR and its mouse syntenic region were identified by homology searches and by gene prediction programs, and their gene structures and expression profiles were characterized. In addition to 12 genes previously mapped, we identified 8 new genes and 10 predicted genes in the SMCR. In the mouse syntenic region of the human SMCR, 16 genes and 6 predicted genes were identified. The SMCR is highly conserved between humans and mice, including 19 genes with the same gene order and orientation. Our findings will facilitate both the identification of gene(s) responsible for the SMS phenotype and the engineering of an SMS mouse model.

Genome architecture, rearrangements and genomic disorders

An increasing number of human diseases are recognized to result from recurrent DNA rearrangements involving unstable genomic regions. These are termed genomic disorders, in which the clinical phenotype is a consequence of abnormal dosage of gene(s) located within the rearranged genomic fragments. Both inter- and intrachromosomal rearrangements are facilitated by the presence of region-specific low-copy repeats (LCRs) and result from nonallelic homologous recombination (NAHR) between paralogous genomic segments. LCRs usually span approximately 10-400 kb of genomic DNA, share >or= 97% sequence identity, and provide the substrates for homologous recombination, thus predisposing the region to rearrangements. Moreover, it has been suggested that higher order genomic architecture involving LCRs plays a significant role in karyotypic evolution accompanying primate speciation.

Meiotic deletion at the BUF1 locus of the fungus Magnaporthe grisea is controlled by interaction with the homologous chromosome

The Magnaporthe grisea BUF1 gene suffers high-frequency mutation in certain genetic crosses, resulting in buff-colored progeny. Analysis of 16 buf1 mutants arising from a cross with a mutation frequency of 25% revealed that, in every case, the BUF1 gene was deleted. The deletions occurred in only one of the parental chromosomes and were due to intrachromosomal recombination. Tetrad analysis revealed that deletions occurred in 44% of meioses and usually affected both chromatids of the mutable chromosome. This suggests that they happen before the premeiotic round of DNA synthesis. However, they were also almost entirely restricted to heteroallelic crosses. This, together with the discovery of numerous repetitive elements that were present only in the mutable BUF1 locus, suggests that the deletion process is sensitive to pairing interactions between homologous chromosomes, such that only unpaired loci are subject to deletion. Given that karyogamy is not supposed to occur until after premeiotic DNA replication in Pyrenomycetous fungi such as M. grisea, this latter observation would place the time of deletion during, or after, DNA synthesis. These conflicting results suggest that karyogamy might actually precede DNA replication in Pyrenomycetous fungi or that parts of the genome remain unreplicated until after karyogamy and subsequent chromosome pairing have taken place.

The genetic convergence of Charcot-Marie-Tooth disease types 1 and 2 and the role of genetics in sporadic neuropathy

Charcot-Marie-Tooth (CMT) disease represents a clinically and genetically heterogeneous group of inherited neuropathies caused by aberration of the intimate relationship between the myelin sheath and the axon; disorders causing demyelination are classified as CMT1 and those causing axonal loss as CMT2. The mechanisms by which mutations disturb the relationship of the myelin sheath and axon are not fully understood; however, we hypothesize that some mutations affect this relationship more profoundly than others, and thus account for the paradox that mutation of a "myelin gene" can present with electrophysiologic features of CMT2 and vice versa. Also, contrary to popular understanding, inherited neuropathies account for a substantial number of chronic peripheral neuropathies. Because of this observation, we propose that molecular diagnosis is a necessary adjunct for differentiating genetic and acquired peripheral neuropathies, even in sporadic chronic neuropathy.

Segmental duplications: an 'expanding' role in genomic instability and disease

The knowledge that specific genetic diseases are caused by recurrent chromosomal aberrations has indicated that genomic instability might be directly related to the structure of the regions involved. The sequencing of the human genome has directed significant attention towards understanding the molecular basis of such recombination 'hot spots'. Segmental duplications have emerged as a significant factor in the aetiology of disorders that are caused by abnormal gene dosage. These observations bring us closer to understanding the mechanisms and consequences of genomic rearrangement.

