Hereditary motor and sensory neuropathy--Lom (HMSNL): refined genetic mapping in Romani (Gypsy) families from several European countries

Identification and Characterization of Novel Founder Mutations in NDRG1: Refining the Genetic Landscape of Charcot-Marie-Tooth Disease Type 4D in Bulgaria

Charcot-Marie-Tooth neuropathy type 4D (CMT4D) is a rare genetic disorder of the peripheral nervous system caused by biallelic mutations in the N-Myc Downstream Regulated 1 gene (NDRG1). Patients present with an early onset demyelinating peripheral neuropathy causing severe distal muscle weakness and sensory loss, leading to loss of ambulation and progressive sensorineural hearing loss. The disorder was initially described in the Roma community due to a common founder mutation, and only a handful of disease-causing variants have been described in this gene so far. Here, we present genetic and clinical findings from a large Bulgarian cohort of demyelinating CMT patients harboring recurrent and novel variants in the NDRG1 gene. Notably, two splice-site variants are exclusive to Bulgarian Muslims and reside in ancestral haplotypes, suggesting a founder effect. Functional characterization of these novel variants implicates a loss-of-function mechanism due to shorter gene products. Our findings contribute to a deeper understanding of the genetic and clinical heterogeneity of CMT4D and highlight novel founder mutations in the ethnic minority of Bulgarian Muslims.

HMSN Lom in 12 Czech patients, with one unusual case due to uniparental isodisomy of chromosome 8

Hereditary motor and sensory neuropathy-type Lom (HMSNL), also known as CMT4D, a demyelinating neuropathy with late-onset deafness is an autosomal recessive disorder threatening Roma population worldwide. The clinical phenotype was reported in several case reports before the gene discovery. HMSNL is caused by a homozygous founder mutation p.Arg148* in the N-Myc downstream-regulated gene 1. Here, we report findings from the Czech Republic, where HMSNL was found in 12 Czech patients from eight families. In these 12 patients, 11 of the causes were due to p.Arg148* mutation inherited from both parents by the autosomal recessive mechanism. But in one case, the recessive mutation was inherited only from one parent (father) and unmasked owing to an uniparental isodisomy of the entire chromosome eight. The inherited peripheral neuropathy owing to an isodisomy of the whole chromosome pointed to an interesting, less frequent possibility of recessive disease and complications with genetic counseling.

Neuropathology of Charcot-Marie-Tooth and related disorders

The peripheral nervous system (PNS), with all its branches and connections, is so complex that it is impossible to study all components at the light or electron microscopic level in any individual case; nevertheless, in certain diseases a simple nerve biopsy may suffice to arrive at a precise diagnosis. Structural changes of the PNS in neuropathies of the Charcot-Marie-Tooth (CMT) type and related disorders comprise various components of the PNS. These include peripheral motor, sensory, and autonomous neurons with their axons, Schwann cells, and myelin sheaths in the radicular and peripheral nerves as well as satellite cells in spinal and autonomous ganglia. Astrocytes, oligodendroglial cells, and microglial cells around motor neurons in the anterior horn and around sensory neurons in other areas of the spinal cord are also involved. In addition, connective tissue elements such as endoneurial, perineurial, and epineurial components including blood and lymph vessels play an important role. This review focuses on the cellular components and organelles involved, that is, myelin sheaths, axons with their micro-tubules and neurofilaments; nuclei, mitochondria, endoplasmic reticulum, and connective tissue including the perineurium and blood vessels. A major role is attributed to recent progress in the pathomorphology of various types of CMT1, 2,4, CMTX, and HMNSL, based on light and electron microscopic findings, morphometry, teased fiber studies, and new immunohisto-chemical results such as staining of certain periaxin domains in CMT4F.

