Updated variant curation expert panel criteria and pathogenicity classifications for 251 variants for RYR1-related malignant hyperthermia susceptibility.

The ClinGen malignant hyperthermia susceptibility (MHS) variant curation expert panel specified the American College of Medical Genetics and Genomics/Association of Molecular Pathologists (ACMG/AMP) criteria for RYR1-related MHS and a pilot analysis of 84 variants was published. We have now classified an additional 251 variants for RYR1-related MHS according to current ClinGen standards and updated the criteria where necessary. Criterion PS4 was modified such that individuals with multiple RYR1 variants classified as pathogenic (P), likely pathogenic (LP), or variant of uncertain significance (VUS) were not considered as providing evidence for pathogenicity. Criteria PS1 and PM5 were revised to consider LP variants at the same amino-acid residue as providing evidence for pathogenicity at reduced strength. Finally, PM1 was revised such that if PS1 or PM5 are used PM1, if applicable, should be downgraded to supporting. Of the 251 RYR1 variants, 42 were classified as P/LP, 16 as B/LB, and 193 as VUS. The primary driver of 175 VUS classifications was insufficient evidence supporting pathogenicity, rather than evidence against pathogenicity. Functional data supporting PS3/BS3 was identified for only 13 variants. Based on the posterior probabilities of pathogenicity and variant frequencies in gnomAD, we estimated the prevalence of individuals with RYR1-related MHS pathogenic variants to be between 1/300 and 1/1075, considerably higher than current estimates. We have updated ACMG/AMP criteria for RYR1-related MHS and classified 251 variants. We suggest that prioritization of functional studies is needed to resolve the large number of VUS classifications and allow for appropriate risk assessment. RYR1-related MHS pathogenic variants are likely to be more common than currently appreciated.

Variant curation expert panel recommendations for RYR1 pathogenicity classifications in malignant hyperthermia susceptibility.

PURPOSE: As a ClinGen Expert Panel (EP) we set out to adapt the American College of Medical Genetics and Genomics (ACMG)/Association for Molecular Pathology (AMP) pathogenicity criteria for classification of RYR1 variants as related to autosomal dominantly inherited malignant hyperthermia (MH). METHODS: We specified ACMG/AMP criteria for variant classification for RYR1 and MH. Proposed rules were piloted on 84 variants. We applied quantitative evidence calibration for several criteria using likelihood ratios based on the Bayesian framework. RESULTS: Seven ACMG/AMP criteria were adopted without changes, nine were adopted with RYR1-specific modifications, and ten were dropped. The in silico (PP3 and BP4) and hotspot criteria (PM1) were evaluated quantitatively. REVEL gave an odds ratio (OR) of 23:1 for PP3 and 14:1 for BP4 using trichotomized cutoffs of ≥0.85 (pathogenic) and ≤0.5 (benign). The PM1 hotspot criterion had an OR of 24:1. PP3 and PM1 were implemented at moderate strength. Applying the revised ACMG/AMP criteria to 44 recognized MH variants, 29 were classified as pathogenic, 13 as likely pathogenic, and 2 as variants of uncertain significance. CONCLUSION: Curation of these variants will facilitate classification of RYR1/MH genomic testing results, which is especially important for secondary findings analyses. Our approach to quantitatively calibrating criteria is generalizable to other variant curation expert panels.

Adult-onset autosomal dominant spastic paraplegia linked to a GTPase-effector domain mutation of dynamin 2.

