Segregation of a totally skewed pattern of X chromosome inactivation in four familial cases of Rett syndrome without MECP2 mutation: implications for the disease

Somatic mosaicism detected by genome-wide sequencing in 500 parent-child trios with suspected genetic disease: clinical and genetic counseling implications

Identifying genetic mosaicism is important in establishing a diagnosis, assessing recurrence risk, and providing accurate genetic counseling. Next-generation sequencing has allowed for the identification of mosaicism at levels below those detectable by conventional Sanger sequencing or chromosomal microarray analysis. The CAUSES Clinic was a pediatric translational trio-based genome-wide (exome or genome) sequencing study of 500 families (531 children) with suspected genetic disease at BC Children's and Women's Hospitals. Here we present 12 cases of apparent mosaicism identified in the CAUSES cohort: nine cases of parental mosaicism for a disease-causing variant found in a child and three cases of mosaicism in the proband for a de novo variant. In six of these cases, there was no evidence of mosaicism on Sanger sequencing—the variant was not detected on Sanger sequencing in three cases, and it appeared to be heterozygous in three others. These cases are examples of six clinical manifestations of mosaicism: a proband with classical clinical features of mosaicism (e.g., segmental abnormalities of skin pigmentation or asymmetrical growth of bilateral body parts), a proband with unusually mild manifestations of a disease, a mosaic proband who is clinically indistinguishable from the constitutive phenotype, a mosaic parent with no clinical features of the disease, a mosaic parent with mild manifestations of the disease, and a family in which both parents are unaffected and two siblings have the same disease-causing constitutional mutation. Our data demonstrate the importance of considering the possibility of mosaicism whenever exome or genome sequencing is performed and that its detection via genome-wide sequencing can permit more accurate genetic counseling.

Technological Improvements in the Genetic Diagnosis of Rett Syndrome Spectrum Disorders

Rett syndrome (RTT) is a severe neurodevelopmental disorder that constitutes the second most common cause of intellectual disability in females worldwide. In the past few years, the advancements in genetic diagnosis brought by next generation sequencing (NGS), have made it possible to identify more than 90 causative genes for RTT and significantly overlapping phenotypes (RTT spectrum disorders). Therefore, the clinical entity known as RTT is evolving towards a spectrum of overlapping phenotypes with great genetic heterogeneity. Hence, simultaneous multiple gene testing and thorough phenotypic characterization are mandatory to achieve a fast and accurate genetic diagnosis. In this review, we revise the evolution of the diagnostic process of RTT spectrum disorders in the past decades, and we discuss the effectiveness of state-of-the-art genetic testing options, such as clinical exome sequencing and whole exome sequencing. Moreover, we introduce recent technological advancements that will very soon contribute to the increase in diagnostic yield in patients with RTT spectrum disorders. Techniques such as whole genome sequencing, integration of data from several “omics”, and mosaicism assessment will provide the tools for the detection and interpretation of genomic variants that will not only increase the diagnostic yield but also widen knowledge about the pathophysiology of these disorders.

Gender Specific Differences in Disease Susceptibility: The Role of Epigenetics

Many complex traits or diseases, such as infectious and autoimmune diseases, cancer, xenobiotics exposure, neurodevelopmental and neurodegenerative diseases, as well as the outcome of vaccination, show a differential susceptibility between males and females. In general, the female immune system responds more efficiently to pathogens. However, this can lead to over-reactive immune responses, which may explain the higher presence of autoimmune diseases in women, but also potentially the more adverse effects of vaccination in females compared with in males. Many clinical and epidemiological studies reported, for the SARS-CoV-2 infection, a gender-biased differential response; however, the majority of reports dealt with a comparable morbidity, with males, however, showing higher COVID-19 adverse outcomes. Although gender differences in immune responses have been studied predominantly within the context of sex hormone effects, some other mechanisms have been invoked: cellular mosaicism, skewed X chromosome inactivation, genes escaping X chromosome inactivation, and miRNAs encoded on the X chromosome. The hormonal hypothesis as well as other mechanisms will be examined and discussed in the light of the most recent epigenetic findings in the field, as the concept that epigenetics is the unifying mechanism in explaining gender-specific differences is increasingly emerging.

Mutation in the MICOS subunit gene APOO (MIC26) associated with an X-linked recessive mitochondrial myopathy, lactic acidosis, cognitive impairment and autistic features

BACKGROUND

Mitochondria provide ATP through the process of oxidative phosphorylation, physically located in the inner mitochondrial membrane (IMM). The mitochondrial contact site and organising system (MICOS) complex is known as the 'mitoskeleton' due to its role in maintaining IMM architecture. APOO encodes MIC26, a component of MICOS, whose exact function in its maintenance or assembly has still not been completely elucidated.

METHODS

We have studied a family in which the most affected subject presented progressive developmental delay, lactic acidosis, muscle weakness, hypotonia, weight loss, gastrointestinal and body temperature dysautonomia, repetitive infections, cognitive impairment and autistic behaviour. Other family members showed variable phenotype presentation. Whole exome sequencing was used to screen for pathological variants. Patient-derived skin fibroblasts were used to confirm the pathogenicity of the variant found in APOO. Knockout models in Drosophila melanogaster and Saccharomyces cerevisiae were employed to validate MIC26 involvement in MICOS assembly and mitochondrial function.

RESULTS

A likely pathogenic c.350T>C transition was found in APOO predicting an I117T substitution in MIC26. The mutation caused impaired processing of the protein during import and faulty insertion into the IMM. This was associated with altered MICOS assembly and cristae junction disruption. The corresponding mutation in MIC26 or complete loss was associated with mitochondrial structural and functional deficiencies in yeast and D. melanogaster models.

CONCLUSION

This is the first case of pathogenic mutation in APOO, causing altered MICOS assembly and neuromuscular impairment. MIC26 is involved in the assembly or stability of MICOS in humans, yeast and flies.

