Systematic TOR1A non-c.907_909delGAG variant analysis in isolated dystonia and controls

BackgroundAn increasing number of rare, functionally relevant non-c.907_909delGAG (non-ΔGAG) variants in TOR1A have been recognized, associated with phenotypic expressions different from classic DYT1 childhood-onset generalized dystonia. Only recently, DYT1 genotype-phenotype correlations have been proposed, awaiting further elucidation in independent cohorts.MethodsWe screened the entire coding sequence and the 5′-UTR region of TOR1A for rare non-ΔGAG sequence variants in a large series of 940 individuals with various forms of isolated dystonia as well as in 376 ancestry-matched controls. The frequency of rare, predicted deleterious non-ΔGAG TOR1A variants was assessed in the European sample of the Exome Aggregation Consortium (ExAC) dataset.ResultsIn the case cohort, we identified a rare 5′-UTR variant (c.-39G > T), a rare splice-region variant (c.445-8T > C), as well as one novel (p.Ile231Asn) and two rare (p.Ala163Val, p.Thr321Met) missense variants, each in a single patient with adult-onset focal/segmental isolated dystonia. Of these variants, only p.Thr321Met qualified as possibly disease-related according to variant interpretation criteria. One novel, predicted deleterious missense substitution (p.Asn208Ser) was detected in the control cohort. Among European ExAC individuals, the carrier rate of rare, predicted deleterious non-ΔGAG variants was 0.4%.ConclusionsOur study does not allow the establishment of genotype-specific clinical correlations for DYT1. Further large-scale genetic screening accompanied by comprehensive segregation and functional studies is required to conclusively define the contribution of TOR1A whole-gene variation to the pathogenesis of isolated dystonia.     

Identification of novel sequence variants in the neurofilament-light gene in a Japanese population: analysis of Charcot-Marie-Tooth disease patients and normal individuals

Mutations of the neurofilament-light (NEFL/NF-L) gene were examined in 124 unrelated Japanese patients with Charcot-Marie-Tooth disease (CMT) without known gene mutations, and 248 normal Japanese individuals. A new method, which can detect basepair mismatches with RNase cleavage on agarose gel electrophoresis, coupled with DNA sequencing, identified 8 novel sequence variations in the NF-L gene. In these sequence variants, 5 variants were polymorphisms, including 3 single nucleotide polymorphisms (SNPs), and 3 other missense mutations (Pro22Thr, Asn97Ser and Ala148Val) were found in the patients with CMT phenotype. The variant alleles in the NF-L gene could influence the developing process of CMT phenotype and also might cause CMT phenotype.     

The GIGYF2 variants are not associated with Parkinson's disease in the mainland Chinese population

In order to determine the prevalence of GIGYF2 (Grb10-Interacting GYF Protein 2) variants in the Chinese population and to better understand the association between GIGYF2 and Parkinson's disease (PD), we conducted the genetic screening of GIGYF2 in the Chinese population. Twelve exonic variants were identified in 52 familial PD probands and 56 healthy controls. Non-synonymous point variants (Thr25Ala, Asn457Th and Pro460Th) were analyzed in 510 PD patients and 481 healthy controls of Chinese Han ethnicity. The insertion and deletion variants in Exon 25 (Ins Q 1212, Ins QQ 1217, Del Q 1210, Del Q 1216 and Del PPQ1217_1219) are not related to the onset of familial PD. Our data indicate the GIGYF2 variants are not associated with PD in the mainland Chinese Population.     

