A novel deletion in the GJA12 gene causes Pelizaeus-Merzbacher-like disease

Mechanisms of Diseases Associated with Mutation in GJC2/Connexin 47

Connexins are members of a family of integral membrane proteins that provide a pathway for both electrical and metabolic coupling between cells. Astroglia express connexin 30 (Cx30)-GJB6 and Cx43-GJA1, while oligodendroglia express Cx29/Cx31.3-GJC3, Cx32-GJB1, and Cx47-GJC2. Connexins organize into hexameric hemichannels (homomeric if all subunits are identical or heteromeric if one or more differs). Hemichannels from one cell then form cell-cell channels with a hemichannel from an apposed cell. (These are termed homotypic if the hemichannels are identical and heterotypic if the hemichannels differ). Oligodendrocytes couple to each other through Cx32/Cx32 or Cx47/Cx47 homotypic channels and they couple to astrocytes via Cx32/Cx30 or Cx47/Cx43 heterotypic channels. Astrocytes couple via Cx30/Cx30 and Cx43/Cx43 homotypic channels. Though Cx32 and Cx47 may be expressed in the same cells, all available data suggest that Cx32 and Cx47 cannot interact heteromerically. Animal models wherein one or in some cases two different CNS glial connexins have been deleted have helped to clarify the role of these molecules in CNS function. Mutations in a number of different CNS glial connexin genes cause human disease. Mutations in GJC2 lead to three distinct phenotypes, Pelizaeus Merzbacher like disease, hereditary spastic paraparesis (SPG44) and subclinical leukodystrophy.

Identification of GJC2 gene mutations in Chinese patients with Pelizaeus-Merzbacher-like disease

BACKGROUND

Clinical and genetic features were analyzed in five pedigrees with Pelizaeus-Merzbacher-like disease (PMLD) to provide bases for genetic counseling and prenatal diagnosis.

CONCLUSIONS

Six patients from five pedigrees were diagnosed with PMLD based on their clinical data. Six GJC2 novel mutations were found in this study, expanding the spectrum of GJC2 mutations. This is the second group of GJC2 mutations reported from six Chinese patients with PMLD.

METHODS

Clinical data including medical history, physical signs, and auxiliary examinations were collected from six patients and their family numbers in five pedigrees with PMLD. Polymerase chain reaction and sequence analysis were used to amplify GJC2 and PLP1 alterations, while multiplex ligation-dependent probe amplification (MLPA) was performed to detect PLP1 dosage changes. The gene mutations were diagnosed for further analysis of the genetic features.

RESULTS

A total of seven GJC2 mutations were identified in these patients, including two novel missense mutations (c.217C>T, p.Pro73Ser; c.1199C>A, p.Ala400Glu), one nonsense mutation (c.735C>A, p.Cys245X), three novel frameshift mutations (c.579delC, p.Gly193fsX17 and c.1296_1297insG, p.Gly433fsX59; c.689delG, p.Gly230AlafsX241), and one known missense mutation (c.814T>G, p.Tyr272Asp). Compound heterozygotes were found for P1-3, while homozygotes were found for P4-6 that were inherited from their parents with normal phenotypes except for P5 and P6, respectively. The c.814T>G (p.Tyr272Asp) mutation in P5 was de novo. A c.1199C>A (p.Ala400Glu) homozygous mutation in GJC2 was identified in P6. A heterozygous variation was found in his father and the wild type was seen in his mother.

The role of oligodendrocyte gap junctions in neuroinflammation

Gap junctions (GJs) provide channels for direct cell-to-cell connectivity serving the homeostasis in several organs of vertebrates including the central (CNS) and peripheral (PNS) nervous systems. GJs are composed of connexins (Cx), which show a highly distinct cellular and subcellular expression pattern. Oligodendrocytes, the myelinating cells of the CNS, are characterized by extensive GJ connectivity with each other as well as with astrocytes. The main oligodendrocyte connexins forming these GJ channels are Cx47 and Cx32. The importance of these channels has been highlighted by the discovery of human diseases caused by mutations in oligodendrocyte connexins, manifesting with leukodystrophy or transient encephalopathy. Experimental models have provided further evidence that oligodendrocyte GJs are essential for CNS myelination and homeostasis, while a strong inflammatory component has been recognized in the absence of oligodendrocyte connexins. Further studies revealed that connexins are also disrupted in multiple sclerosis (MS) brain, and in experimental models of induced inflammatory demyelination. Moreover, induced demyelination was more severe and associated with higher degree of CNS inflammation in models with oligodendrocyte GJ deficiency, suggesting that disrupted connexin expression in oligodendrocytes is not only a consequence but can also drive a pro-inflammatory environment in acquired demyelinating disorders such as MS. In this review, we summarize the current insights from human disorders as well as from genetic and acquired models of demyelination related to oligodendrocyte connexins, with the remaining challenges and perspectives.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