Diagnostic strategy for familial and sporadic cases of neuropathy associated with 17p11.2 deletion

Clinical, electrophysiologic and molecular studies were performed on at-risk members of 14 families with hereditary neuropathy with liability to pressure palsies (HNPP), in order to detect asymptomatic carriers of the 17p11.2 deletion. Sporadic cases due to de novo deletion accounted for 21% of the investigated HNPP families. Approximately one half of deletion carriers were asymptomatic and did not display significant signs on clinical examination. The electrophysiologic hallmark in both symptomatic and asymptomatic deletion carriers was the presence of a nonuniform sensorimotor demyelinating polyneuropathy with conduction abnormalities preferentially located at common entrapment sites and distal nerve segments. A perfect correlation was found between the molecular and electrophysiologic analyses. A reliable screening method to detect clinically unaffected carriers of the deletion in families with HNPP was the evaluation of motor conduction in at least two nerves across usual entrapment sites, especially the ulnar nerve at the elbow, and evaluation of sensory conduction in the sural nerve. In sporadic cases due to a de novo deletion, electrophysiologic studies were suggestive but not sufficient for the diagnosis, and molecular analysis represented the most sensitive diagnostic tool.

The evolutionary chromosome translocation 4;19 in Gorilla gorilla is associated with microduplication of the chromosome fragment syntenic to sequences surrounding the human proximal CMT1A-REP

Many genomic disorders occur as a result of chromosome rearrangements involving low-copy repeats (LCRs). To better understand the molecular basis of chromosome rearrangements, including translocations, we have investigated the mechanism of evolutionary rearrangements. In contrast to several intrachromosomal rearrangements, only two evolutionary translocations have been identified by cytogenetic analyses of humans and greater apes. Human chromosome 2 arose as a result of a telomeric fusion between acrocentric chromosomes, whereas chromosomes 4 and 19 in Gorilla gorilla are the products of a reciprocal translocation between ancestral chromosomes, syntenic to human chromosomes 5 and 17, respectively. Fluorescence in situ hybridization (FISH) was used to characterize the breakpoints of the latter translocation at the molecular level. We identified three BAC clones that span translocation breakpoints. One breakpoint occurred in the region syntenic to human chromosome 5q13.3, between the HMG-CoA reductase gene (HMGCR) and RAS p21 protein activator 1 gene (RASA1). The second breakpoint was in a region syntenic to human chromosome 17p12 containing the 24 kb region-specific low-copy repeat-proximal CMT1A-REP. Moreover, we found that the t(4;19) is associated with a submicroscopic chromosome duplication involving a 19p chromosome fragment homologous to the human chromosome region surrounding the proximal CMT1A-REP. These observations further indicate that higher order genomic architecture involving low-copy repeats resulting from genomic duplication plays a significant role in karyotypic evolution.

The 1.4-Mb CMT1A duplication/HNPP deletion genomic region reveals unique genome architectural features and provides insights into the recent evolution of new genes

Duplication and deletion of the 1.4-Mb region in 17p12 that is delimited by two 24-kb low copy number repeats (CMT1A-REPs) represent frequent genomic rearrangements resulting in two common inherited peripheral neuropathies, Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsy (HNPP). CMT1A and HNPP exemplify a paradigm for genomic disorders wherein unique genome architectural features result in susceptibility to DNA rearrangements that cause disease. A gene within the 1.4-Mb region, PMP22, is responsible for these disorders through a gene-dosage effect in the heterozygous duplication or deletion. However, the genomic structure of the 1.4-Mb region, including other genes contained within the rearranged genomic segment, remains essentially uncharacterized. To delineate genomic structural features, investigate higher-order genomic architecture, and identify genes in this region, we constructed PAC and BAC contigs and determined the complete nucleotide sequence. This CMT1A/HNPP genomic segment contains 1,421,129 bp of DNA. A low copy number repeat (LCR) was identified, with one copy inside and two copies outside of the 1.4-Mb region. Comparison between physical and genetic maps revealed a striking difference in recombination rates between the sexes with a lower recombination frequency in males (0.67 cM/Mb) versus females (5.5 cM/Mb). Hypothetically, this low recombination frequency in males may enable a chromosomal misalignment at proximal and distal CMT1A-REPs and promote unequal crossing over, which occurs 10 times more frequently in male meiosis. In addition to three previously described genes, five new genes (TEKT3, HS3ST3B1, NPD008/CGI-148, CDRT1, and CDRT15) and 13 predicted genes were identified. Most of these predicted genes are expressed only in embryonic stages. Analyses of the genomic region adjacent to proximal CMT1A-REP indicated an evolutionary mechanism for the formation of proximal CMT1A-REP and the creation of novel genes by DNA rearrangement during primate speciation.