BACKGROUND: Hereditary Spastic Paraplegia (HSP) represents a large group of clinically and genetically heterogeneous disorders linked to over 70 different loci and more than 60 recognized disease-causing genes. A heightened vulnerability to disruption of various cellular processes inherent to the unique function and morphology of corticospinal neurons may account, at least in part, for the genetic heterogeneity. METHODS: Whole exome sequencing was utilized to identify candidate genetic variants in a four-generation Siberian kindred that includes nine individuals showing clinical features of HSP. Segregation of candidate variants within the family yielded a disease-associated mutation. Functional as well as in-silico structural analyses confirmed the selected candidate variant to be causative. RESULTS: Nine known patients had young-adult onset of bilateral slowly progressive lower-limb spasticity, weakness and hyperreflexia progressing over two-to-three decades to wheel-chair dependency. In the advanced stage of the disease, some patients also had distal wasting of lower leg muscles, pes cavus, mildly decreased vibratory sense in the ankles, and urinary urgency along with electrophysiological evidence of a mild distal motor/sensory axonopathy. Molecular analyses uncovered a missense c.2155C > T, p.R719W mutation in the highly conserved GTP-effector domain of dynamin 2. The mutant DNM2 co-segregated with HSP and affected endocytosis when expressed in HeLa cells. In-silico modeling indicated that this HSP-associated dynamin 2 mutation is located in a highly conserved bundle-signaling element of the protein while dynamin 2 mutations associated with other disorders are located in the stalk and PH domains; p.R719W potentially disrupts dynamin 2 assembly. CONCLUSION: This is the first report linking a mutation in dynamin 2 to a HSP phenotype. Dynamin 2 mutations have previously been associated with other phenotypes including two forms of Charcot-Marie-Tooth neuropathy and centronuclear myopathy. These strikingly different pathogenic effects may depend on structural relationships the mutations disrupt. Awareness of this distinct association between HSP and c.2155C > T, p.R719W mutation will facilitate ascertainment of additional DNM2 HSP families and will direct future research toward better understanding of cell biological processes involved in these partly overlapping clinical syndromes.

Genome-Wide Analysis of Exertional Rhabdomyolysis in Sickle Cell Trait Positive African Americans.

Sickle cell trait (SCT), although generally a benign carrier state of hemoglobin S (HbAS), is a risk factor for exertional rhabdomyolysis (ERM), a rare but potentially fatal consequence of highly intense physical exercise, particularly among active-duty military personnel and high-performance athletes. The association between SCT and ERM is poorly understood. The objective of this study was to elucidate the genetic basis of ERM in an SCT-positive African American cohort. SCT-positive African Americans with a personal history of ERM (cases, n = 30) and without history of ERM (controls, n = 53) were enrolled in this study. Whole-genome sequencing was performed on DNA samples isolated from peripheral white blood cells. Participants' demographic, behavioral, and medical history information was obtained. An additional 131 controls were extracted from SCT-positive subjects of African descent from the 1000 Genomes Project. SCT carriers with ERM were characterized by myotoxicity features, significant muscle involvement dominated by muscle weakness, and severe pain and substantial increase in serum creatine kinase, with a mean value of 50,480 U/L. A distinctive feature of the SCT individuals with ERM was exertional collapse, which was reported in 53.3% of the cases in the study cohort. An important factor for the development of ERM was the duration and frequency of strenuous physical activity in the cases compared to the controls. Whole-genome sequencing identified 79,696 protein-coding variants. Genome-wide association analysis revealed that the p.C477R, rs115958260 variant in the SLC44A3 gene was significantly associated with ERM event in SCT-positive African Americans. The study results suggest that a combination of vigorous exercise and a genetic predisposing factor is involved in ERM.

The impact of heat stress on the human plasma lipidome.

The year of 2023 displayed the highest average global temperatures since it has been recorded-the duration and severity of extreme heat are projected to increase. Rising global temperatures represent a major public health threat, especially to occupations exposed to hot environments, such as construction and agricultural workers, and first responders. Despite efforts of the scientific community, there is still a need to characterize the pathophysiological processes leading to heat related illness and develop biomarkers that can predict its onset. Here, we performed a plasma lipidomic analysis on male and female subjects who underwent heat tolerance testing (HTT), consisting of a 2-h treadmill walk at 5 km/h with 2% inclination at a controlled temperature of 40°C. We identified 995 lipids from 27 classes, with nearly half of all detected lipids being responsive to HTT. Lipid classes related to substrate utilization were predominantly affected by HTT, with a downregulation of triacylglycerols and upregulation of free fatty acids and acyl-carnitines (CARs). We additionally examined correlations between changes in plasma lipids by using the physiological strain index (PSI). Here, even chain CAR 4:0, 14:0 and 16:1, suggested by-products of incomplete beta oxidation, and diacylglycerols displayed the highest correlation to PSI. PSI did not correlate with plasma lactate levels, suggesting that correlations between even chain CARs and PSI is related to metabolic efficiency versus physical exertion. Overall, our results show that HTT has a strong impact on the plasma lipidome and that metabolic inefficiencies may underlie heat intolerance.

Dominant mutations in KBTBD13, a member of the BTB/Kelch family, cause nemaline myopathy with cores.