Gene therapy targeting oligodendrocytes provides therapeutic benefit in a leukodystrophy model

Pelizaeus-Merzbacher-like disease or hypomyelinating leukodystrophy-2 is an autosomal recessively inherited leukodystrophy with childhood onset resulting from mutations in the gene encoding the gap junction protein connexin 47 (Cx47, encoded by GJC2). Cx47 is expressed specifically in oligodendrocytes and is crucial for gap junctional communication throughout the central nervous system. Previous studies confirmed that a cell autonomous loss-of-function mechanism underlies hypomyelinating leukodystrophy-2 and that transgenic oligodendrocyte-specific expression of another connexin, Cx32 (GJB1), can restore gap junctions in oligodendrocytes to achieve correction of the pathology in a disease model. To develop an oligodendrocyte-targeted gene therapy, we cloned the GJC2/Cx47 gene under the myelin basic protein promoter and used an adeno-associated viral vector (AAV.MBP.Cx47myc) to deliver the gene to postnatal Day 10 mice via a single intracerebral injection in the internal capsule area. Lasting Cx47 expression specifically in oligodendrocytes was detected in Cx47 single knockout and Cx32/Cx47 double knockout mice up to 12 weeks post-injection, including the corpus callosum and the internal capsule but also in more distant areas of the cerebrum and in the spinal cord. Application of this oligodendrocyte-targeted somatic gene therapy at postnatal Day 10 in groups of double knockout mice, a well characterized model of hypomyelinating leukodystrophy-2, resulted in significant improvement in motor performance and coordination at 1 month of age in treated compared to mock-treated mice, as well as prolonged survival. Furthermore, immunofluorescence and morphological analysis revealed improvement in demyelination, oligodendrocyte apoptosis, inflammation, and astrogliosis, all typical features of this leukodystrophy model in both brain and spinal cord. Functional dye transfer analysis confirmed the re-establishment of oligodendrocyte gap junctional connectivity in treated as opposed to untreated mice. These results provide a significant advance in the development of oligodendrocyte-cell specific gene therapy. Adeno-associated viral vectors can be used to target therapeutic expression of a myelin gene to oligodendrocytes. We show evidence for the first somatic gene therapy approach to treat hypomyelinating leukodystrophy-2 preclinically, providing a potential treatment for this and similar forms of leukodystrophies.

GJC2 promoter mutations causing Pelizaeus-Merzbacher-like disease

OBJECTIVE

Pelizaeus-Merzbacher-like disease is a rare hypomyelinating leukodystrophy caused by autosomal recessive mutations in GJC2, encoding a gap junction protein essential for production of a mature myelin sheath. A previously identified GJC2 mutation (c.-167A>G) in the promoter region is hypothesized to disrupt a putative SOX10 binding site; however, the lack of additional mutations in this region and contradictory functional data have limited the interpretation of this variant.

METHODS

We describe two independent Pelizaeus-Merzbacher-like disease families with a novel promoter region mutation and updated in vitro functional assays.

RESULTS

A novel GJC2 mutation (c.-170A>G) in the promoter region was identified in Pelizaeus-Merzbacher-like disease patients. In vitro functional assays using human GJC2 promoter constructs demonstrated that this mutation and the previously described c.-167A>G mutation similarly diminished the transcriptional activity driven by SOX10 and the binding affinity for SOX10.

INTERPRETATION

These findings support the role of GJC2 promoter mutations in Pelizaeus-Merzbacher-like disease. GJC2 promoter region mutation screening should be included in the evaluation of patients with unexplained hypomyelinating leukodystrophies.