Molecular mechanisms for constitutional chromosomal rearrangements in humans

Cytogenetic imbalance in the newborn is a frequent cause of mental retardation and birth defects. Although aneuploidy accounts for the majority of imbalance, structural aberrations contribute to a significant fraction of recognized chromosomal anomalies. This review describes the major classes of constitutional, structural cytogenetic abnormalities and recent studies that explore the molecular mechanisms that bring about their de novo occurrence. Genomic features flanking the sites of recombination may result in susceptibility to chromosomal rearrangement. One such substrate for recombination is low-copy region-specific repeats. The identification of genome architectural features conferring susceptibility to rearrangements has been accomplished using methods that enable investigation of regions of the genome that are too small to be visualized by traditional cytogenetics and too large to be resolved by conventional gel electrophoresis. These investigations resulted in the identification of previously unrecognized structural cytogenetic anomalies, which are associated with genetic syndromes and allowed for the molecular basis of some chromosomal rearrangements to be delineated.

Structure of chromosomal duplicons and their role in mediating human genomic disorders

Chromosome-specific low-copy repeats, or duplicons, occur in multiple regions of the human genome. Homologous recombination between different duplicon copies leads to chromosomal rearrangements, such as deletions, duplications, inversions, and inverted duplications, depending on the orientation of the recombining duplicons. When such rearrangements cause dosage imbalance of a developmentally important gene(s), genetic diseases now termed genomic disorders result, at a frequency of 0.7-1/1000 births. Duplicons can have simple or very complex structures, with variation in copy number from 2 to >10 repeats, and each varying in size from a few kilobases in length to hundreds of kilobases. Analysis of the different duplicons involved in human genomic disorders identifies features that may predispose to recombination, including large size and high sequence identity between the recombining copies, putative recombination promoting features, and the presence of multiple genes/pseudogenes that may include genes expressed in germ cells. Most of the chromosome rearrangements involve duplicons near pericentromeric regions, which may relate to the propensity of such regions to accumulate duplicons. Detailed analyses of the structure, polymorphic variation, and mechanisms of recombination in genomic disorders, as well as the evolutionary origin of various duplicons will further our understanding of the structure, function, and fluidity of the human genome.

The peripheral myelin protein 22 and epithelial membrane protein family

The peripheral myelin protein 22 (PMP22) and the epithelial membrane proteins (EMP-1, -2, and -3) comprise a subfamily of small hydrophobic membrane proteins. The putative four-transmembrane domain structure as well as the genomic structure are highly conserved among family members. PMP22 and EMPs are expressed in many tissues, and functions in cell growth, differentiation, and apoptosis have been reported. EMP-1 is highly up-regulated during squamous differentiation and in certain tumors, and a role in tumorigenesis has been proposed. PMP22 is most highly expressed in peripheral nerves, where it is localized in the compact portion of myelin. It plays a crucial role in normal physiological and pathological processes in the peripheral nervous system. Progress in molecular genetics has revealed that genetic alterations in the PMP22 gene, including duplications, deletions, and point mutations, are responsible for several forms of hereditary peripheral neuropathies, including Charcot-Marie-Tooth disease type 1A (CMT1A), Dejerine-Sottas syndrome (DDS), and hereditary neuropathy with liability to pressure palsies (HNPP). The natural mouse mutants Trembler and Trembler-J contain a missense mutation in different hydrophobic domains of PMP22, resulting in demyelination and Schwann cell proliferation. Transgenic mice carrying many copies of the PMP22 gene and PMP22-null mice display a variety of defects in the initial steps of myelination and/or maintenance of myelination, whereas no pathological alterations are detected in other tissues normally expressing PMP22. Further characterization of the interactions of PMP22 and EMPs with other proteins as well as their regulation will provide additional insight into their normal physiological function and their roles in disease and possibly will result in the development of therapeutic tools.