We identified a member of the BTB/Kelch protein family that is mutated in nemaline myopathy type 6 (NEM6), an autosomal-dominant neuromuscular disorder characterized by the presence of nemaline rods and core lesions in the skeletal myofibers. Analysis of affected families allowed narrowing of the candidate region on chromosome 15q22.31, and mutation screening led to the identification of a previously uncharacterized gene, KBTBD13, coding for a hypothetical protein and containing missense mutations that perfectly cosegregate with nemaline myopathy in the studied families. KBTBD13 contains a BTB/POZ domain and five Kelch repeats and is expressed primarily in skeletal and cardiac muscle. The identified disease-associated mutations, C.742C>A (p.Arg248Ser), c.1170G>C (p.Lys390Asn), and c.1222C>T (p.Arg408Cys), located in conserved domains of Kelch repeats, are predicted to disrupt the molecule's beta-propeller blades. Previously identified BTB/POZ/Kelch-domain-containing proteins have been implicated in a broad variety of biological processes, including cytoskeleton modulation, regulation of gene transcription, ubiquitination, and myofibril assembly. The functional role of KBTBD13 in skeletal muscle and the pathogenesis of NEM6 are subjects for further studies.

Exome sequencing reveals SCO2 mutations in a family presented with fatal infantile hyperthermia.

We applied whole-exome sequencing (WES) for identification of an underlying genetic cause of a disease in a family presented with fatal infantile hyperthermia. Analysis of WES results revealed novel, deleterious compound missense mutations, Val160Ala and Pro233Thr, in the synthesis of cytochrome C oxidase 2 gene (SCO2) encoding a mitochondrial protein, Sco2, which is important for cytochrome C oxidase (COX) synthesis. Autosomal recessive mutations in SCO2 are known to be associated with COX deficiency recognized as fatal infantile cardio-encephalomyopathy (604272, OMIM). The Val160Ala and Pro233Thr mutations occurred in the conserved thioredoxin domain of Sco2 and predicted to disrupt protein folding and interaction of Sco2 with other proteins. Our results show applicability of WES in identification of disease-causing mutations and in establishing molecular diagnosis of severe, infantile onset disorder with a challenging diagnosis.

Exome sequencing identifies titin mutations causing hereditary myopathy with early respiratory failure (HMERF) in families of diverse ethnic origins.

BACKGROUND: Hereditary myopathy with early respiratory failure (HMERF) was described in several North European families and recently linked to a titin gene (TTN) mutation. We independently studied HMERF-like diseases with the purpose to identify the cause, refine diagnostic criteria, and estimate the frequency of this disease among myopathy patients of various ethnic origins. METHODS: Whole exome sequencing analysis was carried out in a large U.S. family that included seven members suffering from skeletal muscle weakness and respiratory failure. Subsequent mutation screening was performed in further 45 unrelated probands with similar phenotypes. Studies included muscle strength evaluation, nerve conduction studies and concentric needle EMG, respiratory function test, cardiologic examination, and muscle biopsy. RESULTS: A novel TTN p.Gly30150Asp mutation was identified in the highly conserved A-band of titin that co-segregated with the disease in the U.S. family. Screening of 45 probands initially diagnosed as myofibrillar myopathy (MFM) but excluded based on molecular screening for the known MFM genes led to the identification of a previously reported TTN p.Cys30071Arg mutation in one patient. This same mutation was also identified in a patient with suspected HMERF. The p.Gly30150Asp and p.Cys30071Arg mutations are localized to a side chain of fibronectin type III element A150 of the 10th C-zone super-repeat of titin. CONCLUSIONS: Missense mutations in TTN are the cause of HMERF in families of diverse origins. A comparison of phenotypic features of HMERF caused by the three known TTN mutations in various populations allowed to emphasize distinct clinical/pathological features that can serve as the basis for diagnosis. The newly identified p.Gly30150Asp and the p.Cys30071Arg mutation are localized to a side chain of fibronectin type III element A150 of the 10th C-zone super-repeat of titin.

Case report: Death in the emergency department: an unrecognized awake malignant hyperthermia-like reaction in a six-year-old.

A healthy 6-year-old boy developed lower extremity rigidity, trismus, and fever after playing in a splash pool. On arrival in the emergency department, he appeared to be seizing. An endotracheal tube was emergently placed using succinylcholine. Cardiac arrest followed. He could not be resuscitated. Postmortem genetic analysis found a novel RYR1 variant. Family testing revealed the same variant in his father who also had muscle contracture testing diagnostic for susceptibility to malignant hyperthermia and central core disease diagnosed histologically. Because there was no exposure to volatile anesthetics before the onset of symptoms, this is a case of "awake" malignant hyperthermia worsened by succinylcholine.