Pelizaeus-Merzbacher-Like Disease in a Family With Variable Phenotype and a Novel Splicing GJC2 Mutation

Pelizaeus-Merzbacher-like disease is an autosomal recessive disorder characterized by neonatal nystagmus, ataxia, progressive spasticity, and development delay and is rarely caused by GJC2 mutations. We report 7 patients from a large consanguineous family who had variable severity of Pelizaeus-Merzbacher-like disease. The 3 youngest of branch A were bedridden by their first year because of permanent scissoring of their legs and had severe frontal lobe epilepsy. The single patient from branch B was the least affected, being able to walk until 12 years of age and had no epilepsy. Brain magnetic resonance imaging (MRI) showed hypomyelination. The patients had a novel canonical splicing GJC2 c.-20+1G>C mutation with a predicted loss of the coding connexin 47 protein. The exceptionally large number of patients in this unique family enabled to describe the intrafamilial variability of Pelizaeus-Merzbacher-like disease. The predicted functional loss of connexin 47 might be associated with a severe form of Pelizaeus-Merzbacher-like disease.

Gap junctions in inherited human disorders of the central nervous system

CNS glia and neurons express connexins, the proteins that form gap junctions in vertebrates. We review the connexins expressed by oligodendrocytes and astrocytes, and discuss their proposed physiologic roles. Of the 21 members of the human connexin family, mutations in three are associated with significant central nervous system manifestations. For each, we review the phenotype and discuss possible mechanisms of disease. Mutations in GJB1, the gene for connexin 32 (Cx32) cause the second most common form of Charcot-Marie-Tooth disease (CMT1X). Though the only consistent phenotype in CMT1X patients is a peripheral demyelinating neuropathy, CNS signs and symptoms have been found in some patients. Recessive mutations in GJC2, the gene for Cx47, are one cause of Pelizaeus-Merzbacher-like disease (PMLD), which is characterized by nystagmus within the first 6 months of life, cerebellar ataxia by 4 years, and spasticity by 6 years of age. MRI imaging shows abnormal myelination. A different recessive GJC2 mutation causes a form of hereditary spastic paraparesis, which is a milder phenotype than PMLD. Dominant mutations in GJA1, the gene for Cx43, cause oculodentodigital dysplasia (ODDD), a pleitropic disorder characterized by oculo-facial abnormalities including micropthalmia, microcornia and hypoplastic nares, syndactyly of the fourth to fifth fingers and dental abnormalities. Neurologic manifestations, including spasticity and gait difficulties, are often but not universally seen. Recessive GJA1 mutations cause Hallermann-Streiff syndrome, a disorder showing substantial overlap with ODDD. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and functions.

Changes in object recognition and anxiety-like behaviour in mice expressing a Cx47 mutation that causes Pelizaeus-Merzbacher-like disease

Pelizaeus-Merzbacher-like disease is characterized by impaired psychomotor development, ataxia, progressive spasticity and mental retardation. It is induced by mutations in the gap junction gene GJC2 that encodes for the gap junction protein connexin 47. Mice bearing a human Cx47M283T missense mutation have been generated as a transgenic mouse model of Pelizaeus-Merzbacher-like disease. Homozygous expression of the mutant connexin 47 gene in oligodendrocytes resulted in a complex and variable neuropathologic phenotype, which was associated with impairments in motor coordination in juvenile, but not adult mice. In the present study, we have investigated anxiety-like behaviour and spatial working memory in juvenile (P23) and adult (3-month-old) Cx47M282T mutant mice. Adult Cx47M282T mice were also evaluated in terms of neuromotor functions and in the novel object recognition test. Juvenile Cx47M282T mutant mice exhibited an increase in anxiety-like behaviour in the open field test, but no changes in spatial working memory performance. No significant changes in anxiety-like behaviour, spatial working memory or neuromotor functions were observed in the adult Cx47M282T mutant mice. However, novel object recognition was significantly impaired in adult Cx47M282T mice. Our results suggest that the expression of the human Cx47M282T mutation in the mouse causes changes in anxiety-like behaviour in juvenile and novel object recognition impairments in adult mice. It appears that the distortion of panglial gap junction coupling in white and grey matter tissue in the Cx47M282T mice is associated with a complex age-dependent behavioural phenotype including changes in psychomotor, emotional and memory functions.