Homologous DNA exchanges in humans can be explained by the yeast double-strand break repair model: a study of 17p11.2 rearrangements associated with CMT1A and HNPP

Rearrangements in 17p11.2, responsible for the 1.5 Mb duplications and deletions associated, respectively, with autosomal dominant Charcot-Marie-Tooth type 1A disease (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) are a suitable model for studying human recombination. Rearrangements in 17p11.2 are caused by unequal crossing-over between two homologous 24 kb sequences, the CMT1A-REPs, that flank the disease locus and occur in most cases within a 1.7 kb hotspot. We sequenced this hotspot in 28 de novo patients (25 CMT1A and three HNPP), in order to localize precisely, at the DNA sequence level, the crossing-overs. We show that some chimeric CMT1A-REPs in de novo patients (10/28) present conversion of DNA segments associated with the crossing-over. These rearrangements can be explained by the double-strand break (DSB) repair model described in yeast. Fine mapping of the de novo rearrangements provided evidence that the successive steps of this model, heteroduplex DNA formation, mismatch correction and gene conversion, occurred in patients. Furthermore, the model explains 17p11.2 recombinations between chromosome homologues as well as between sister chromatids. In addition, defective mismatch repair of the heteroduplex DNA, observed in two patients, resulted in two heterozygous chimeric CMT1A-REPs which can be explained, as in yeast, by post-meiotic segregation. This work supports the hypothesis that the DSB repair model of DNA exchange may apply universally from yeasts to humans.

Chromosome breakage in the Prader-Willi and Angelman syndromes involves recombination between large, transcribed repeats at proximal and distal breakpoints

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are distinct neurobehavioral disorders that most often arise from a 4-Mb deletion of chromosome 15q11-q13 during paternal or maternal gametogenesis, respectively. At a de novo frequency of approximately.67-1/10,000 births, these deletions represent a common structural chromosome change in the human genome. To elucidate the mechanism underlying these events, we characterized the regions that contain two proximal breakpoint clusters and a distal cluster. Novel DNA sequences potentially associated with the breakpoints were positionally cloned from YACs within or near these regions. Analyses of rodent-human somatic-cell hybrids, YAC contigs, and FISH of normal or rearranged chromosomes 15 identified duplicated sequences (the END repeats) at or near the breakpoints. The END-repeat units are derived from large genomic duplications of a novel gene (HERC2), many copies of which are transcriptionally active in germline tissues. One of five PWS/AS patients analyzed to date has an identifiable, rearranged HERC2 transcript derived from the deletion event. We postulate that the END repeats flanking 15q11-q13 mediate homologous recombination resulting in deletion. Furthermore, we propose that active transcription of these repeats in male and female germ cells may facilitate the homologous recombination process.

Proteolipoprotein gene analysis in 82 patients with sporadic Pelizaeus-Merzbacher Disease: duplications, the major cause of the disease, originate more frequently in male germ cells, but point mutations do not. The Clinical European Network on Brain Dysmyelinating Disease

Pelizaeus-Merzbacher Disease (PMD) is an X-linked developmental defect of myelination affecting the central nervous system and segregating with the proteolipoprotein (PLP) locus. Investigating 82 strictly selected sporadic cases of PMD, we found PLP mutations in 77%; complete PLP-gene duplications were the most frequent abnormality (62%), whereas point mutations in coding or splice-site regions of the gene were involved less frequently (38%). We analyzed the maternal status of 56 cases to determine the origin of both types of PLP mutation, since this is relevant to genetic counseling. In the 22 point mutations, 68% of mothers were heterozygous for the mutation, a value identical to the two-thirds of carrier mothers that would be expected if there were an equal mutation rate in male and female germ cells. In sharp contrast, among the 34 duplicated cases, 91% of mothers were carriers, a value significantly (chi2=9. 20, P<.01) in favor of a male bias, with an estimation of the male/female mutation frequency (k) of 9.3. Moreover, we observed the occurrence of de novo mutations between parental and grandparental generations in 17 three-generation families, which allowed a direct estimation of the k value (k=11). Again, a significant male mutation imbalance was observed only for the duplications. The mechanism responsible for this strong male bias in the duplications may involve an unequal sister chromatid exchange, since two deletion events, responsible for mild clinical manifestations, have been reported in PLP-related diseases.