Ryanodine receptor type 1 gene variants in the malignant hyperthermia-susceptible population of the United States.

BACKGROUND: Mutations in the ryanodine receptor type 1 gene (RYR1) that encodes the skeletal muscle-specific intracellular calcium (Ca(2+)) release channel are a cause of malignant hyperthermia (MH). In this study, we examined RYR1 mutations in a large number of North American MH-susceptible (MHS) subjects without prior genetic diagnosis. METHODS: RYR1 was examined in 120 unrelated MHS subjects from the United States in a tiered manner. The α-1 subunit of the dihydropyridine receptor gene (CACNA1S) was screened for 4 variants in subjects in whom no abnormality was found in ≥ 100 exons of RYR1. RESULTS: Ten known causative MH mutations were found in 26 subjects. Variants of uncertain significance in RYR1 were found in 36 subjects, 16 of which are novel. Novel variants in both RYR1 and CACNA1S were found in the 1 subject who died of MH. Two RYR1 variants were found in 4 subjects. Variants of uncertain significance were found outside and inside the hotspots of RYR1. Maximal contractures in the caffeine-halothane contracture test were greater in those who had a known MH mutation or variant of uncertain significance in RYR1 than in those who did not. CONCLUSIONS: The identification of novel RYR1 variants and previously observed RYR1 variants of uncertain significance in independent MHS families is necessary for demonstrating the significance of these variants for MH susceptibility and supports the need for functional studies of these variants. Continued reporting of the clinical phenotypes of MH is necessary for interpretation of genetic findings, especially because the pathogenicity of most of these genetic variants associated with MHS remains to be elucidated.

KBTBD13 interacts with Cullin 3 to form a functional ubiquitin ligase.

Autosomal dominant mutations in BTB and Kelch domain containing 13 protein (KBTBD13) are associated with a new type of Nemaline Myopathy (NEM). NEM is a genetically heterogeneous group of muscle disorders. Mutations causing phenotypically distinct NEM variants have previously been identified in components of muscle thin filament. KBTBD13 is a muscle specific protein composed of an N terminal BTB domain and a C terminal Kelch-repeat domain. The function of this newly identified protein in muscle remained unknown. In this study, we show that KBTBD13 interacts with Cullin 3 (Cul3) and the BTB domain mediates this interaction. Using ubiquitination assays, we determined that KBTBD13 participates in the formation of a Cul3 based RING ubiquitin ligase (Cul3-RL) capable of ubiquitin conjugation. Confocal microscopy of transiently expressed KBTBD13 revealed its co-localization with ubiquitin. Taken together, our results demonstrate that KBTBD13 is a putative substrate adaptor for Cul3-RL that functions as a muscle specific ubiquitin ligase, and thereby implicate the ubiquitin proteasome pathway in the pathogenesis of KBTBD13-associated NEM.

Investigation of the relationship between serum creatine kinase and genetic polymorphisms in military recruits.

Genetic polymorphisms may explain why certain individuals will develop exertional rhabdomyolysis (ER) or markedly elevated serum creatine kinase (CK) levels following exertion, while others in the same environment, performing the same exertion, do not. Prospectively, 499 recruits were evaluated during the initial fortnight of Army basic training. Serum CK levels were determined before and during that time. Eleven candidate genetic polymorphisms were studied and compared to CK levels. No subjects developed ER. Baseline CK was significantly greater in interleukin-6 G174C GG and myosin light chain kinase 2 (MLCK 2) AA subjects. Intertraining levels were significantly greater in angiotensin I-converting enzyme D/D and interleukin-6 GG subjects. Among African-Americans, those with MLCK2 AA had greater baseline CK (1,352 +/- 1,102.8 IU/L) than AC and CC genotypes (536.9 +/- 500.6). African-American men have the highest baseline levels and are more likely to have MLCK AA genotype. Whether this finding is associated with an increased incidence of ER requires further study.

Optimization of Omega-3 Index Levels in Athletes at the US Naval Academy: Personalized Omega-3 Fatty Acid Dosage and Molecular Genetic Approaches.