A newly discovered founder population: the Roma/Gypsies

The Gypsies (a misnomer, derived from an early legend about Egyptian origins) defy the conventional definition of a population: they have no nation-state, speak different languages, belong to many religions and comprise a mosaic of socially and culturally divergent groups separated by strict rules of endogamy. Referred to as "the invisible minority", the Gypsies have for centuries been ignored by Western medicine, and their genetic heritage has only recently attracted attention. Common origins from a small group of ancestors characterise the 8-10 million European Gypsies as an unusual trans-national founder population, whose exodus from India played the role of a profound demographic bottleneck. Social and economic pressures within Europe led to gradual fragmentation, generating multiple genetically differentiated subisolates. The string of population bottlenecks and founder effects have shaped a unique genetic profile, whose potential for genetic research can be met only by study designs that acknowledge cultural tradition and self-identity.

Vlax Roma history: what do coalescent-based methods tell us?

Three coalescent-based methods allowed us to infer some aspects of the history of three Bulgarian Gypsies populations belonging to the Vlax linguistic group: the Lom, Rudari and Kalderas. We used several kinds of genetic markers: HV1 sequences of the maternally inherited mitochondrial genome and microsatellites of the paternally inherited Y chromosome and of the biparentally inherited chromosome 8. This allowed us to infer several parameters for men and women: the splitting order of the populations and the ages of the splitting events, the growth rate in each population and the migration rates between populations. Altogether, they enabled us to infer a demographic scenario that could explain the genetic diversity of Vlax Roma: recent splits occurring after the arrival in Europe, asymmetric migration flows especially for males and unequal growth rates. This represents a considerable contribution to the Vlax Roma history in comparison with the inferences from classical population genetics.

Mutation history of the roma/gypsies

The 8-10 million European Roma/Gypsies are a founder population of common origins that has subsequently split into multiple socially divergent and geographically dispersed Gypsy groups. Unlike other founder populations, whose genealogy has been extensively documented, the demographic history of the Gypsies is not fully understood and, given the lack of written records, has to be inferred from current genetic data. In this study, we have used five disease loci harboring private Gypsy mutations to examine some missing historical parameters and current structure. We analyzed the frequency distribution of the five mutations in 832-1,363 unrelated controls, representing 14 Gypsy populations, and the diversification of chromosomal haplotypes in 501 members of affected families. Sharing of mutations and high carrier rates supported a strong founder effect, and the identity of the congenital myasthenia 1267delG mutation in Gypsy and Indian/Pakistani chromosomes provided the best evidence yet of the Indian origins of the Gypsies. However, dramatic differences in mutation frequencies and haplotype divergence and very limited haplotype sharing pointed to strong internal differentiation and characterized the Gypsies as a founder population comprising multiple subisolates. Using disease haplotype coalescence times at the different loci, we estimated that the entire Gypsy population was founded approximately 32-40 generations ago, with secondary and tertiary founder events occurring approximately 16-25 generations ago. The existence of multiple subisolates, with endogamy maintained to the present day, suggests a general approach to complex disorders in which initial gene mapping could be performed in large families from a single Gypsy group, whereas fine mapping would rely on the informed sampling of the divergent subisolates and searching for the shared genomic region that displays the strongest linkage disequilibrium with the disease.

Refined mapping of the HMSNR critical gene region--construction of a high-density integrated genetic and physical map

Hereditary motor and sensory neuropathy russe, a form of autosomal recessive Charcot-Marie-Tooth disease, is a rare disorder found in several Roma families from Europe. The gene has been mapped to a 1Mb region on 10q22. Detailed analysis led to the exclusion of 22 candidate genes and the assembly of a high-density genetic map comprising 141 polymorphic markers. Extensive genotyping in an extended sample of affected families resulted in a 10-fold reduction of the critical hereditary motor and sensory neuropathy russe gene region, which is now contained within a single completely sequenced BAC clone. The fact that no sequence variant has been detected in the known genes in the critical region indicates that the hereditary motor and sensory neuropathy russe mutation affects a novel gene that remains to be identified.