Case Report: Prenatal Diagnosis for a Rett Syndrome Family Caused by a Novel MECP2 Deletion With Heteroduplexes of PCR Product

Rett syndrome is an X-linked dominant, postnatal neurological disorder. Approximately 80-90% of classic Rett syndrome patients harbor mutations in the coding region of MECP2. Somatic or germline MECP2 mosaicism is not rare, and paternal germline MECP2 mosaicism occurs in especially high proportions. Here, we report the case of a Chinese girl with Rett syndrome in whom a heterozygous deletion was found in exon 4 of MECP2 using multiplex ligation-dependent probe amplification. To obtain an accurate region of deletion, we narrowed down the deletion region using real-time quantitative PCR, and subsequent long-range PCR was performed to detect the deletion breakpoints. Surprisingly, three DNA bands from long-range PCR products were observed after gel electrophoresis. To exclude somatic mosaicism, we performed T-A cloning and DNA sequencing, the middle DNA band was proved to be a heteroduplex of the PCR product in vitro. Meanwhile, a prenatal diagnosis was performed for the pregnant mother of the patient. Our study showed that the patient was heterozygous for the deletion of 713-base pairs in exon 4 of MECP2 (MECP2: c.441_1153del713), resulting in a frameshift and premature termination of the 487 amino acid protein at the 154th codon. In summary, we reported a novel heterozygous deletion in the MECP2 gene with heteroduplexes of the PCR product in vitro, which can help in the genetic counseling and prenatal diagnosis of disorders of MECP2 defects.

A new sex-specific underlying mechanism for female schizophrenia: accelerated skewed X chromosome inactivation

BACKGROUND

X chromosome inactivation (XCI) is the mechanism by which the X-linked gene dosage is adjusted between the sexes. Evidence shows that many sex-specific diseases have their basis in X chromosome biology. While female schizophrenia patients often have a delayed age of disease onset and clinical phenotypes that are different from those of males, it is unknown whether the sex differences in schizophrenia are associated with X-linked gene dosage and the choice of X chromosome silencing in female cells. Previous studies demonstrated that sex chromosome aneuploidies may be related to the pathogeneses of some psychiatric diseases. Here, we examined the changes in skewed XCI in patients with schizophrenia.

METHODS

A total of 109 female schizophrenia (SCZ) patients and 80 age- and sex-matched healthy controls (CNTLs) were included in this study. We evaluated clinical features including disease onset age, disease duration, clinical symptoms by the Positive and Negative Syndrome Scale (PANSS) and antipsychotic treatment dosages. The XCI skewing patterns were analyzed by the methylation profile of the HUMARA gene found in DNA isolated from SCZ patient and CNTL leukocytes in the three age groups.

RESULTS

First, we found that the frequency of skewed XCI in SCZ patients was 4 times more than that in the age- and sex-matched CNTLs (p < 0.01). Second, we found an earlier onset of severe XCI skewing in the SCZ patients than in CNTLs. Third, we demonstrated a close relationship between the severity of skewed XCI and schizophrenic symptoms (PANSS score ≥ 90) as well as the age of disease onset. Fourth, we demonstrated that the skewed XCI in SCZ patients was not transmitted from the patients' mothers.

LIMITATIONS

The XCI skewing pattern might differ depending on tissues or organs. Although this is the first study to explore skewed XCI in SCZ, in the future, samples from different tissues or cells in SCZ patients might be important for understanding the impact of skewed XCI in this disease.

CONCLUSION

Our study, for the first time, investigated skewed XCI in female SCZ patients and presented a potential mechanism for the sex differences in SCZ. Our data also suggested that XCI might be a potential target for the development of female-specific interventions for SCZ.

Disturbed redox homeostasis and oxidative stress: Potential players in the developmental regression in Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder affecting mostly girls. A seemingly normal initial development is followed by developmental stagnation and regression, leading to severe mental impairment with autistic features, motor dysfunction, irregular breathing and epilepsy. Currently, a cure does not exist. Due to the close association of RTT with mitochondrial alterations, cellular redox-impairment and oxidative stress, compounds stabilizing mitochondrial function, cellular redox-homeostasis, and oxidant detoxification are increasingly considered as treatment concepts. Indeed, antioxidants and free-radical scavengers ameliorate certain aspects of the complex and severe clinical presentation of RTT. To further evaluate these strategies, reliable biosensors are needed to quantify redox-conditions in brain and peripheral organs of mouse models or in patient-derived cells. Genetically-encoded redox-sensors meet these requirements. Expressed in transgenic mouse-models such as our unique Rett-redox indicator mice, they will report for any cell type desired the severity of oxidant stress throughout the various disease stages of RTT. Furthermore, these sensors will be crucial to evaluate in vitro and in vivo the outcome of mitochondria- and redox-balance targeted treatments.

Silent angels the genetic and clinical aspects of Rett syndrome

Rett syndrome is a neurodevelopmental genetic disorder and, because of some behavioral characteristics, individuals affected by the disease are known as silent angels. Girls with Rett syndrome perform stereotyped movements, they have learning difficulties, their reaction time is prolonged, and they seem alienated in the environment. These children require constant pediatric, neurological and orthopedic care. In the treatment of Rett syndrome physical therapy, music therapy, hydrotherapy, hippotherapy, behavioral methods, speech therapy and diet, are also used. In turn, psychological therapy of the syndrome is based on the sensory integration method, using two or more senses simultaneously. In 80% of cases, the syndrome is related to mutations of the MECP2 gene, located on chromosome X. The pathogenesis of Rett syndrome is caused by the occurrence of a non-functional MeCP2 protein, which is a transcription factor of many genes, i.e. Bdnf, mef2c, Sgk1, Uqcrc1. Abnormal expression of these genes reveals a characteristic disease phenotype. Clinical symptoms relate mainly to the nervous, respiratory, skeletal and gastrointestinal systems. Currently causal treatment is not possible. However, researchers are developing methods by which, perhaps in the near future, it will be possible to eliminate the mutations in the MECP2 gene, and this will give a chance to the patient for normal functioning.

The paper presents the etiology and pathogenesis of the disease, genetic, clinical, pharmacological aspects and other forms of Rett syndrome treatment.

X Chromosome Inactivation in Opioid Addicted Women

INTRODUCTION

X chromosome inactivation (XCI) is a process during which one of the two X chromosomes in female human is silenced leading to equal gene expression with males who have only one X chromosome. Here we have investigated XCI ratio in females with opioid addiction to see whether XCI skewness in women could be a risk factor for opioid addiction.

METHODS

30 adult females meeting DSM IV criteria for opioid addiction and 30 control females with no known history of addiction were included in the study. Digested and undigested DNA samples which were extracted from blood were analyzed after amplification of the polymorphic androgen receptor (AR) gene located on the X chromosome. XCI skewness was studied in 3 ranges: 50:50-64:36 (random inactivation), 65:35-80:20 (moderately skewed) and >80:20 (highly skewed).

RESULTS

XCI from informative females in control group was 63% (N=19) random, 27% (N=8) moderately skewed and 10% (N=3) highly skewed. Addicted women showed 57%, 23% and 20%, respectively. The distribution and frequency of XCI status in women with opioid addiction was not significantly different from control group (P=0.55).