The Dietary Guidelines for Americans recommend increasing the intake of omega-3 polyunsaturated fatty acids. The Omega-3 Index (O3I) is one marker used to assess omega-3 status. The O3I national average is 4.3%, which translates into a high risk for developing cardiovascular disease. Research has reported an association between variants in the two desaturase encoding genes, fatty acid desaturase 1 and fatty acid desaturase 2 (FADS1/2), and the concentration of O3I. The aim of this study was to assess whether a personalized dosage of omega-3 supplementation would lead to an O3I ≥ 8%. A secondary aim was to identify if changes in O3I levels would be associated with either of the two FADS1/2 variants. METHODS: This interventional study had a pre- and post-intervention design to assess changes in O3I. Ninety participants completed demographic, biometrics, O3I, and genetic testing. Participants were provided a personalized dose of omega-3 supplements based on their baseline O3I. RESULTS: The majority (63%) of participants were 20 year old white males with an average O3I at baseline of 4.6%; the post-supplementation average O3I was 5.6%. The most frequent genetic variants expressed in the full sample for FADS1/2 were GG (50%) and CA/AA (57%). CONCLUSIONS: O3I was significantly increased following omega-3 supplementation. However, it was not possible to conclude whether the two FADS1/2 variants led to differential increases in OI3 or if a personalized dosage of omega-3 supplementation led to an O3I ≥ 8%, due to our study limitations.

Identical de novo mutation in the type 1 ryanodine receptor gene associated with fatal, stress-induced malignant hyperthermia in two unrelated families.

BACKGROUND: Mutations in the type 1 ryanodine receptor gene (RYR1) result in malignant hyperthermia, a pharmacogenetic disorder typically triggered by administration of anesthetics. However, cases of sudden death during exertion, heat challenge, and febrile illness in the absence of triggering drugs have been reported. The underlying causes of such drug-free fatal "awake" episodes are unknown. METHODS: De novo R3983C variant in RYR1 was identified in two unrelated children who experienced fatal, nonanesthetic awake episodes associated with febrile illness and heat stress. One of the children also had a second novel, maternally inherited D4505H variant located on a separate haplotype. Effects of all possible heterotypic expression conditions on RYR1 sensitivity to caffeine-induced Ca release were determined in expressing RYR1-null myotubes. RESULTS: Compared with wild-type RYR1 alone (EC50 = 2.85 ± 0.49 mM), average (± SEM) caffeine sensitivity of Ca release was modestly increased after coexpression with either R3983C (EC50 = 2.00 ± 0.39 mM) or D4505H (EC50 = 1.64 ± 0.24 mM). Remarkably, coexpression of wild-type RYR1 with the double mutant in cis (R3983C-D4505H) produced a significantly stronger sensitization of caffeine-induced Ca release (EC50 = 0.64 ± 0.17 mM) compared with that observed after coexpression of the two variants on separate subunits (EC50 = 1.53 ± 0.18 mM). CONCLUSIONS: The R3983C mutation potentiates D4505H-mediated sensitization of caffeine-induced RYR1 Ca release when the mutations are in cis (on the same subunit) but not when present on separate subunits. Nevertheless, coexpression of the two variants on separate subunits still resulted in a ∼2-fold increase in caffeine sensitivity, consistent with the observed awake episodes and heat sensitivity.

Nemaline myopathy type 6: clinical and myopathological features.

Nemaline myopathy (NEM) is one of the most common congenital myopathies. A unique subtype, NEM6, maps to chromosome 15q21-q23 in two pedigrees, but the causative gene has not been determined. We conducted clinical examination and myopathological studies in a new NEM family. Genotyping and gene screening were accomplished by searching known and 18 new candidate genes. The disease started in childhood by affecting proximal and distal muscles and causing slowness of movements. Muscle biopsies showed numerous nemaline rods and core-like formations. Suggestive linkage to chromosome 15q22-q23 was established. Genes known to be mutated in NEM or core-rod myopathy were screened and excluded. No pathogenic mutations were identified in other candidate genes. The disease in this Spanish family was classified as NEM6. It is phenotypically similar and probably allelic to the two previously reported NEM6 pedigrees. Further studies of these families will lead to the identification of the NEM6 gene.

Trauma, systemic inflammatory response syndrome, dietary supplements, illicit steroid use and a questionable malignant hyperthermia reaction.