A new alpha-synuclein missense variant (Thr72Met) in two Turkish families with Parkinson's disease

IntroductionMissense variants and multiplications of the alpha-synuclein gene (SNCA) are established as rare causes of autosomal dominant forms of Parkinson's Disease (PD).MethodsTwo families of Turkish origins with PD were studied; the SNCA coding region was analyzed by Sanger sequencing, and by whole exome sequencing (WES) in the index patient of the first and the second family, respectively. Co-segregation studies and haplotype analysis across the SNCA locus were carried out. Functional studies included in vitro thioflavin-T aggregation assay and in silico structural modelling of the alpha-synuclein (α-syn) protein.ResultsWe identified a novel heterozygous SNCA variant, c.215C > T (p.Thr72Met), segregating with PD in a total of four members in the two families. A shared haplotype across the SNCA locus was found among variant carriers, suggestive of a common ancestor. We next showed that the Thr72Met α-syn displays enhanced aggregation in-vitro, compared to the wild-type species. In silico analysis of a tetrameric α-syn structural model revealed that Threonine 72 lies in the tetrameric interface, and substitution with the much larger methionine residue could potentially destabilize the tetramer.ConclusionWe present clinical, genetic, and functional data supporting a causative role of the SNCA c.215C > T (p.Thr72Met) variant in familial PD. Testing for this variant in patients with PD, especially of Turkish origin, might detect additional carriers. Further functional analyses might offer new insights into the shared biochemical properties of the PD-causing SNCA missense variants, and how they lead to neurodegeneration.     

The role of the Ala746Thr variant in the ATP13A2 gene among Chinese patients with Parkinson’s disease

The association between idiopathic Parkinson’s disease (PD) and the ATP13A2 (PARK9) Ala746Thr variant, associated with Kufor-Rakeb syndrome, is controversial. We investigated this association in 69 patients with early onset PD (EOPD; ≦50 years of age), 192 patients with late onset PD (LOPD; >50 years of age), and 180 healthy controls in the Chinese population in Hong Kong. The presence of the Ala746Thr variant in the ATP13A2 locus was examined in all participants. We detected the heterozygous Ala746Thr variant in one healthy control (0.6%), one patient with EOPD (1.4%, p = 0.50), and one patient with LOPD (0.5%, p = 0.96). We suggest that the ATP13A2 Ala746Thr variant is not a common risk factor for PD in the Chinese population in Hong Kong.     

Additional observation of a de novo pathogenic variant in KCNT2 leading to epileptic encephalopathy with clinical features of frontal lobe epilepsy

IntroductionKCNT2 was recently recognized as a gene associated with neurodevelopmental disorder and epilepsy.Case reportWe present an additional observation of a 16-year-old male patient with a novel de novo KCNT2 likely pathogenic variant and review the five previously reported cases of de novo variants in this gene.DiscussionWhole exome sequencing identified the missense variant c.725C > A p.(Thr242Asn), which was confirmed by Sanger sequencing. Our patient has a refractory stereotyped and monomorphic type of hyperkinetic focal motor seizure, similar to what is seen in frontal lobe epilepsy, occurring only during sleep. This type of seizure is not usually seen in epileptic encephalopathies.     

Evaluation of PINK1 variants in Indian Parkinson's disease patients

Mutations in PINK1 have been identified in familial and sporadic cases of early onset Parkinson's disease (PD). To determine the contribution of PINK1 variants in Indian PD patients, the gene was screened in 250 patients and 205 ethnically matched controls by polymerase chain reaction, single-stranded conformation polymorphism and DNA sequencing. Two potentially pathogenic variants (Arg246Gln & Arg276Gln) were detected in the heterozygous state in 5 patients; none of the patients carried homozygous or compound heterozygous mutations. In addition, 13 other variants were identified, including a known polymorphism (Ala340Thr), a few synonymous or intronic changes, none of which are likely to be pathogenic. Unlike the Chinese population, the Ala340Thr variant did not show any association with PD in Indian population. Six single nucleotide polymorphisms (SNPs) selected from dbSNP were genotyped in 531 normal, healthy individuals representing different ethnic groups of India. Most of the SNP markers were observed to be highly heterozygous among Indians, which could be used for segregation analysis of PINK1 alleles in familial PD cases.     