Expanded spectrum of Pelizaeus-Merzbacher-like disease: literature revision and description of a novel GJC2 mutation in an unusually severe form

Homozygous or compound heterozygous mutations in the GJC2 gene, encoding the gap junction protein connexin47 (Cx47), cause the autosomal recessive hypomyelinating Pelizaeus-Merzbacher-like disease (PMLD1, MIM# 608804). Although clinical and neuroradiological findings resemble those of the classic Pelizaeus-Merzbacher disease, PMLD patients usually show a greater level of cognitive and motor functions. Unpredictably a homozygous missense GJC2 mutation (p.Glu260Lys) was found in a patient presenting with a very severe clinical picture characterised by congenital nystagmus and severe neurological impairment. Also magnetic resonance imaging was unusually severe, showing an abnormal supra- and infratentorial white matter involvement extending to the spinal cord. The novel p.Glu260Lys (c.778G>A) mutation, occurring in a highly conserved motif (SRPTEK) of the Cx47 extracellular loop-2 domain, was predicted, by modelling analysis, to break a 'salt bridge network', crucial for a proper connexin-connexin interaction to form a connexon, thus hampering the correct formation of the connexon pore. The same structural analysis, extended to the previously reported missense mutations, predicted that most changes were expected to have less severe impact on protein functions, correlating with the mild PMLD1 form of the patients. Our study expands the spectrum of PMLD1 and provides evidence that the extremely severe clinical and neuroradiological PMLD1 form of our patient likely correlates with the predicted impairment of gap junction channel assembly resulting from the detrimental effect of the new p.Glu260Lys mutant allele on Cx47 protein.

Connexin 47 mutations increase risk for secondary lymphedema following breast cancer treatment

PURPOSE

Secondary lymphedema is a frequent complication of breast cancer associated with surgery, chemotherapy, or radiation following breast cancer treatment. The potential contribution of genetic susceptibility to risk of developing secondary lymphedema following surgical trauma, radiation, and other tissue insults has not been studied.

EXPERIMENTAL DESIGN

To determine whether women with breast cancer and secondary lymphedema had mutations in candidate lymphedema genes, we undertook a case-control study of 188 women diagnosed with breast cancer recruited from the University of Pittsburgh Breast Cancer Program (http://www.upmccancercenter.com/breast/index.cfm) between 2000 and 2010. Candidate lymphedema genes, GJC2 (encoding connexin 47 [Cx47]), FOXC2, HGF, MET, and FLT4 (encoding VEGFR3), were sequenced for mutation. Bioinformatics analysis and in vitro functional assays were used to confirm significance of novel mutations.

RESULTS

Cx47 mutations were identified in individuals having secondary lymphedema following breast cancer treatment but not in breast cancer controls or normal women without breast cancer. These novel mutations are dysfunctional as assessed through in vitro assays and bioinformatics analysis and provide evidence that altered gap junction function leads to lymphedema.

CONCLUSIONS

Our findings challenge the view that secondary lymphedema is solely due to mechanical trauma and support the hypothesis that genetic susceptibility is an important risk factor for secondary lymphedema. A priori recognition of genetic risk (i) raises the potential for early detection and intervention for a high-risk group and (ii) allows the possibility of altering surgical approach and/or chemo- and radiation therapy, or direct medical treatment of secondary lymphedema with novel connexin-modifying drugs.

High frequency of GJA12/GJC2 mutations in Turkish patients with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is an early onset dysmyelinating leukodystrophy. About 80% of PMD cases have been associated with duplications and mutations of the proteolipid protein 1 (PLP1) gene. Pelizaeus-Merzbacher-like disease is a genetically heterogeneous autosomal recessive disease and rarely caused by mutations in gap junction protein α12 (GJA12/GJC2) gene. The molecular basis of the disease was investigated in a cohort of 19 Turkish families. This study identified novel chromosomal rearrangements proximal and distal to, and exclusive of the PLP1 gene, showed equal frequencies of PLP1 and GJA12/GJC2 mutations at least in our cohort, and suggested further genetic heterogeneity.

Promoter mutation is a common variant in GJC2-associated Pelizaeus-Merzbacher-like disease

Pelizaeus-Merzbacher-like disease (PMLD) is a clinically and genetically heterogeneous neurological disorder of cerebral hypomyelination. It is clinically characterised by early onset (usually infantile) nystagmus, impaired motor development, ataxia, choreoathetoid movements, dysarthria and progressive limb spasticity. We undertook autozygosity mapping studies in a large consanguineous family of Pakistani origin in which affected children had progressive lower limb spasticity and features of cerebral hypomyelination on MR brain imaging. SNP microarray and microsatellite marker analysis demonstrated linkage to chromosome 1q42.13-1q42.2. Direct sequencing of the gap junction protein gamma-2 gene, GJC2, identified a promoter region mutation (c.-167A>G) in the non-coding exon 1. The c.-167A>G promoter mutation was identified in a further 4 individuals from two families (who were also of Pakistani origin) with clinical and radiological features of PMLD in whom previous routine diagnostic screening of GJC2 had been reported as negative. A common haplotype was identified at the GJC2 locus in the three mutation-positive families, consistent with a common origin for the mutation and likely founder effect. This promoter mutation has only recently been reported in GJC2-PMLD but it has been postulated to affect the binding of the transcription factor SOX10 and appears to be a prevalent mutation, accounting for ~29% of reported patients with GJC2-PMLD. We propose that diagnostic screening of GJC2 should include sequence analysis of the non-coding exon 1, as well as the coding regions to avoid misdiagnosis or diagnostic delay in suspected PMLD.