BACKGROUND: Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated primarily, but not exclusively, with mutations in the skeletal muscle ryanodine receptor. Associated environmental factors, however, may also be important for expression of the syndrome. METHODS AND RESULTS: A 24-yr-old trauma patient developed a fulminant MH crisis after a 3 minute exposure to sevoflurane. A thorough evaluation of underlying co-morbidities revealed a number of environmental factors that could have altered skeletal muscle calcium regulation, and may have potentially influenced the effects of volatile inhaled anesthetics. Since MH is a syndrome characterized by abnormal skeletal muscle calcium regulation, other factors that alter calcium homeostasis may exacerbate the impact of inhaled MH-triggering drugs. CONCLUSIONS: While a thorough history of MH episodes in a proband and family is emphasized as part of a complete preanesthetic evaluation, obtaining a history of other environmental entities that may alter calcium regulation may be equally important to knowing the family history.

Pathogenic and rare deleterious variants in multiple genes suggest oligogenic inheritance in recurrent exertional rhabdomyolysis.

Exertional rhabdomyolysis is a metabolic event characterized by the release of muscle content into the circulation due to exercise-driven breakdown of skeletal muscle. Recurrent exertional rhabdomyolysis has been associated with metabolic myopathies and mitochondrial disorders, a clinically and genetically heterogeneous group of predominantly autosomal recessive, monogenic conditions. Although genetics factors are well recognized in recurrent rhabdomyolysis, the underlying causes and mechanisms of exercise-driven muscle breakdown remain unknown in a substantial number of cases. We present clinical and genetic study results from seven adult male subjects with recurrent exertional rhabdomyolysis. In all subject, whole exome sequencing identified multiple heterozygous variants in genes associated with monogenic metabolic and/or mitochondrial disorders. These variants consisted of known pathogenic and/or new likely pathogenic variants in combination with other rare deleterious alleles. The presence of heterozygous pathogenic and rare deleterious variants in multiple genes suggests an oligogenic inheritance for exertional rhabdomyolysis etiology. Our data imply that exertional rhabdomyolysis can reflect cumulative effects or synergistic interactions of deleterious variants in multiple genes that are likely to compromise muscle metabolism under the stress of exercise.

Multifactorial Origin of Exertional Rhabdomyolysis, Recurrent Hematuria, and Episodic Pain in a Service Member with Sickle Cell Trait.

Individuals with Sickle Cell Trait (SCT), generally considered a benign carrier state of hemoglobin S (HbAS), are thought to be at risk for exertional rhabdomyolysis and hematuria, conditions that can also be caused by various other acquired and inherited factors. We report an SCT positive service member with an exertional rhabdomyolysis event, recurrent hematuria with transient proteinuria, and episodic burning pain in the lower extremities. Clinical and genetic studies revealed the multifactorial nature of his complex phenotype. The service member was taking prescription medications known to be associated with exertional rhabdomyolysis. He carried a pathogenic mutation, NPHS2 p.V260E, reported in nephropathy and a new variant p.R838Q in SCN11A, a gene involved in familial episodic pain syndrome. Results suggest that drug-to-drug interactions coupled with the stress of exercise, coinheritance of HbAS and NPHS2 p.V260E, and p. R838Q in SCN11A contributed to exertional rhabdomyolysis, recurrent hematuria with proteinuria, and episodic pain, respectively. This case underscores the importance of comprehensive clinical and genetic evaluations to identify underlying causes of health complications reported in SCT individuals.

Genomewide scans in North American families reveal genetic linkage of essential tremor to a region on chromosome 6p23.

Essential tremor (ET) is the most prevalent adult-onset movement disorder showing evidence of non-random accumulation in some families. ET has previously been mapped to genetic loci on chromosomes 2p and 3q, but no causative genes identified. We conducted genomewide linkage screening with subsequent fine mapping in seven large North American families comprising a total of 325 genotyped individuals that included 65 patients diagnosed as definite ET. Linkage analysis was based on methodology implemented in SimWalk2 and LINKAGE programs. A multigenerational family revealed suggestive linkage to a locus on chromosome 6p23 with maximal nonparametric linkage (NPL) multipoint score 3.281 (P = 0.0005) and parametric multipoint log of the odds (LOD) score 2.983. A second family showed positive linkage to the same 6p23 region with a maximal NPL score 2.125 (P = 0.0075) and LOD score 1.265. Haplotype analysis led to the identification of a 600 kb interval shared by both families. Sequencing of coding regions of 15 genes located in the linked region detected numerous sequence variants, some of them predicting a change of the encoded amino acid, but each was also found in controls. Our findings provide evidence for linkage to a novel susceptibility locus on chromosome 6p23. Analysis of additional ET-affected families is needed to confirm linkage and identify the underlying gene.