Phenotype–genotype correlations in patients with GNB1 gene variants,including the first three reported Japanese patients to exhibit spastic diplegia,dyskinetic quadriplegia,and infantile spasms

We report the first three Japanese patients with missense variants in the GNB1 gene. Patients exhibited severe dyskinetic quadriplegia with cortical blindness and epileptic spasms, West syndrome (but with good outcomes), and hypotonic quadriplegia that later developed into spastic diplegia. Whole-exome sequencing revealed two recurrent GNB1 variants (p.Leu95Pro and p.Ile80Thr) and one novel variant (p.Ser74Leu). A recent investigation revealed large numbers of patients with GNB1 variants. Functional studies of such variants and genotype–phenotype correlation are required to enable future precision medicine.     

Analysis of the GIGYF2 gene in familial and sporadic Parkinson disease in the Spanish population

Background and purpose: Linkage analysis in familial Parkinson’s disease (PD) identified a locus in 2q36‐37 (PARK11). Sequencing of GIGYF2 identified several variants only present amongst PD individuals. Methods: We analyzed the presence of disease‐associated GIGYF2 variants in familial and sporadic PD from Spanish origin by sequencing of 147 PD individuals. The entire GIGYF2 coding sequence was analyzed in 122 familial PD individuals and exons 2, 4, 8–11, 14 and 25–26 were sequenced in 25 sporadic PD to identify disease‐associated variants. Results: We found no variants associated with PD and failed to identify any of previously PD‐associated GIGYF2 variants in our sample. We identified four novel missense changes in GIGYF2. p.Met48Ile was found in a PD individual who also was a carrier of two PARKIN mutations. p.Q1244_Q1247del variant was present only in one PD individual but not found in 70 controls. However, its location in the highly polymorphic GIGYF2 glutamine/proline‐rich region does not support a role in PD. Two variants (p.P1238insAGC and p.Q1249del) were present both in PD subjects and in controls. Additionally, the p.L1230_Q1237del variant, which was previously considered as a PD‐associated change, was found in one control. Conclusion: Our findings suggest that GIGYF2 mutations are not a frequent cause of PD in the Spanish population, since we found no clearly segregating variants. We propose further analyses in PD subjects from different populations to define the role of GIGYF2. A clear pathogenic mutation in other gene at 2q36‐37 in the PARK11‐linked PD families would definitively disprove GIGYF2 as the responsible gene.     

Characterisation of blood coagulation factor XI T475I

PCR-SSCP and DNA sequence analysis of a factor XI (FXI) deficient patient (FXI:C 39 U/dL; FXI:Ag 27 U/dL) identified a C to T transition in exon 12 of the FXI gene (F11 c.1521C>T) that predicts the substitution of Thr475 by Ile (FXI T475I) within the serine protease domain of FXI. This mutation destroys a consensus sequence for N-linked glycosylation, N473-Y-T475, known to be utilized in vivo. The FXIT475I variant was generated by site-directed mutagenesis, together with other variants that could help explain the phenotype, and recombinant FXI variants were expressed in Chinese hamster ovary cells. FXI:Ag expression was analysed by Western blot analysis, ELISA and immunocytochemical staining. Wild-type FXI:Ag was secreted at high levels, however the mutant (FXI T475I) was secreted very poorly. Substitution of Thr475 by Ala, Pro, Lys or Arg (all of which abolish the glycosylation consensus sequence) also severely reduced the level of secreted FXI:Ag suggesting that glycosylation at Asn473 is required for folding or secretion. Concordant with this hypothesis the conservative substitution of Thr475 by Ser (which preserves the glycosylation consensus sequence) had no effect on FXI secretion. Thr/Ser475 is highly conserved in serine protease domains but the glycosylation site (Asn473) is not. Surprisingly, substitution of Asn473 by Ala (which removes the N-linked glycosylation site) had no effect on the levels of FXI:Ag secreted. In conclusion, although the FXI-T475I mutation destroys an N-linked glycosylation consensus sequence, the cause of failure to secrete FXI is not the loss of a glycosylation site but rather a direct effect of the substitution of this highly conserved residue.     