DISCUSSION

Our data did not approve our hypothesis of increased XCI skewness among women with opioid addiction or unbalanced (non-random) expression of genes associated with X chromosome in female opioid addicted subjects.

Intragenic rearrangements in X-linked intellectual deficiency: results of a-CGH in a series of 54 patients and identification of TRPC5 and KLHL15 as potential XLID genes

High-resolution array comparative genomic hybridization (a-CGH) enables the detection of intragenic rearrangements, such as single exon deletion or duplication. This approach can lead to the identification of new disease genes. We report on the analysis of 54 male patients presenting with intellectual deficiency (ID) and a family history suggesting X-linked (XL) inheritance or maternal skewed X-chromosome inactivation (XCI), using a home-made X-chromosome-specific microarray covering the whole human X-chromosome at high resolution. The majority of patients had whole genome array-CGH prior to the selection and we did not include large rearrangements such as MECP2 and FMR1 duplications. We identified four rearrangements considered as causative or potentially pathogenic, corresponding to a detection rate of 8%. Two CNVs affected known XLID genes and were therefore considered as causative (IL1RAPL1 and OPHN1 intragenic deletions). Two new CNVs were considered as potentially pathogenic as they affected interesting candidates for ID. The first CNV is a deletion of the first exon of the TRPC5 gene, encoding a cation channel implicated in dendrite growth and patterning, in a child presenting with ID and an autism spectrum disorder (ASD). The second CNV is a partial deletion of KLHL15, in a patient with severe ID, epilepsy, and anomalies of cortical development. In both cases, in spite of strong arguments for clinical relevance, we were not able at this stage to confirm pathogenicity of the mutations, and the causality of the variants identified in XLID remains to be confirmed.

Distinct de novo deletions in a brother-sister pair with RTT: a case report

Rett syndrome (RTT), a neurodevelopmental disorder caused by mutations in the X-linked gene encoding methyl-CpG-binding protein2 (MeCP2), is a leading cause of mental retardation in females. Majority of cases are sporadic (99%) but some familial cases have also been observed. We describe a familial study with a brother-sister pair with symptoms of RTT and exhibiting distinct deletions in the MECP2. The non-shared de novo deletion in the two sibs provides important insights into the disease etiology, especially for male sibs showing varied phenotypes as compared to the classical ones seen in the females.

MeCP2 and Rett syndrome: reversibility and potential avenues for therapy

Mutations in the X-linked gene MECP2 (methyl CpG-binding protein 2) are the primary cause of the neurodevelopmental disorder RTT (Rett syndrome), and are also implicated in other neurological conditions. The expression product of this gene, MeCP2, is a widely expressed nuclear protein, especially abundant in mature neurons of the CNS (central nervous system). The major recognized consequences of MECP2 mutation occur in the CNS, but there is growing awareness of peripheral effects contributing to the full RTT phenotype. MeCP2 is classically considered to act as a DNA methylation-dependent transcriptional repressor, but may have additional roles in regulating gene expression and chromatin structure. Knocking out Mecp2 function in mice recapitulates many of the overt neurological features seen in RTT patients, and the characteristic postnatally delayed onset of symptoms is accompanied by aberrant neuronal morphology and deficits in synaptic physiology. Evidence that reactivation of endogenous Mecp2 in mutant mice, even at adult stages, can reverse aspects of RTT-like pathology and result in apparently functionally mature neurons has provided renewed hope for patients, but has also provoked discussion about traditional boundaries between neurodevelopmental disorders and those involving dysfunction at later stages. In the present paper we review the neurobiology of MeCP2 and consider the various genetic (including gene therapy), pharmacological and environmental interventions that have been, and could be, developed to attempt phenotypic rescue in RTT. Such approaches are already providing valuable insights into the potential tractability of RTT and related conditions, and are useful pointers for the development of future therapeutic strategies.

Two new Rett syndrome families and review of the literature: expanding the knowledge of MECP2 frameshift mutations

BACKGROUND

Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder, which is usually caused by de novo mutations in the MECP2 gene. More than 70% of the disease causing MECP2 mutations are eight recurrent C to T transitions, which almost exclusively arise on the paternally derived X chromosome. About 10% of the RTT cases have a C-terminal frameshift deletion in MECP2. Only few RTT families with a segregating MECP2 mutation, which affects female carriers with a phenotype of mental retardation or RTT, have been reported in the literature. In this study we describe two new RTT families with three and four individuals, respectively, and review the literature comparing the type of mutations and phenotypes observed in RTT families with those observed in sporadic cases. Based on these observations we also investigated origin of mutation segregation to further improve genetic counselling.

METHODS

MECP2 mutations were identified by direct sequencing. XCI studies were performed using the X-linked androgen receptor (AR) locus. The parental origin of de novo MECP2 frameshift mutations was investigated using intronic SNPs.

RESULTS

In both families a C-terminal frameshift mutation segregates. Clinical features of the mutation carriers vary from classical RTT to mild mental retardation. XCI profiles of the female carriers correlate to their respective geno-/phenotypes. The majority of the de novo frameshift mutations occur on the paternally derived X chromosome (7/9 cases), without a paternal age effect.

CONCLUSIONS

The present study suggests a correlation between the intrafamilial phenotypic differences observed in RTT families and their respective XCI pattern in blood, in contrast to sporadic RTT cases where a similar correlation has not been demonstrated. Furthermore, we found de novo MECP2 frameshift mutations frequently to be of paternal origin, although not with the same high paternal occurrence as in sporadic cases with C to T transitions. This suggests further investigations of more families. This study emphasizes the need for thorough genetic counselling of families with a newly diagnosed RTT patient.

De novo deletion in MECP2 in a monozygotic twin pair: a case report

BACKGROUND

Rett syndrome (RTT) is a severe, progressive, neurodevelopmental disorder predominantly observed in females that leads to intellectual disability. Mutations and gross rearrangements in MECP2 account for a large proportion of cases with RTT. A limited number of twin pairs with RTT have also been reported in literature.

CASE PRESENTATION

We investigated 13 year old, monozygotic twin females with RTT and some noticeable differences in development using a combinatorial approach of sequencing and Taqman assay. Monozygosity status of the twins was confirmed by informative microsatellite markers.

CONCLUSIONS

The twins shared a de novo deletion in exon 3 in the MBD domain of MECP2. To the best of our knowledge, this is only the second report of genetic analysis of a monozygotic twin pair.