Pathologic and phenotypic alterations in a mouse expressing a connexin47 missense mutation that causes Pelizaeus-Merzbacher-like disease in humans

Gap junction channels are intercellular conduits that allow diffusional exchange of ions, second messengers, and metabolites. Human oligodendrocytes express the gap junction protein connexin47 (Cx47), which is encoded by the GJC2 gene. The autosomal recessive mutation hCx47M283T causes Pelizaeus-Merzbacher-like disease 1 (PMLD1), a progressive leukodystrophy characterized by hypomyelination, retarded motor development, nystagmus, and spasticity. We introduced the human missense mutation into the orthologous position of the mouse Gjc2 gene and inserted the mCx47M282T coding sequence into the mouse genome via homologous recombination in embryonic stem cells. Three-week-old homozygous Cx47M282T mice displayed impaired rotarod performance but unchanged open-field behavior. 10-15-day-old homozygous Cx47M282T and Cx47 null mice revealed a more than 80% reduction in the number of cells participating in glial networks after biocytin injections into oligodendrocytes in sections of corpus callosum. Homozygous expression of mCx47M282T resulted in reduced MBP expression and astrogliosis in the cerebellum of ten-day-old mice which could also be detected in Cx47 null mice of the same age. Three-month-old homozygous Cx47M282T mice exhibited neither altered open-field behavior nor impaired rotarod performance anymore. Adult mCx47M282T expressing mice did not show substantial myelin alterations, but homozygous Cx47M282T mice, additionally deprived of connexin32, which is also expressed in oligodendrocytes, died within six weeks after birth and displayed severe myelin defects accompanied by astrogliosis and activated microglia. These results strongly suggest that PMLD1 is caused by the loss of Cx47 channel function that results in impaired panglial coupling in white matter tissue.

Pelizaeus-Merzbacher-like disease caused by AIMP1/p43 homozygous mutation

Pelizaeus-Merzbacher disease is an X-linked hypomyelinating leukodystrophy caused by PLP1 mutations. A similar autosomal-recessive phenotype, Pelizaeus-Merzbacher-like disease (PMLD), has been shown to be caused by homozygous mutations in GJC2 or HSPD1. We report a consanguineous Israeli Bedouin kindred with clinical and radiological findings compatible with PMLD in which linkage to PLP1, GJC2, and HSPD1 was excluded. Through genome-wide homozygosity mapping and mutation analysis, we demonstrated in all affected individuals a homozygous frameshift mutation that fully abrogates the main active domain of AIMP1, encoding ARS-interacting multifunctional protein 1. The mutation fully segregates with the disease-associated phenotype and was not found in 250 Bedouin controls. Our findings are in line with the previously demonstrated inability of mutant mice lacking the AIMP1/p43 ortholog to maintain axon integrity in the central and peripheral neural system.

Magnetic resonance imaging pattern recognition in hypomyelinating disorders

Hypomyelination is observed in the context of a growing number of genetic disorders that share clinical characteristics. The aim of this study was to determine the possible role of magnetic resonance imaging pattern recognition in distinguishing different hypomyelinating disorders, which would facilitate the diagnostic process. Only patients with hypomyelination of known cause were included in this retrospective study. A total of 112 patients with Pelizaeus-Merzbacher disease, hypomyelination with congenital cataract, hypomyelination with hypogonadotropic hypogonadism and hypodontia, Pelizaeus-Merzbacher-like disease, infantile GM1 and GM2 gangliosidosis, Salla disease and fucosidosis were included. The brain scans were rated using a standard scoring list; the raters were blinded to the diagnoses. Grouping of the patients was based on cluster analysis. Ten clusters of patients with similar magnetic resonance imaging abnormalities were identified. The most important discriminating items were early cerebellar atrophy, homogeneity of the white matter signal on T(2)-weighted images, abnormal signal intensity of the basal ganglia, signal abnormalities in the pons and additional T(2) lesions in the deep white matter. Eight clusters each represented mainly a single disorder (i.e. Pelizaeus-Merzbacher disease, hypomyelination with congenital cataract, hypomyelination with hypogonadotropic hypogonadism and hypodontia, infantile GM1 and GM2 gangliosidosis, Pelizaeus-Merzbacher-like disease and fucosidosis); only two clusters contained multiple diseases. Pelizaeus-Merzbacher-like disease was divided between two clusters and Salla disease did not cluster at all. This study shows that it is possible to separate patients with hypomyelination disorders of known cause in clusters based on magnetic resonance imaging abnormalities alone. In most cases of Pelizaeus-Merzbacher disease, hypomyelination with congenital cataract, hypomyelination with hypogonadotropic hypogonadism and hypodontia, Pelizaeus-Merzbacher-like disease, infantile GM1 and GM2 gangliosidosis and fucosidosis, the imaging pattern gives clues for the diagnosis.