Novel A18T and pA29S substitutions in α-synuclein may be associated with sporadic Parkinson's disease

ObjectiveMutations in the α-synuclein-encoding gene SNCA are considered as a rare cause of Parkinson's disease (PD). Our objective was to examine the frequency of the SNCA point mutations among PD patients of Polish origin.MethodsDetection of the known SNCA point mutations A30P (c.88G>C), E46K (c.136G>A) and A53T (c.157A>T) was performed either using the Sequenom MassArray iPLEX platform or by direct sequencing of the SNCA exons 2 and 3. As the two novel substitutions A18T (c.52G>A) and A29S (c.85G>T) were identified, their frequency in a control population of Polish origin was assessed and in silico analysis performed to investigate the potential impact on protein structure and function.ResultsWe did not observe the previously reported point mutations in the SNCA gene in our 629 PD patients; however, two novel potentially pathogenic substitutions A18T and A29S were identified. Each variant was observed in a single patient presenting with a typical late-onset sporadic PD phenotype. Although neither variant was observed in control subjects and in silico protein analysis predicts a damaging effect for A18T and pA29S substitutions, the lack of family history brings into question the true pathogenicity of these rare variants.ConclusionsLarger population based studies are needed to determine the pathogenicity of the A18T and A29S substitutions. Our findings highlight the possible role of rare variants contributing to disease risk and may support further screening of the SNCA gene in sporadic PD patients from different populations.     

Role of LRP10 in Parkinson's disease in a Taiwanese cohort

IntroductionVariants in the low-density lipoprotein receptor–related protein 10 (LRP10), linked to inherited forms of α-synucleinopathies, have been reported. Nine variants of LRP10 were identified in the first such report, and subsequent studies have identified possible pathogenic variants in patients with sporadic Parkinson's disease (PD). Few studies have investigated the role of LRP10 in PD. We sought to validate the role of this gene in Taiwanese patients with PD.MethodsIn total, 1277 individuals were included in this study (669 had PD and 608 were controls). The entire LRP10 coding exons and exon–intron boundaries were sequenced in 103 probands with early-onset PD or familial PD. We then genotyped the newly identified variants from the 103 patients and previously reported potential pathogenic variants in our cohort. The frequencies of variants were analyzed.ResultsFive new and possibly pathogenic variants were identified initially. In total, 14 potentially pathogenic variants (including nine previously reported and five newly identified variants) were analyzed thereafter. We did not find any significant associations between any variant and the risk of PD. However, c.1424+5delG was identified in a patient with sporadic PD who was diagnosed as having PD and dementia and who had prominent psychiatric symptoms.ConclusionAlthough we identified a patient with sporadic PD and dementia carrying a c.1424+5delG variant, our data did not provide sufficient evidence to support the role of LRP10 in PD in Taiwanese adults.     

常染色体显性遗传性帕金森病家系临床特征及LRRK2基因分析

目的 研究常染色体显性遗传家族性帕金森病(PD)患者的临床特征及其LRRK2基因突变.方法 对16例常染色体显性遗传家族史的PD患者LRRK2基因的外显子5、13、31、32、35、37、41、48进行测序检测.利用限制性片段长度多态性(RFLP)分析所发现的新突变c.1432 G＞TAsp478Tyr在240例散发性PD患者及214名健康者中的发生频率.结果 常染色体显性遗传家系PD患者与散发性PD患者比较临床特点以晚发为主,首发症状以静止性震颤(9例,56.25%,t=0.558,P=0.679)和动作迟缓(9例,56.25%,t=0.369,P=0.454)最为常见,其次是肌强直(6例,37.50%,t=1.324,P=0.735)和姿势障碍(5例,31.25%,t=2.369,P=0.956),对左旋多巴治疗反应良好,左旋多巴诱导的异动症少见.LRRK2基因突变检测发现6个突变:c.457 T＞C Leu153Leu、c.1432 G＞T Asp478Tyr、c.5457 T＞C Gly1819Gly、c.7153 G＞A Gly2385Arg、IVS31+28 T＞G、IVS37+162 T＞C.其中,c.1432 G＞T Asp478Tyr是未报道过的新突变.在240例散发性PD患者及214名健康者中均未发现有此突变.未发现与PD相关的LRRK2基因突变Arg1441Cys/Gly/His、Arg1514Gln、Tyr1699Cys、Ile2012Thr、Gly2019Ser和Ile2020Thr.结论 常染色体显性遗传性PD患者临床特征表现为典型的PD临床特征,存在LRRK2基因Asp478Tyr和Gly2385Arg突变.Asp478Tyr是未报道过的新突变.     