Role of polymorphism of the androgen receptor gene andnon-random x chromosome inactivation in the pathogenesisof androgenic alopecia

The article presents data on molecular and genetic studies of mechanisms of development of androgenic alopecia as well as correlation between polymorphism of the androgen receptor gene by the CAG repeat length in exon 1 and androgen-dependent diseases including alopecia.

APOE epsilon4: a potential modulation factor in Rett syndrome

Rett syndrome is a neurodevelopmental disorder mainly caused by de novo mutations in the MECP2 (methyl-CpG-binding protein 2) gene. There is considerable variation in the severity of clinical features among Rett syndrome patients, even among patients with the same MECP2 mutation. In addition to X-chromosome inactivation pattern, the genetic background of the affected individual might also have a role in determining the severity of the disorder. We suggest that APOE is one of the genetic modulating factors. We analyzed clinical phenotypes of 46 patients with Rett syndrome, with confirmed MECP2 mutation. We discovered that among epsilon4 carriers, some clinical features were more severe, and the developmental regression occurred 4 months earlier on average than in those without the epsilon4 allele. Earlier onset of regression suggests a possible trend; however, it did not achieve distinctive statistical significance. Nevertheless, the epsilon4 allele of APOE may serve as a candidate modulation factor for the Rett syndrome phenotype.

A novel MECP2 gene mutation in a Tunisian patient with Rett syndrome

Patients with classical Rett show an apparently normal psychomotor development during the first 6-18 months of life. Thereafter, they enter a short period of developmental stagnation followed by a rapid regression in language and motor development. Purposeful hand use is often lost and replaced by repetitive, stereotypic movements. Rett syndrome (RTT) is an X-linked dominant disorder caused frequently by mutations in the methyl-CpG-binding protein 2 gene (MECP2). The aim of this study was to search for mutations in MECP2 gene in two Tunisian patients affected with RTT. The results of mutation analysis revealed mutations in exon 4 of MECP2 gene in the two patients. In one patient we identified a new mutation consisting of a deletion of four bases (c.810-813delAAAG), which led to a frame shift and generated a premature stop codon (p.Lys271Arg fs X15) in transcriptional repression domain-nuclear localization signal (TRD-NLS) domain of MeCP2 protein. With regard to the second patient, a previously described transition (c.916C>T) that changed an arginine to a cysteine residue (p.R306C) in TRD domain of MeCP2 protein was revealed. In conclusion, a new and a known de novo mutation in MECP2 gene were revealed in two Tunisian patients affected with RTT.

Two sisters with Rett syndrome and non-identical paternally-derived microdeletions in the MECP2 gene

The unique case of two sisters with symptoms of RTT and two quite distinct, novel, and apparently de novo microdeletions of the MECP2 gene is described. One sister possessed an 18 base-pair (bp) deletion (c.1155_1172del18) within the deletion hotspot region of exon 4, whereas the other sister exhibited a 43 bp deletion at a different location in the same exon (c.1448_1461del14+29). Although these lesions occurred on the same paternally-derived X chromosome, this is probably due to chance co-occurrence owing to the relatively high mutation rate of the MECP2 gene rather than to a constitutional mutator phenotype.

Delineation of large deletions of the MECP2 gene in Rett syndrome patients, including a familial case with a male proband

Comprehensive genetic screening programs have led to the identification of pathogenic methyl-CpG-binding protein 2 (MECP2) mutations in up to 95% of classical Rett syndrome (RTT) patients. This high rate of mutation detection can partly be attributed to specialised techniques that have enabled the detection of large deletions in a substantial fraction of otherwise mutation-negative patients. These cases would normally be missed by the routine PCR-based screening strategies. Here, we have identified large multi-exonic deletions in 12/149 apparently mutation-negative RTT patients using multiplex ligation-dependent probe amplification (MLPA). These deletions were subsequently characterised using real-time quantitative PCR (qPCR) and long-range PCR with the ultimate aim of defining the exact nucleotide positions of the breakpoints and rearrangements. We detected an apparent deletion in one further patient using MLPA; however, this finding was contradicted by subsequent qPCR and long-range PCR results. The patient group includes an affected brother and sister with a large MECP2 deletion also present in their carrier mother. The X chromosome inactivation pattern of all female patients in this study was determined, which, coupled with detailed clinical information, allowed meaningful genotype-phenotype correlations to be drawn. This study reaffirms the view that large MECP2 deletions are an important cause of both classical and atypical RTT syndrome, and cautions that apparent deletions detected using high-throughput diagnostic techniques require further characterisation.

X-chromosome inactivation patterns in females with Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a complex neurodevelopmental disorder caused by loss of paternally expressed genes from the 15q11-q13 region generally due to a paternally-derived deletion of the 15q11-q13 region or maternal disomy 15 (UPD). Maternal disomy 15 is usually caused by maternal meiosis I non-disjunction associated with advanced maternal age and after fertilization with a normal sperm leading to trisomy 15, a lethal condition unless trisomy rescue occurs with loss of the paternal chromosome 15. To further characterize the pathogenesis of maternal disomy 15 process in PWS, the status of X-chromosome inactivation was calculated to determine whether non-random skewing of X-inactivation is present indicating a small pool of early embryonic cells. We studied X-chromosome inactivation in 25 females with PWS-UPD, 35 with PWS-deletion, and 50 controls (with similar means, medians, and age ranges) using the polymorphic androgen receptor (AR) gene assay. A significant positive correlation (r = 0.5, P = 0.01) was seen between X-chromosome inactivation and age for only the UPD group. Furthermore, a significantly increased level (P = 0.02) of extreme X-inactivation skewness (>90%) was detected in our PWS-UPD group (24%) compared to controls (4%). This observation could indicate that trisomy 15 occurred at conceptus with trisomy rescue in early pregnancy leading to extreme skewness in several PWS-UPD subjects. Extreme X-inactivation skewness may also lead to additional risks for X-linked recessive disorders in PWS females with UPD and extreme X-chromosome skewness.