Pelizaeus-Merzbacher-like disease is caused not only by a loss of connexin47 function but also by a hemichannel dysfunction

Autosomal recessive mutations in the GJA12/GJC2 gene encoding the gap junction protein connexin47 (C x 47) cause a form of Pelizaeus-Merzbacher-like disease (PMLD) with hypomyelination, nystagmus, impaired psychomotor development and progressive spasticity. We investigated the functional consequences of four C x 47 missense mutations (G149S, G236R, T265A, and T398I) and one C x 47 complex mutation (A98G_V99insT) by immunoblot analysis and immunocytochemistry in transfected communication-incompetent HeLa cells and in OLI-neu cells. All studied C x 47 mutants, except G236R, generated stable proteins in transfected HeLa cells and OLI-neu cells. The mutants T265A and A98G_V99insT were retained in the ER, T398I formed gap junctional plaques at the plasma membrane, and G149S showed both, structures at the plasma membrane and ER localization. Two-microelectrode voltage clamp analyses in Xenopus laevis oocytes injected with wild-type and mutant C x 47 cRNA revealed reduced hemichannel currents for G236R, T265A, and A98G_V99insT. In contrast, T398I revealed hemichannel currents comparable to wild-type. For C x 47 mutant T398I, our results indicate a defect in hemichannel function, whereas C x 47 mutants G149S, G236R, T265A, and A98G_V99insT are predicted to result in a loss of C x 47 hemichannel function. Thus, PMLD is likely to be caused by two different disease mechanisms: a loss of function and a dysfunction.

[A case of autosomal recessive hypomyelinating leukodystrophy without GJA12 mutation presenting a novel phenotype]

A 50-year-old woman, who had consanguineous parents, developed gait disturbance at age 3, and revealed nystagmus, cerebellar ataxia, peripheral neuropathy, and spastic tetraparesis. She admitted to our hospital at age 14, and the symptoms progressed very slowly. MRI of this case at age 45 showed a remarkable, diffuse hypomyelination of the cerebrum. Her older sister who already deceased at age 16 showed neurological symptoms similar to this case. The patient was found to have no proteolipid protein-1 gene duplications and deletions and base substitution. Her symptoms were considered to be different from those of typical HLD2, 3, 4 and 5. She carried no GJA12 mutations. These facts suggested that this disease is a novel, autosomal recessive hypomyelinating leukodystrophy.

Two novel gap junction protein alpha 12 gene mutations in two Chinese patients with Pelizaeus-Merzbacher-like disease

Pelizaeus-Merzbacher-like disease (PMLD) is a hypomyelinating disorder of the central nervous system caused by mutation in the gap junction protein alpha 12 (GJA12) gene. Uniparental disomy (UPD) is defined as the presence of a chromosome pair, in a diploid individual, that derives from only one parent. Here, we analyzed GJA12 gene mutations in two Chinese PMLD patients and two novel mutations of GJA12 c.216delGinsAA (c.P73fsX106) caused by paternal UPD for chromosome 1 and c.138C>G (p.I46M) were identified. The patient 1 harbored a homozygous frameshift mutation at c.216delGinsAA (p.P73fsX106) in the GJA12. Haplotype analysis of the entire chromosome 1 of the patient revealed that this chromosome was exclusively derived from her father. The GJA12 gene is located on chromosome 1q41-42 and falls within the region of paternal isodisomy on the q arm. Thus, a novel homozygous frameshift mutation p.P73fsX106, caused by paternal UPD for chromosome 1, was identified in patient 1 with PMLD. Patient 2 was found a homozygous missense mutation at c.138C>G (p.I46M). This is the first GJA12 gene mutations reported from two Chinese PMLD patients and one mutation was associated with UPD for chromosome 1.