Comprehensive LRRK2 and GBA screening in Portuguese patients with Parkinson's disease: Identification of a new family with the LRRK2 p.Arg1441His mutation and novel missense variants

Mutations in the LRRK2 and GBA genes are increasingly recognized as frequent determinants of familial and sporadic Parkinson's disease (PD). However, for several populations, accurate data on the prevalence and types of mutations are not available, because previous studies have not investigated the complete coding regions of these genes in large samples.We studied 312 PD patients ascertained at a single centre in Lisbon, Portugal. In 61 patients, with familial PD, we sequenced the entire open reading frames and exon-intron boundaries of LRRK2 and GBA. In LRRK2, we identified ten heterozygous p.Gly2019Ser (16.4%), and two heterozygous p.Arg1441His carriers (3.3%); furthermore, six patients each carried a novel LRRK2 heterozygous variant (five coding and one 3′-UTR variants) of undetermined pathogenic role. Segregation of the p.Arg1441His mutation with PD was observed in the families of both carriers. None of these variants were identified in 138 healthy controls. Screening of GBA revealed no mutations. In the remaining 251 PD patients (25 familial and 226 sporadic) we found ten additional carriers of the heterozygous p.Gly2019Ser and no carriers of the other mutations. Thus, the p.Gly2019Ser mutation was detected in a total number of 20 carriers out of 312 patients (6.4%), including twelve familial (14%) and eight sporadic patients (3.5%).This comprehensive study confirms that p.Gly2019Ser is the most important genetic cause of PD known so far in Portugal and supports the contention that p.Arg1441His is also a PD-causing mutation. These findings have relevance for the genetic testing and counseling of PD patients in this population.     

Novel and reported variants in Parkinson's disease genes confer high disease burden among Indians

BackgroundGenetic heterogeneity in Parkinson's disease (PD) has been unambiguously reported across different populations. Assuming a higher genetic load, we tested variant burden in PD genes to an early onset PD cohort from India.MethodsWhole exome sequencing was performed in 250 PD patients recruited following MDS-UPDRS criteria. The number of rare variants in the 20 known PD genes per exome were used to calculate average rare variant burden with the 616 non-PD exomes available in-house as a comparison group. SKAT-O test was used for gene level analysis.Results80 patients harboured rare variants in 20 PD genes, of which six had known pathogenic variants accounting for 2.4% of the cohort. Of 80 patients, 12 had homozygous and nine had likely compound heterozygous variants in recessive PD genes and 59 had heterozygous variants in only dominant PD genes. Of the 16 novel variants of as yet unknown significance identified, four homozygous across ATP13A2, PRKN, SYNJ1 and PARK7; and 12 heterozygous among LRRK2, VPS35, EIF4G1 and CHCHD2 were observed. SKAT-O test suggested a higher burden in GBA (p_unadjusted = 0.002). Aggregate rare variant analysis including 75 more individuals with only heterozygous variants in recessive PD genes (excluding GBA), with an average of 0.85 protein-altering rare variants per PD patient exome versus 0.51 in the non-PD group, revealed a significant enrichment (p < 0.0001).ConclusionThis first study in an early onset PD cohort among Indians identified 16 novel variants in known genes and also provides evidence for a high genetic burden in this ethnically distinct population.     