An explanation for another familial case of Rett syndrome: maternal germline mosaicism

Rett syndrome (RTT; OMIM#312750) is a severe neurodevelopmental disorder that affects mainly girls. It has an estimated incidence of 1:10,000-15,000 females. Mutations in the X-linked gene methyl CpG-binding protein 2 (MECP2) have been found in most patients. The most accepted explanation for the sex bias is that the Rett mutation in sporadic cases has its origin in the paternal germline X chromosome and can thus only be transmitted to females. The majority of cases are sporadic (99.5%) but some familial cases have been described. These cases can either be explained by germline mosaicism or by asymptomatic carrier mothers with skewing of X-inactivation towards the wild-type MECP2 allele. We describe one of the few familial cases of RTT in which a maternal germline mosaicism is the most likely explanation. The mutation p.Arg270fs (c.808delC) was identified in both a girl with classical RTT and her brother who had the severe neurological phenotype usually described in males. The mutation was absent in DNA extracted from blood of both parents. These type of events must be taken into consideration in the genetic counselling of families after the diagnosis of a first case of RTT in a female or a MECP2 mutation in a male.

Rett syndrome: new clinical and molecular insights

In this review, we give a clinical overview of Rett syndrome (RTT), and provide a framework for clinical and molecular approaches to the diagnosis of this severe neurodevelopmental disorder. We also discuss issues that need to be considered in the management of RTT patients, and raise some of the challenges associated with genetic counselling.

Germline mosaicism for a MECP2 mutation in a man with two Rett daughters

Rett syndrome is a severe neurodevelopmental disorder that is caused by mutations in the X-linked gene, methyl-CpG binding protein 2 (MECP2). The majority of cases are sporadic, but rarely germline mosaicism can lead to familial cases. Here, we report the first case where germline mosaicism for a MECP2 mutation has been shown in a man. He has two affected daughters who are half sisters, and both have the c.808delC mutation. We show that this mutation is present at a low level in DNA extracted from the patient's semen. This case has implications for genetic counseling, and pre-natal testing should be offered for the partners of men who have a daughter with Rett syndrome.

Correlation between clinical severity in patients with Rett syndrome with a p.R168X or p.T158M MECP2 mutation, and the direction and degree of skewing of X-chromosome inactivation

INTRODUCTION

Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder that is usually associated with mutations in the MECP2 gene. The most common mutations in the gene are p.R168X and p.T158M. The influence of X-chromosome inactivation (XCI) on clinical severity in patients with RTT with these mutations was investigated, taking into account the extent and direction of skewing.

METHODS

Female patients and their parents were recruited from the UK and Australia. Clinical severity was measured by the Pineda Severity and Kerr profile scores. The degree of XCI and its direction relative to the X chromosome parent of origin were measured in DNA prepared from peripheral blood leucocytes, and allele-specific polymerase chain reaction was used to determine the parental origin of mutation. Combining these, the percentage of cells expected to express the mutant allele was calculated.

RESULTS

Linear regression analysis was undertaken for fully informative cases with p.R168X (n = 23) and p.T158M (n = 20) mutations. A statistically significant increase in clinical severity with increase in the proportion of active mutated allele was shown for both the p.R168X and p.T158M mutations.

CONCLUSIONS

XCI may vary in neurological and haematological tissues. However, these data are the first to show a relationship between the degree and direction of XCI in leucocytes and clinical severity in RTT, although the clinical utility of this in giving a prognosis for individual patients is unclear.

Increased skewing of X chromosome inactivation in Rett syndrome patients and their mothers

Rett syndrome is a largely sporadic, X-linked neurological disorder with a characteristic phenotype, but which exhibits substantial phenotypic variability. This variability has been partly attributed to an effect of X chromosome inactivation (XCI). There have been conflicting reports regarding incidence of skewed X inactivation in Rett syndrome. In rare familial cases of Rett syndrome, favourably skewed X inactivation has been found in phenotypically normal carrier mothers. We have investigated the X inactivation pattern in DNA from blood and buccal cells of sporadic Rett patients (n=96) and their mothers (n=84). The mean degree of skewing in blood was higher in patients (70.7%) than controls (64.9%). Unexpectedly, the mothers of these patients also had a higher mean degree of skewing in blood (70.8%) than controls. In accordance with these findings, the frequency of skewed (XCI > or =80%) X inactivation in blood was also higher in both patients (25%) and mothers (30%) than in controls (11%). To test whether the Rett patients with skewed X inactivation were daughters of skewed mothers, 49 mother-daughter pairs were analysed. Of 14 patients with skewed X inactivation, only three had a mother with skewed X inactivation. Among patients, mildly affected cases were shown to be more skewed than more severely affected cases, and there was a trend towards preferential inactivation of the paternally inherited X chromosome in skewed cases. These findings, particularly the greater degree of X inactivation skewing in Rett syndrome patients, are of potential significance in the analysis of genotype-phenotype correlations in Rett syndrome.

Mutations in PHD-like domain of the ATRX gene correlate with severe psychomotor impairment and severe urogenital abnormalities in patients with ATRX syndrome

Mutations in ATRX are associated with a wide and clinically heterogeneous spectrum of X-linked mental retardation syndromes. The ATRX protein, involved in chromatin remodelling, belongs to the family of SWI/SNF DNA helicases and contains a plant homeodomain (PHD)-like domain. To date, more than 60 different mutations have been reported in ATRX. One of them is recurrent and accounts for 20% of all the reported mutations, whereas all others are private. Most mutations are clustered in the two major functional domains, the helicase and the PHD-like domain. So far, no clear genotype-phenotype correlation has been established, with exception to the rare truncating mutations located at the C-terminal part of the protein, which are consistently associated with severe urogenital defects. In this study, we report the molecular analysis performed in 16 families positive for ATRX. Our findings indicate that, in addition to the previously described mutation 'hotspot' in the PHD-like domain, two other protein sections emerge as minor 'hotspots' in the helicase region encoded by exons 18-20 and 26-29, respectively, gathering 33% of all described mutations. Additionally, based on the clinical data collected for 22 patients from the 16 families, we observe that mutations in the PHD-like domain produce severe and permanent psychomotor deficiency, usually preventing patients from walking, as well as constant urogenital abnormalities, while mutations in the helicase domain lead to delayed but correct psychomotor acquisitions together with mild or absent urogenital abnormalities. In summary, mutations in the helicase domain are associated with milder phenotypes than mutations in the PHD-like domain.

Brief report: non-random X chromosome inactivation in females with autism

Autism is a heterogeneous neurodevelopmental disorder with a 3-4 times higher sex ratio in males than females. X chromosome genes may contribute to this higher sex ratio through unusual skewing of X chromosome inactivation. We studied X chromosome skewness in 30 females with classical autism and 35 similarly aged unaffected female siblings as controls using the polymorphic androgen receptor (AR) gene. Significantly, increased X chromosome skewness (e.g., >80:20%) was detected in our autism group (33%) compared to unaffected females (11%). X chromosome skewness was also seen in 50% of the mothers with autistic daughters. No mutation was seen in the promoter region of the XIST gene reported to be involved in X chromosome inactivation in our subjects. X chromosome skewness has been reported in female carriers of other neurological disorders such as X-linked mental retardation, adrenoleukodystrophy and Rett syndrome.