Mutations in the peripheral myelin genes and associated genes in inherited peripheral neuropathies

The peripheral myelin protein 22 gene (PMP22), the myelin protein zero gene (MPZ, P0), and the connexin 32 gene (Cx32, GJB1) code for membrane proteins expressed in Schwann cells of the peripheral nervous system (PNS). The early growth response 2 gene (EGR2) encodes a transcription factor that may control myelination in the PNS. Mutations in the respective genes, located on human chromosomes 17p11.2, 1q22-q23, Xq13.1, and 10q21.1-q22.1, are associated with several inherited peripheral neuropathies. To date, a genetic defect in one of these genes has been identified in over 1,000 unrelated patients manifesting a wide range of phenotypes, i.e., Charcot-Marie-Tooth disease type 1 (CMT1) and type 2 (CMT2), Dejerine-Sottas syndrome (DSS), hereditary neuropathy with liability to pressure palsies (HNPP), and congenital hypomyelination (CH). This large number of genetically defined patients provides an exceptional opportunity to examine the correlation between phenotype and genotype.

Genomic disorders: structural features of the genome can lead to DNA rearrangements and human disease traits

Molecular medicine began with Pauling's seminal work, which recognized sickle-cell anemia as a molecular disease, and with Ingram's demonstration of a specific chemical difference between the hemoglobins of normal and sickled human red blood cells. During the four decades that followed, investigations have focused on the gene--how mutations specifically alter DNA and how these changes affect the structure and expression of encoded proteins. Recently, however, the advances of the human genome project and the completion of total genome sequences for yeast and many bacterial species, have enabled investigators to view genetic information in the context of the entire genome. As a result, we recognize that the mechanisms for some genetic diseases are best understood at a genomic level. The evolution of the mammalian genome has resulted in the duplication of genes, gene segments and repeat gene clusters. This genome architecture provides substrates for homologous recombination between nonsyntenic regions of chromosomes. Such events can result in DNA rearrangements that cause disease.

Pelizaeus-Merzbacher disease: identification of Xq22 proteolipid-protein duplications and characterization of breakpoints by interphase FISH

Pelizaeus-Merzbacher disease (PMD) is an X-linked, dysmyelinating disorder of the CNS. Duplications of the proteolipid protein (PLP) gene have been found in a proportion of patients, suggesting that, in addition to coding-region or splice-site mutations, overdosage of the gene can cause PMD. We show that the duplication can be detected by interphase FISH, using a PLP probe in five patients and their four asymptomatic carrier mothers. The extent of the duplication was analyzed in each family by interphase FISH, with probes from a 1. 7-Mb region surrounding the PLP gene between markers DXS83 and DXS94. A large duplication >=500 kb was detected, with breakpoints that differed, between families, at the proximal end. Distinct separation of the duplicated PLP signals could be seen only on metaphase chromosomes in one family, providing further evidence that different duplication events are involved. Quantitative fluorescent multiplex PCR was used to confirm the duplication in patients, by the detection of increased copy number of the PLP gene. Multiallelic markers from the duplicated region were analyzed, since the identification of two alleles in an affected boy would indicate a duplication. The majority of boys were homozygous for all four markers, compared with their mothers, who were heterozygous for one to three of the markers. These results suggest that intrachromosomal rearrangements may be a common mechanism by which duplications arise in PMD. One boy was heterozygous for the PLP marker, indicating a duplication and suggesting that interchromosomal rearrangements of maternal origin also can be involved. Since duplications are a major cause of PMD, we propose that interphase FISH is a reliable method for diagnosis and identification of female carriers.

Many facets of the peripheral myelin protein PMP22 in myelination and disease

Peripheral myelin protein 22 (PMP22) is a small, hydrophobic glycoprotein, which is most prominently expressed by Schwann cells as a component of compact myelin of the peripheral nervous system (PNS). Recent progress in molecular genetics revealed that mutations affecting the PMP22 gene including duplications, deletions, and point mutations are responsible for the most common forms of hereditary peripheral neuropathies including Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), and a subtype of Dejerine-Sottas Syndrome (DSS). Functionally, PMP22 is involved in correct myelination during development of peripheral nerves, the stability of myelin, and the maintenance of axons. While most of these functions relate to a role of PMP22 as a structural component of myelin, PMP22 has also been proposed as a regulator of Schwann cell proliferation and differentiation. In this review, we will discuss our current knowledge of PMP22 and its related proteins in the normal organism as well as in disease. In particular, we will focus on how the function of PMP22 and its regulation may be relevant to particular disease mechanisms.