Gap junctions in inherited human disease

Gap junctions (GJ) provide direct intercellular communication. The structures underlying these cell junctions are membrane-associated channels composed of six integral membrane connexin (Cx) proteins, which can form communicating channels connecting the cytoplasms of adjacent cells. This provides coupled cells with a direct pathway for sharing ions, nutrients, or small metabolites to establish electrical coupling or balancing metabolites in various tissues. Genetic approaches have uncovered a still growing number of mutations in Cxs related to human diseases including deafness, skin disease, peripheral and central neuropathies, cataracts, or cardiovascular dysfunctions. The discovery of a growing number of inherited human disorders provides an unequivocal demonstration that gap junctional communication is crucial for diverse physiological processes.

Analysis of human alternative first exons and copy number variation of the GJA12 gene in patients with Pelizaeus-Merzbacher-like disease

Pelizaeus-Merzbacher-like disease (PMLD) is a heterogeneous disease with primary hypomyelination of the central nervous system. Only the minority of patients have mutations in the coding region of the GJA12 gene encoding gap junction protein alpha 12, a subunit of intercellular channels highly expressed by oligodendrocytes, the myelin forming cells of the central nervous system. No other gene has been found so far to be mutated in PMLD besides GJA12. We therefore extended the mutational screening in the GJA12 gene, searched for alternative first exons-as described in mice-determined the human 5'-end of the gene, screened therein for mutations and analyzed for copy number variations of the GJA12 gene in 14 patients with PMLD. Unlike in mice we did not find alternative first exons but detected a unique 79 bp first exon in human adolescent brain and spinal cord. No mutation in this non-coding region was found in our cohort. Copy number variation of the GJA12 gene was assessed by real-time PCR TaqMan gene expression technology, but neither patient showed an aberrant copy number. These data confirm that GJA12 alterations are a rare cause of PMLD-even after extending the screening for copy number variation and for mutations in the non-coding region of GJA12. Full genome scans in informative families and further screenings of candidate genes are feasible approaches to elucidate the genetic background of the majority of patients with PMLD.

Connexin-caused genetic diseases and corresponding mouse models

The human and mouse genomes contain 21 and 20 connexin genes, respectively. During the last 10-year period, genetic research on connexins has been stimulated by two parallel approaches: first, the characterization of genetic diseases that are caused by connexin mutations and, second, the generation and characterization of connexin knockout (null) mutated mice in which the coding region of nearly all connexin genes has been deleted. We summarize the current results of each of these two approaches. More recently, first results have been published in which connexin point mutations in human connexin genes were inserted at the corresponding position of the orthologous mouse gene. Under these conditions, the mutated connexin protein is expressed, in contrast to a connexin null mutation, and its interaction with other connexin isoforms or other connexin-binding proteins can be maintained. In this review, we discuss advantages and problems of such an approach and possible implications regarding the mechanism of the disease. The long-term goal is to understand the biologic function of each connexin isoform and the contribution of these proteins to the physiology of the corresponding organs in health and disease.

Hereditary spastic paraplegia is a novel phenotype for GJA12/GJC2 mutations

Recessive mutations in GJA12/GJC2, the gene that encodes the gap junction protein connexin47 (Cx47), cause Pelizaeus-Merzbacher-like disease (PMLD), an early onset dysmyelinating disorder of the CNS, characterized by nystagmus, psychomotor delay, progressive spasticity and cerebellar signs. Here we describe three patients from one family with a novel recessively inherited mutation, 99C>G (predicted to cause an Ile>Met amino acid substitution; I33M) that causes a milder phenotype. All three had a late-onset, slowly progressive, complicated spastic paraplegia, with normal or near-normal psychomotor development, preserved walking capability through adulthood, and no nystagmus. MRI and MR spectroscopy imaging were consistent with a hypomyelinating leukoencephalopathy. The mutant protein forms gap junction plaques at cell borders similar to wild-type (WT) Cx47 in transfected cells, but fails to form functional homotypic channels in scrape-loading and dual whole-cell patch clamp assays. I33M forms overlapping gap junction plaques and functional channels with Cx43, however, I33M/Cx43 channels open only when a large voltage difference is applied to paired cells. These channels probably do not function under physiological conditions, suggesting that Cx47/Cx43 channels between astrocytes and oligodendrocytes are disrupted, similar to the loss-of-function endoplasmic reticulum-retained Cx47 mutants that cause PMLD. Thus, GJA12/GJC2 mutations can result in a milder phenotype than previously appreciated, but whether I33M retains a function of Cx47 not directly related to forming functional gap junction channels is not known.