Does genotype predict phenotype in Rett syndrome?

Mutations in the X-linked gene encoding the methyl-CpG binding protein MeCP2 are the primary cause of classic and atypical Rett syndrome and have recently been shown to contribute to other neurodevelopmental disorders of varying severity. To determine whether there are molecular correlates to the phenotypic heterogeneity, numerous groups have performed genotype-phenotype correlation studies. These studies have yielded conflicting results, in part because they used different criteria for determining severity and classifying mutations. Evolution of the phenotype with age and variable expressivity arising from individual variability in X-chromosome inactivation patterns are among other reasons the findings varied. Nonetheless, evidence of differences in the phenotypic consequences of specific types of mutations is emerging. This review analyzes the available literature and makes recommendations for future studies.

Newborn screening and prenatal diagnosis for Rett syndrome: implications for therapy

Most girls with Rett syndrome develop normally prior to the appearance of the typical symptoms. A presymptomatic phase is also observed in many inborn errors of metabolism that are included in newborn screening programs. Diagnostic testing for mutations or large genomic rearrangements involving methyl-CpG binding protein 2 gene (MECP2) is highly sensitive and identifies mutations in up to 95% of female individuals with classic Rett syndrome. This has prompted some to ask whether MECP2 testing should be included in newborn and prenatal screening programs. We review current and evolving practices in these programs, emphasizing their relevance to Rett syndrome. The availability of a reliable test and the characteristic early latent phase, which creates a window of opportunity for early treatment, favor universal newborn screening for Rett syndrome. However, the high cost and the lack of an effective presymptomatic treatment make universal newborn screening for Rett syndrome impractical at present. In contrast, prenatal diagnosis should be offered to the parents of an affected child if the responsible mutation has been identified in the index case.

X chromosome inactivation patterns in brain in Rett syndrome: implications for the disease phenotype

Skewed X chromosome inactivation (XCI) has been implicated in modulating the severity of Rett syndrome (RTT), although studies by different groups have yielded conflicting results. In this study we have characterised the XCI pattern in various neuroanatomical regions of nine RTT brains and non-neural tissue in two of these patients to determine whether or not variable XCI patterns occur in different brain regions or somatic tissues of the same patient. The mean XCI patterns for frontal and occipital cortex were compared between RTT and control subjects, and showed no significant differences when comparing RTT frontal to control frontal cortex or RTT occipital to control occipital cortex. However, one RTT subject displayed variability across the different neuroanatomical regions of the brain and skewing in some non-neural tissues. This observation adds another dimension to the epigenetic factors that may contribute to the phenotype in RTT. It also mandates that caution should be exercised in factoring XCI, including assumptions based on the blood XCI pattern, into the development of phenotype-genotype correlations.

Reduced proportion of Purkinje cells expressing paternally derived mutant Mecp2308 allele in female mouse cerebellum is not due to a skewed primary pattern of X-chromosome inactivation

Rett syndrome (RTT) is an X-linked disorder caused by mutations in the methyl CpG binding protein 2 (MECP2) gene. The pattern of X-chromosome inactivation (XCI) is thought to play a role in phenotypic severity. In the present study, patterns of XCI were assessed by lacZ staining of embryos and adult brains of mice heterozygous for a X-linked Hmgcr-nls-lacZ transgene on a mutant mouse model of RTT. We found that there was no difference between the lacZ staining patterns in the brain of wild-type and heterozygous mutant embryos at embryonic day 9.5 (E9.5) suggesting that Mecp2 has no effect on the primary pattern of XCI. At 20 weeks of age, there was no significant difference between XCI patterns in the Purkinje cells in the cerebellum of heterozygous mutant and wild-type mice when the mutant allele was inherited from the mother. However, when the mutant allele was paternally inherited, a significant difference was detected. Thus, parental origin of the mutation may have a bearing on phenotype through XCI patterns. An estimation of the Purkinje cell precursor number based on XCI mosaicism revealed that, when the mutation was paternally inherited, the precursor number was less than that in the wild-type mice. Therefore, it is likely that the number of precursor cells allocated to the Purkinje cell lineage is affected by a paternally inherited mutation in Mecp2. We also observed that the pattern of XCI in cultured fibroblasts was significantly correlated with patterns in the Purkinje cells in mutant animals but not in wild-type mice.

Germline mosaicism in Rett syndrome identified by prenatal diagnosis

Rett syndrome is an X-linked neurodevelopmental dominant disorder that affects almost exclusively girls. The vast majority of cases are sporadic and are caused by de novo mutations in the MECP2 gene, located in Xq28. Only few familial cases have been reported: in four cases, the mother was an asymptomatic carrier and in other four cases, the germline mosaicism in the mother was postulated. Owing to the above reported cases of germline mosaicism, we decided to offer prenatal diagnosis to all expectant mothers with a Rett daughter despite the absence of the causative mutation in parents' blood. We describe here the outcome of the first nine cases of prenatal diagnosis followed by our center. In eight cases, the fetus did not carry the mutation. In one case, the female fetus did carry the same mutation of the affected sister. The couple decided to interrupt the pregnancy and to devolve fetal tissues for research purposes. Our results indicate that prenatal diagnosis should be proposed to all couples with a Rett daughter, even when the mutation is apparently de novo. Moreover, one positive prenatal test among the first nine cases indicates that germline mosaicism may be seriously considered for the assessment of recurrence risk during genetic counseling.