VPS35 gene variants are not associated with Parkinson's disease in the mainland Chinese population

VPS35 gene mutation has recently been reported in autosomal-dominant, late-onset Parkinson disease (PD). There are no reports regarding the association between VPS35 and Parkinson's disease (PD) in the Chinese population. We conducted a comprehensive genetic analysis of VPS35 gene in a cohort of twenty seven probands belonging to families with autosomal-dominant, late-onset PD, followed up with screening of specific variants in a separate group of 1011 sporadic PD patients and 1016 healthy controls. Our analysis revealed two exonic variants and three intronic variants across the entire VPS35 gene. There was no statistical difference in genotype or allele frequencies of rs3743928 and IVS14?24 t > c variants in VPS35 gene between sporadic PD group and healthy control group. None of the 1011 sporadic PD patients and 1016 controls carried the VPS35 gene c.1858G > A (p.Asp620Asn) mutation. Our data indicated that the VPS35 variants are not associated with PD in the mainland Chinese population.     

Genetic screening for mutations in the Nrdp1 gene in Parkinson disease patients in a Chinese population

Strong evidence has shown that a defect in the Parkin gene is known to be a common, genetic cause of Parkinson disease (PD). The E3 ubiquitin ligase Nrdp1 is shown to interact with the N terminal of Parkin (the first 76 amino acids) and catalyze degradation of Parkin via the ubiquitin–proteasome pathway, suggesting that Nrdp1 may be involved in the development of PD via the regulation of Parkin, We believe we are the first to have screened PD patients for mutations in the Nrdp1 gene to determine the association between these variants and PD. By direct sequencing, we analysed the entire coding regions and 5′ UTR of Nrdp1 in 209 Chinese PD patients and 302 unrelated healthy individuals. No variant was detected in the coding regions (exons 3–7); only 2 variants (c.−206 T > A and c.−208–8 A > G) were identified in the 5′ UTR (exon 2) and intron 1. Furthermore, a study of the allelic and genotypic association between patients and controls showed no significant association between the c.−206 T > A polymorphism and PD; c.−208–8 A > G was identified in one PD patient and not in controls. Our data do not support the hypothesis of a major role for the Nrdp1 gene in PD development in the Chinese population.     

Phenotypic expansion of KCNH1 ‐associated disorders to include isolated epilepsy and its associations with genotypes and molecular sub‐regional locations

PurposeGenotype‐phenotypic correlation of KCNH1 variant remains elusive. This study aimed to expand the phenotypic spectrum of KCNH1 and explore the correlations between epilepsy and molecular sub‐regional locations.MethodsWe performed whole‐exome sequencing in a cohort of 98 patients with familiar febrile seizure (FS) or epilepsy with unexplained etiologies. The damaging effects of variants were predicted by protein modeling and multiple in silico tools. All reported patients with KCNH1 pathogenic variants with detailed neurological phenotypes were analyzed to evaluate the genotype‐phenotype correlation.ResultsTwo novel KCNH1 variants were identified in three cases, including two patients with FS with inherited variant (p.Ile113Thr) and one boy with epilepsy with de novo variant (p.Arg357Trp). Variant Ile113Thr was located within the eag domain, and variant p.Arg357Trp was located in transmembrane domain 4 of KCNH1, respectively. Two patients experienced refractory status epilepticus (SE), of which one patient died of acute encephalopathy induced by SE. Further analysis of 30 variants in 51 patients demonstrated that de novo variants were associated with epileptic encephalopathy, while mosaic/somatic or germline variants cause isolated epilepsy/FS. All hotspot variants associated with epileptic encephalopathy clustered in transmembrane domain (S4 and S6), while those with isolated epilepsy/seizures or TBS/ZLS without epilepsy were scattered in the KCNH1.ConclusionsWe found two novel missense variants of KCNH1 in three individuals with isolated FS/epilepsy. Variants in the KCNH1 cause a spectrum of epileptic disorders ranging from a benign form of genetic isolated epilepsy/FS to intractable form of epileptic encephalopathy. The genotypes and variant locations help explaining the phenotypic variation of patients with KCNH1 variant.     