Mutation screening of the N-myc downstream-regulated gene 1 (NDRG1) in patients with Charcot-Marie-Tooth Disease

In a previous study, we have shown that N-myc downstream-regulated gene 1 (NDRG1), classified in databases as a tumor suppressor and heavy metal-response protein, is mutated in hereditary motor and sensory neuropathy Lom (HMSNL), a severe autosomal recessive form of Charcot-Marie-Tooth (CMT) disease. The private founder mutation R148X, causing HMSNL in patients of Romani ethnicity, has so far remained the only molecular defect linking NDRG1 to a specific disease phenotype. Here we report the first study aiming to assess the overall contribution of this gene to the pathogenesis of peripheral neuropathies, in cases where the most common causes of CMT disease have been excluded. Sequence analysis of NDRG1 in 104 CMT patients of diverse ethnicity identified one novel disease-causing mutation, IVS8-1G>A (g.2290787G>A), which affects the splice-acceptor site of IVS8 and results in the skipping of exon 9. The phenotype of the IVS8-1G>A homozygote was very closely related to that of HMSNL patients. In addition, we have detected homozygosity for the known R148X mutation in two affected individuals. Mutations in NDRG1 thus accounted for 2.88% of our overall group of patients, and for 4.68% of cases with demyelinating neuropathies. No other variants were identified in the coding sequence, whereas 12 single nucleotide polymorphisms were observed in the introns. Hum Mutat 22:129-135, 2003.

Dejerine-Sottas disease and hereditary demyelinating polyneuropathy of infancy

Dejerine-Sottas disease (DSD) was originally described as a hypertrophic polyneuropathy characterized by onset in infancy or early childhood in patients born to unaffected parents. The clinical features included distal sensory changes with ataxia; pes cavus, at times with kyphoscoliosis; motor deficit and atrophy predominating in the distal lower limbs and progressing toward the proximal limbs following a length-dependent pattern; palpable nerve hypertrophy; and Argyll-Robertson pupils. The morphological hallmark was the extensive nerve and root hypertrophy associated with demyelination-remyelination of surviving, originally myelinated axons and profuse Schwann-cell proliferation forming onion bulbs. Wide variations in clinical manifestations of chronic demyelinating polyneuropathies of early onset in children born to unaffected parents have now been reported, with only some of the characteristics required in the original study, and at least seven genes encoding the myelin proteins P0, PMP22, the transcriptional factor EGR2, and others have been implicated. Thus, DSD is now a component of the hereditary demyelinating polyneuropathies of infancy that also include subsets of the recently individualized CMT4 neuropathies. The presumed recessive transmission of patients with DSD should be confirmed by molecular genetic analysis, which is still negative in a significant proportion of patients. The nerve biopsy can be useful in patients in whom genealogical or DNA abnormalities in favor of a genetic disorder are missing, because in a few patients with a progressive or relapsing course the diagnosis of early-onset chronic inflammatory demyelinating polyneuropathy must be considered.

Electrocochleography in auditory neuropathy

Auditory neuropathy (AN) is a disorder characterized by the absence or the severe impairment of the auditory brainstem responses (ABRs) together with the preservation of otoacoustic emissions and/or cochlear microphonic (CM). We recorded transtympanic electrocochleography (ECohG) evoked by 0.1 ms clicks in one young adult and in four children having distortion product otoacoustic emissions and absent ABRs. In all but one patient CM and summating potential (SP) were present with normal threshold, and their amplitudes appeared comparable to or higher than the values obtained from subjects with normal hearing. The compound action potential (CAP) was absent in two patients while in one subject CM and SP were followed by a highly desynchronized neural activity. A broad CAP was found in two children and the threshold appeared clearly elevated in one of them, while it showed only a mild elevation in the other. No correlation was found between CAP and behavioral thresholds. These results suggest that ECohG can be useful in AN diagnoses since it is the only reliable tool in evaluating the auditory peripheral function in the presence of a desynchronized ABR.

Genetic studies of the Roma (Gypsies): a review

BACKGROUND

Data provided by the social sciences as well as genetic research suggest that the 8-10 million Roma (Gypsies) who live in Europe today are best described as a conglomerate of genetically isolated founder populations. The relationship between the traditional social structure observed by the Roma, where the Group is the primary unit, and the boundaries, demographic history and biological relatedness of the diverse founder populations appears complex and has not been addressed by population genetic studies.