Mutations of CDKL5 cause a severe neurodevelopmental disorder with infantile spasms and mental retardation

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused, in most classic cases, by mutations in the X-linked methyl-CpG-binding protein 2 gene (MECP2). A large degree of phenotypic variation has been observed in patients with RTT, both those with and without MECP2 mutations. We describe a family consisting of a proband with a phenotype that showed considerable overlap with that of RTT, her identical twin sister with autistic disorder and mild-to-moderate intellectual disability, and a brother with profound intellectual disability and seizures. No pathogenic MECP2 mutations were found in this family, and the Xq28 region that contains the MECP2 gene was not shared by the affected siblings. Three other candidate regions were identified by microsatellite mapping, including 10.3 Mb at Xp22.31-pter between Xpter and DXS1135, 19.7 Mb at Xp22.12-p22.11 between DXS1135 and DXS1214, and 16.4 Mb at Xq21.33 between DXS1196 and DXS1191. The ARX and CDKL5 genes, both of which are located within the Xp22 region, were sequenced in the affected family members, and a deletion of nucleotide 183 of the coding sequence (c.183delT) was identified in CDKL5 in the affected family members. In a screen of 44 RTT cases, a single splice-site mutation, IVS13-1G-->A, was identified in a girl with a severe phenotype overlapping RTT. In the mouse brain, Cdkl5 expression overlaps--but is not identical to--that of Mecp2, and its expression is unaffected by the loss of Mecp2. These findings confirm CDKL5 as another locus associated with epilepsy and X-linked mental retardation. These results also suggest that mutations in CDKL5 can lead to a clinical phenotype that overlaps RTT. However, it remains to be determined whether CDKL5 mutations are more prevalent in specific clinical subgroups of RTT or in other clinical presentations.

Clinical variability in Rett syndrome

The clinical variability of Rett syndrome, associated with mutations in the MECP2 gene, varies from classically symptomatic female patients to asymptomatic female patients, and male patients who have none of the diagnostic features considered pathognomonic of this disease. Multiple factors contribute to this variability. In our studies, mutations closer to the amino-terminus, prior to amino acid 255, led to severe clinical manifestations, such as inability to walk, severe dysphagia, and urinary organic acid abnormalities, compared with mutations toward the carboxyl-terminus. However, we found no correlation between severity and mutation type (missense versus nonsense). Despite the importance of mutation location to clinical severity, the widely varying severity within the same mutation suggests that in females, X-chromosome inactivation or other epigenetic phenomena also have roles in determining severity. We propose that stages 1 and 2 of the disease are a consequence of failed, time-linked, postnatal expression of MeCP2 in cerebellar neurons. This, in association with glutamate N-methyl-D-aspartate receptor-mediated neuroexcitotoxic injury to the differentiating neurons, results in the transient age-specific autistic-like behavior, motor, and cognitive dysfunction associated with these stages.

Mutations and polymorphisms in the human methyl CpG-binding protein MECP2

Rett syndrome (RTT or RS) is a neurodevelopmental disorder and one of the most frequent genetic diseases in girls. Mutations of the MECP2 gene have been found in a variety of different RTT phenotypes. The MECP2 gene (Xq28) has been described in 1992. Up to now, 218 different mutations have been reported in a total group, of more than 2,100 patients. Mutations in the MECP2 gene are responsible for up to 75% of the classical RTT cases. The mutations, are distributed along the whole gene and are comprised of all types of mutations. Several polymorphisms and benign genetic variants have also been described. Apart from spared reported familial cases, almost all cases are sporadic. RTT syndrome has been considered to be a lethal trait in males. Studying the parental origin of the mutations, however, we and others have found a very high prevalence of de novo mutations on the paternal chromosome. In this work we summarize the mutational reports published until now. One of our aims was to check the mutations' descriptions for consistency and particularly to rename them according to the recommended mutation nomenclature. The increasing number of investigations on the functions of the MeCP2 can help to gain more information about the neuropathogenetic mechanisms causing RTT. Hum Mutat 22:107-115, 2003.

Rett syndrome: the complex nature of a monogenic disease

Rett syndrome (RTT) is a severe neurodevelopmental disorder affecting almost exclusively girls. It is currently considered a monogenic X-linked dominant disorder due to mutations in MECP2 gene, encoding the methyl-CpG binding protein 2. A few RTT male cases, resulting from mosaicism for MECP2 mutations, have been reported. Male germline MECP2 mutations cause either severe encephalopathy with death at birth (usually in brothers of classical RTT females) or X-linked recessive mental retardation (XLMR). To date the wide phenotypic heterogeneity associated with MECP2 mutations in females (from classical RTT to healthy carriers) has been explained by differences in X chromosome inactivation. However, conflicting results have been obtained in different studies, with both random and highly skewed X-inactivation reported in healthy carrier females. Consequently it is possible that mechanisms other than X-inactivation play a role in the expressivity of MECP2 mutations. To explain the phenotypic heterogeneity associated with MECP2 mutations we propose a digenic model in which the presence of a "mutated" allele in a second gene, leading to a less functional protein, determines the clinical severity of the MECP2 mutation. The model is supported by the identification of the same mutation in XLMR and RTT cases. The carrier mothers of XLMR families are clinically asymptomatic and present balanced X chromosome inactivation. Therefore the same mutation arising in different genetic backgrounds can cause XLMR in males, remain silent in the carrier females and cause classic RTT in females. MECP2 mutations account for approximately 70-80% of classic RTT cases. MECP2 negative cases might result from mutations in noncoding regions of MECP2 gene. Alternatively, these cases might be due to mutations in other genes (locus heterogeneity). This hypothesis is supported by the identification of several chromosomal rearrangements in MECP2 negative patients with RTT and RTT-like phenotypes. MeCP2 is considered a general transcriptional repressor. However, conditional mouse mutants with selective loss of Mecp2 in the brain develop clinical manifestations similar to RTT, indicating that MECP2 is exclusively required for central nervous system function. The involvement of MeCP2 in methylation-specific transcriptional repression suggests that MECP2 related disorders result from dysregulated gene expression. Studies on gene expression have been performed in mouse and human brains. A relatively small number of gene expression changes were identified. It is possible that MeCP2 causes dysregulation of a very small subset of genes that are not detected with this method of analysis, or that very subtle changes in many genes cause the neuronal phenotype.

Genetic basis of Rett syndrome

The origin of Rett syndrome has long been debated, but several observations have suggested an X-linked dominant inheritance pattern. We and others have pursued an exclusion-mapping strategy using DNA from a small number of familial Rett syndrome cases. This work resulted in the narrowing of the region likely to harbor the mutated gene to Xq27.3-Xqter. After systematic exclusion of several candidate genes, we discovered mutations in MECP2, the gene that encodes the transcriptional repressor, methyl-CpG-binding protein 2. Since then, nonsense, missense, or frameshift mutations have been found in at least 80% of girls affected with classic Rett syndrome. Sixty-four percent of mutations are recurrent C > T transitions at eight CpG dinucleotides mutation hotspots, while the C-terminal region of the gene is prone to recurrent multinucleotide deletions (11%). Most mutations are predicted to result in total or partial loss of function of MeCP2. There is no clear correlation between the type and position of the mutation and the phenotypic features of classic and variant Rett syndrome patients, and XCI appears to be a major determinant of phenotypic severity. Further research focuses on the pathogenic consequences of these mutations along the hypothesis of loss of transcriptional repression of a small number of genes that are essential for neuronal function in the maturing brain.