Mutation screening and association analysis of the parkin gene in Parkinson's disease patients from South-West China

Various deletions and point mutations in the Parkin gene have been strongly associated with Parkinson's disease (PD) and parkinsonism, especially when the onset occurs at a young age. In this study, we screened 25 "early-onset" (<49 years at onset) and 91 later-onset PD patients from a Han Chinese population from South-West China for deletions and mutations in the Parkin gene. We found no deletions or point mutations in exons 1-12 of the Parkin gene using direct sequence analysis and only detected the common Ser167Asn polymorphism. We analysed Ser167Asn in 116 patients with sporadic PD and 124 controls, matched for age and gender. There were significant differences in allele and genotype frequency between PD patients, with the 167Asn allele more common in cases than controls (46.6 vs. 35.1%; chi(2) = 6.54, p = 0.011, odds ratio = 1.61, 95% confidence interval, CI, 1.10-2.37), as was the 167Asn genotype (17.3 vs. 11.3%; p = 0.04). The frequency of the 167Ser genotype was significantly lower in PD patients than in controls when compared with that of the other two genotypes combined (chi(2) = 7.84, p = 0.005, odds ratio = 0.46, 95% CI 0.25 - 0.82). No significant differences in the frequencies of the allele and genotypes were found between patients classified into two groups according to symptoms at onset (chi(2) = 0.191, p = 0.66, odds ratio = 1.12, 95% CI 0.65-1.95; chi(2) = 0.24, p = 0.887) or age of onset (p = 0.787). In summary, homozygous deletion mutations and point mutations in exons 1-12 of the Parkin gene were not detected in this Han Chinese population, although we cannot exclude compound heterozygous deletions. In addition, our study suggests that the variant 167Asn increases the risk of developing PD.     

The polynucleotide kinase 3′-phosphatase gene (<Emphasis Type="Italic">PNKP</Emphasis>) is involved in Charcot-Marie-Tooth disease (CMT2B2) previously related to <Emphasis Type="Italic">MED25</Emphasis>

Charcot-Marie-Tooth disease (CMT) represents a heterogeneous group of hereditary peripheral neuropathies. We previously reported a CMT locus on chromosome 19q13.3 segregating with the disease in a large Costa Rican family with axonal neuropathy and autosomal recessive pattern of inheritance (CMT2B2). We proposed a homozygous missense variant in the Mediator complex 25 (MED25) gene as causative of the disease. Nevertheless, the fact that no other CMT individuals with MED25 variants were reported to date led us to reevaluate the original family. Using exome sequencing, we now identified a homozygous nonsense variant (p.Gln517ter) in the last exon of an adjacent gene, the polynucleotide kinase 3′-phosphatase (PNKP) gene. It encodes a DNA repair protein recently associated with recessive ataxia with oculomotor apraxia type 4 (AOA4) and microcephaly, seizures, and developmental delay (MCSZ). Subsequently, five unrelated Costa Rican CMT2 subjects initially identified as being heterozygous for the same MED25 variant were found to be also compound heterozygote for PNKP. All were heterozygous for the same variant found homozygous in the large family and a second one previously associated with ataxia (p.Thr408del). Detailed clinical reassessment of the initial family and the new individuals revealed in all an adult-onset slowly progressive CMT2 associated with signs of cerebellar dysfunction such as slurred speech and oculomotor involvement, but neither microcephaly, seizures, nor developmental delay. We propose that PKNP variants are the major causative variant for the CMT2 phenotype in these individuals and that the milder clinical manifestation is due to an allelic effect.