RESULTS

Recent medical genetic research has identified a number of novel, or previously known but rare conditions, caused by private founder mutations. A summary of the findings, provided in this review, should assist diagnosis and counselling in affected families, and promote future collaborative research. The available incomplete epidemiological data suggest a non-random distribution of disease-causing mutations among Romani groups.

CONCLUSION

Although far from systematic, the published information indicates that medical genetics has an important role to play in improving the health of this underprivileged and forgotten people of Europe. Reported carrier rates for some Mendelian disorders are in the range of 5-15%, sufficient to justify newborn screening and early treatment, or community-based education and carrier testing programs for disorders where no therapy is currently available. To be most productive, future studies of the epidemiology of single gene disorders should take social organisation and cultural anthropology into consideration, thus allowing the targeting of public health programs and contributing to the understanding of population structure and demographic history of the Roma.

Hereditary motor and sensory neuropathy-Lom (HMSNL) in a Spanish family: clinical, electrophysiological, pathological and genetic studies

The clinical, electrophysiological, pathological and genetic findings are described in the first Spanish family diagnosed with hereditary motor and sensory neuropathy type Lom (HMSNL) initially identified by Kalaydjeva et al. in 1996. The three affected patients belong to a non-consanguineous family with Gypsy background that were followed up over 10 years. Serial clinical and neurophysiological examinations and genetic analysis were undertaken in every patient. Sural nerve biopsy was performed in the oldest patient. The clinical features are similar to those previously described in HMSNL and all of them showed abnormal brain auditory evoked potentials. The oldest brother developed sensorineural deafness at the age of 20. Conduction velocities were unobtainable in all patients and nerves tested except for the median nerve in the youngest child in whom conduction was severely slowed. Neuropathological examination revealed a severely depleted nerve with very few surviving myelinated fibers which possessed thin myelin sheaths. Schwann cell processes were arranged in circular configurations without typical onion bulb configuration. Genetic analysis showed that the maternal chromosome inherited by all three affected siblings displayed a very unusual haplotype. Our patients show the characteristic clinical, electrophysiological and pathological findings described in HMSNL and represent the first reported Spanish family affected from the disease. The genetic findings in this family have contributed to refine the HMSNL critical linkage region.

N-myc downstream-regulated gene 1 is mutated in hereditary motor and sensory neuropathy-Lom

Hereditary motor and sensory neuropathies, to which Charcot-Marie-Tooth (CMT) disease belongs, are a common cause of disability in adulthood. Growing awareness that axonal loss, rather than demyelination per se, is responsible for the neurological deficit in demyelinating CMT disease has focused research on the mechanisms of early development, cell differentiation, and cell-cell interactions in the peripheral nervous system. Autosomal recessive peripheral neuropathies are relatively rare but are clinically more severe than autosomal dominant forms of CMT, and understanding their molecular basis may provide a new perspective on these mechanisms. Here we report the identification of the gene responsible for hereditary motor and sensory neuropathy-Lom (HMSNL). HMSNL shows features of Schwann-cell dysfunction and a concomitant early axonal involvement, suggesting that impaired axon-glia interactions play a major role in its pathogenesis. The gene was previously mapped to 8q24.3, where conserved disease haplotypes suggested genetic homogeneity and a single founder mutation. We have reduced the HMSNL interval to 200 kb and have characterized it by means of large-scale genomic sequencing. Sequence analysis of two genes located in the critical region identified the founder HMSNL mutation: a premature-termination codon at position 148 of the N-myc downstream-regulated gene 1 (NDRG1). NDRG1 is ubiquitously expressed and has been proposed to play a role in growth arrest and cell differentiation, possibly as a signaling protein shuttling between the cytoplasm and the nucleus. We have studied expression in peripheral nerve and have detected particularly high levels in the Schwann cell. Taken together, these findings point to NDRG1 having a role in the peripheral nervous system, possibly in the Schwann-cell signaling necessary for axonal survival.