Mutation analysis in Rett syndrome

Rett syndrome is an X-linked dominant neurodevelopmental disorder caused by mutations in the MECP2 gene. Mutations have been demonstrated in more than 80% of females with typical features of Rett syndrome. We identified mutations in the MECP2 gene and documented the clinical manifestations in 65 Rett syndrome patients to characterize the genotype-phenotype spectrum. Bidirectional sequencing of the entire MECP2 coding region was performed. We diagnosed 65 patients with MECP2 mutations. Of these, 15 mutations had been reported previously and 13 are novel. Two patients have multiple deletions within the MECP2 gene. Eight common mutations were found in 43 of 65 patients (66.15%). The majority of patients with identified mutations have the classic Rett phenotype, and several had atypical phenotypes. MECP2 analysis identified mutations in almost all cases of typical Rett syndrome, as well as in some with atypical phenotypes. Eleven (20.4%) of the 54 patients with defined mutations and in whom phenotypic data were obtained did not develop acquired microcephaly. Hence, microcephaly at birth or absence of acquired microcephaly does not obviate the need for MECP2 analysis. We have initiated cascade testing starting with PCR analysis for common mutations followed by sequencing, when necessary. Analysis of common mutations before sequencing the entire gene is anticipated to be the most efficacious strategy to identify Rett syndrome gene mutations.

A locus for bilateral perisylvian polymicrogyria maps to Xq28

Polymicrogyria (PMG) is one of a large group of human cortical malformations that collectively account for a significant percentage of patients with epilepsy, congenital neurological deficits, and intellectual disability. PMG is characterized by an excess of small gyri and abnormal cortical lamination. The most common distribution is bilateral, symmetrical, and maximal, in the region surrounding the sylvian fissures, and is known as "bilateral perisylvian polymicrogyria" (BPP). Most cases are sporadic, although several families have been observed with multiple affected members, usually following an X-linked inheritance pattern. Here we report the first genetic locus for BPP mapped by linkage analysis in five families. Linkage places the critical region for BPP at Xq28 (LOD score 3.08 in Xq28, distal to DXS8103 by multipoint analysis). We suggest that this region contains a gene that is necessary for correct neuronal organization and that the identification of this gene will both enhance our understanding of normal cortical development and accelerate the identification of other genes responsible for PMG.

Homozygous defects in LMNA, encoding lamin A/C nuclear-envelope proteins, cause autosomal recessive axonal neuropathy in human (Charcot-Marie-Tooth disorder type 2) and mouse

The Charcot-Marie-Tooth (CMT) disorders comprise a group of clinically and genetically heterogeneous hereditary motor and sensory neuropathies, which are mainly characterized by muscle weakness and wasting, foot deformities, and electrophysiological, as well as histological, changes. A subtype, CMT2, is defined by a slight or absent reduction of nerve-conduction velocities together with the loss of large myelinated fibers and axonal degeneration. CMT2 phenotypes are also characterized by a large genetic heterogeneity, although only two genes---NF-L and KIF1Bbeta---have been identified to date. Homozygosity mapping in inbred Algerian families with autosomal recessive CMT2 (AR-CMT2) provided evidence of linkage to chromosome 1q21.2-q21.3 in two families (Zmax=4.14). All patients shared a common homozygous ancestral haplotype that was suggestive of a founder mutation as the cause of the phenotype. A unique homozygous mutation in LMNA (which encodes lamin A/C, a component of the nuclear envelope) was identified in all affected members and in additional patients with CMT2 from a third, unrelated family. Ultrastructural exploration of sciatic nerves of LMNA null (i.e., -/-) mice was performed and revealed a strong reduction of axon density, axonal enlargement, and the presence of nonmyelinated axons, all of which were highly similar to the phenotypes of human peripheral axonopathies. The finding of site-specific amino acid substitutions in limb-girdle muscular dystrophy type 1B, autosomal dominant Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy type 1A, autosomal dominant partial lipodystrophy, and, now, AR-CMT2 suggests the existence of distinct functional domains in lamin A/C that are essential for the maintenance and integrity of different cell lineages. To our knowledge, this report constitutes the first evidence of the recessive inheritance of a mutation that causes CMT2; additionally, we suggest that mutations in LMNA may also be the cause of the genetically overlapping disorder CMT2B1.

Homozygous defects in LMNA, encoding lamin A/C nuclear-envelope proteins, cause autosomal recessive axonal neuropathy in human (Charcot-Marie-Tooth disorder type 2) and mouse

The Charcot-Marie-Tooth (CMT) disorders comprise a group of clinically and genetically heterogeneous hereditary motor and sensory neuropathies, which are mainly characterized by muscle weakness and wasting, foot deformities, and electrophysiological, as well as histological, changes. A subtype, CMT2, is defined by a slight or absent reduction of nerve-conduction velocities together with the loss of large myelinated fibers and axonal degeneration. CMT2 phenotypes are also characterized by a large genetic heterogeneity, although only two genes---NF-L and KIF1Bbeta---have been identified to date. Homozygosity mapping in inbred Algerian families with autosomal recessive CMT2 (AR-CMT2) provided evidence of linkage to chromosome 1q21.2-q21.3 in two families (Zmax=4.14). All patients shared a common homozygous ancestral haplotype that was suggestive of a founder mutation as the cause of the phenotype. A unique homozygous mutation in LMNA (which encodes lamin A/C, a component of the nuclear envelope) was identified in all affected members and in additional patients with CMT2 from a third, unrelated family. Ultrastructural exploration of sciatic nerves of LMNA null (i.e., -/-) mice was performed and revealed a strong reduction of axon density, axonal enlargement, and the presence of nonmyelinated axons, all of which were highly similar to the phenotypes of human peripheral axonopathies. The finding of site-specific amino acid substitutions in limb-girdle muscular dystrophy type 1B, autosomal dominant Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy type 1A, autosomal dominant partial lipodystrophy, and, now, AR-CMT2 suggests the existence of distinct functional domains in lamin A/C that are essential for the maintenance and integrity of different cell lineages. To our knowledge, this report constitutes the first evidence of the recessive inheritance of a mutation that causes CMT2; additionally, we suggest that mutations in LMNA may also be the cause of the genetically overlapping disorder CMT2B1.