Increased level of N-acetylaspartylglutamate (NAAG) in the CSF of a patient with Pelizaeus-Merzbacher-like disease due to mutation in the GJA12 gene

Autosomal recessive Pelizaeus-Merzbacher-like disease 1 (PMLD1) is a hypomyelinating disorder of the central nervous system (CNS) with virtually identical phenotype to Pelizaeus-Merzbacher disease (PMD). PMLD1 is caused by mutations in GJA12 gene, PMD is due to mutations in PLP1 gene. Elevated levels of N-acetylaspartylglutamate (NAAG), the most abundant peptide neuromodulator in the human brain, have been recently reported in cerebral spinal fluid (CSF) of patients with PMD. Using capillary electrophoresis, we analyzed for the first time, the CSF from a girl with PMLD1 and detected high concentrations of NAAG. This finding confirms the hypothesis that NAAG may be involved in myelination-related processes and can be considered as a useful diagnostic marker not only for patients with the PLP1 related disorder, but also in those with Pelizaeus-Merzbacher like hypomyelinating disease due to other defined genetic causes, such as PMLD1.

The molecular and cellular defects underlying Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a recessive X-linked dysmyelinating disorder of the central nervous system (CNS). The most frequent cause of PMD is a genomic duplication of chromosome Xq22 including the region encoding the dosage-sensitive proteolipid protein 1 (PLP1) gene. The PLP1 duplications are heterogeneous in size, unlike duplications causing many other genomic disorders, and arise by a distinct molecular mechanism. Other causes of PMD include PLP1 deletions, triplications and point mutations. Mutations in the PLP1 gene can also give rise to spastic paraplegia type 2 (SPG2), an allelic form of the disease. Thus, there is a spectrum of CNS disorder from mild SPG2 to severe connatal PMD. PLP1 encodes a major protein in CNS myelin and is abundantly expressed in oligodendrocytes, the myelinating cells of the CNS. Significant advances in our understanding of PMD have been achieved by investigating mutant PLP1 in PMD patients, animal models and in vitro studies. How the different PLP1 mutations and dosage effects give rise to PMD is being revealed. Interestingly, the underlying causes of pathogenesis are distinct for each of the different genetic abnormalities. This article reviews the genetics of PMD and summarises the current knowledge of causative molecular and cellular mechanisms.

Update on leukodystrophies

Leukodystrophies, heritable white matter disorders, have long presented significant diagnostic and therapeutic challenges to the practicing neurologist. Recent advances, however, have identified many of the unclassified leukodystrophies, and work in understanding the underlying pathophysiology of these diverse disorders has progressed. Several hypomyelination disorders have been identified, Aicardi–Goutières syndrome-associated genes have been found and autosomal-dominant leukodystrophy has been established as a genetically distinct disorder. In addition, unifying mechanisms in certain leukodystrophies have been found, including an abnormal endoplasmic reticulum stress response and astrocytic dysfunction. Finally, novel diagnostic approaches have established techniques for rapid identification of affected patients. This review provides an update of these changes and discusses the likely impact they will have on the evaluation and management of leukodystrophies.

Leukodystrophies: clinical and genetic aspects

The leukodystrophies comprise an ever-expanding group of rare central nervous system disorders with defined clinical, pathological, and genetic characteristics. The broader term, leukoencephalopathy, is applied to all brain white matter diseases, whether their molecular cause is known. Magnetic resonance imaging has helped to elucidate new forms of leukodystrophy as well as to permit longitudinal studies of disease progression. The white matter abnormality may appear similar in different forms of leukodystrophy so that in most cases, further studies such as magnetic resonance spectroscopy, tissue biopsies, enzyme studies, and molecular DNA analyses are needed to pinpoint the specific diagnosis. The primary inherited leukoencephalopathies include dysmyelinating, hypomyelinative, and vacuolating forms. Metabolic and vascular causes account for most of the secondary forms, but other inherited syndromes are recognized that have their onset in childhood or adult life and are characterized by distinctive clinical and neuropathologic features. This review discusses some of the mechanisms that have been proposed to explain deficiencies of myelin and the molecular genetic bases underlying these disorders.