Testicular adrenal rest tumours in boys, adolescents and adult men with congenital adrenal hyperplasia may be associated with the CYP21A2 mutation.

Congenital adrenal hyperplasia (CAH) is an autosomal recessive disorder with impaired function of the adrenal cortex caused by mutations in the CYP21A2 gene. Deficiency of steroid 21-hydroxylase accounts for 80-95% of CAH cases. Testicular adrenal rest tumours (TART) may be prevalent in up to 95% of CAH adults and may already appear during childhood. Whether genotype sub-types can account for the development of TART has not been investigated previously. We therefore investigated this by coupling clinical information of CAH patients with information of their genetic mutation. In 49 male patients (age 2.6-40.3 years) with 21-hydroxylase deficiency, testicular ultrasound examinations were performed and CYP21A2 genotypes determined. These were grouped according to the residual 21-hydroxylase activity: group Null (complete enzyme impairment), group A (almost complete enzyme impairment), group B (severe enzyme impairment) and group C (partial impairment). TART were observed in 27 of 49 patients (55%). For the 23 patients younger than 18 years, TART were present in 11 (48%), the youngest patient being 7.5 years old. The presence of TART was dependent on the CYP21A2 genotype: 27 of 37 patients (73%) with the most severe mutations (groups Null and A) had TART, whereas none of 12 patients with the milder mutations (groups B and C) had TART. We conclude that TART were most frequently detected in patients with severe CYP21A2 mutations, and may occur already in early childhood in such patients.

Limited diagnostic value of enzyme analysis in patients with mitochondrial tRNA mutations.

We evaluated the diagnostic value of respiratory chain (RC) enzyme analysis of muscle in adult patients with mitochondrial myopathy (MM). RC enzyme activity was measured in muscle biopsies from 39 patients who carry either the 3243A>G mutation, other tRNA point mutations, or single, large-scale deletions of mtDNA. Findings were compared with those obtained from asymptomatic relatives with the 3243A>G mutation, myotonic dystrophy patients, and healthy subjects. Plasma lactate concentration, maximal oxygen uptake, and ragged-red fibers/cytochrome c-negative fibers in muscle were also determined. Only 10% of patients with the 3243A>G point mutation had decreased enzyme activity of one or more RC complexes, whereas this was the case for 83% of patients with other point mutations and 62% of patients with deletions. Abnormal muscle histochemistry was found in 65%, 100%, and 85% of patients, respectively, in these three groups. The results indicate that RC enzyme analysis in muscle is not a sensitive test for MM in adults. In these patients, abnormal muscle histochemistry appears to be a better predictor ofMM.

Splice mutations preserve myophosphorylase activity that ameliorates the phenotype in McArdle disease.

Over 100 mutations in the myophosphorylase gene, which cause McArdle disease, are known. All these mutations have resulted in a complete block of muscle glycogenolysis, and accordingly, no genotype-phenotype correlation has been identified in this condition. We evaluated physiologic and genetic features of two patients with a variant form of McArdle disease, associated with unusually high exercise capacity. Physiologic findings were compared to those in 47 patients with typical McArdle disease, and 17 healthy subjects. Subjects performed an ischaemic forearm exercise test to assess lactate and ammonia production. Peak oxidative capacity (VO2max) and cardiac output were determined, using cycle ergometry as the exercise modality. The two patients with atypical McArdle disease carried common mutations on one allele (R50X and G205S), and novel splice mutations in introns 3 [IVS3-26A>G (c.425-26A>G)] and 5 [IVS5-601G>A (c.856-601G>A)] on the other allele. Plasma lactate after ischaemic exercise decreased in all typical McArdle patients, but increased in the two atypical McArdle patients (10% of that in healthy subjects). Peak workload and oxidative capacity were 2-fold higher in patients with atypical McArdle disease compared to typical McArdle patients. Oxygen uptake, relative to cardiac output, was severely impaired in the 47 patients with typical McArdle disease, and partially normalized in the milder affected McArdle patients. These findings identify the first distinct genotype-phenotype relationship in McArdle disease, and indicate that minimal myophosphorylase activity ameliorates the typical McArdle disease phenotype by augmenting muscle oxidative capacity. The milder form of McArdle disease provides important clues to the level of functional myophosphorylase needed to support muscle oxidative metabolism.

A novel missense mutation in SUCLG1 associated with mitochondrial DNA depletion, encephalomyopathic form, with methylmalonic aciduria.

Mitochondrial DNA depletion, encephalomyopathic form, with methylmalonic aciduria is associated with mutations in SUCLA2, the gene encoding a beta subunit of succinate-CoA ligase, where 17 patients have been reported. Mutations in SUCLG1, encoding the alpha subunit of the enzyme, have been reported in only one family, where a homozygous 2 bp deletion was associated with fatal infantile lactic acidosis. We here report a patient with a novel homozygous missense mutation in SUCLG1, whose phenotype is similar to that of patients with SUCLA2 mutations.

Mutations in the amiloride-sensitive epithelial sodium channel in patients with cystic fibrosis-like disease.

We investigated whether mutations in the genes that code for the different subunits of the amiloride-sensitive epithelial sodium channel (ENaC) might result in cystic fibrosis (CF)-like disease. In a small fraction of the patients, the disease could be potentially explained by an ENaC mutation by a Mendelian mechanism, such as p.V114I and p.F61L in SCNN1A. More importantly, a more than three-fold significant increase in incidence of several rare ENaC polymorphisms was found in the patient group (30% vs. 9% in controls), indicating an involvement of ENaC in some patients by a polygenetic mechanism. Specifically, a significantly higher number of patients carried c.-55+5G>C or p.W493R in SCNN1A in the heterozygous state, with odds ratios (ORs) of 13.5 and 2.7, respectively.The p.W493R-SCNN1A polymorphism was even found to result in a four-fold more active ENaC channel when heterologously expressed in Xenopus laevis oocytes. About 1 in 975 individuals in the general population will be heterozygous for the hyperactive p.W493R-SCNN1A mutation and a cystic fibrosis transmembrane conductance regulator (CFTR) gene that results in very low amounts (0-10%) functional CFTR. These ENaC/CFTR genotypes may play a hitherto unrecognized role in lung diseases.

Mitochondrial haplogroups: ischemic cardiovascular disease, other diseases, mortality, and longevity in the general population.

BACKGROUND: Rare mutations in the mitochondrial genome may cause disease. Mitochondrial haplogroups defined by common polymorphisms have been associated with risk of disease and longevity. We tested the hypothesis that common haplogroups predict risk of ischemic cardiovascular disease, morbidity from other causes, mortality, and longevity in a general population of European descent. METHODS AND RESULTS: We followed 9254 individuals from the Danish general population, in the Copenhagen City Heart Study, prospectively for risk of ischemic cardiovascular disease, morbidity from other causes, and mortality during 25 and 11 years, respectively. Haplogroup frequencies were as follows: H (45.9%), U (15.9%), T (9.9%), J (9.1), K (6.2%), V (4.5%), W/I (3.8%), and Z (3.5%). Hazard ratios for hospitalization due to all cardiovascular disorders (haplogroup U: 1.0 [95% confidence interval{CI}, 0.9 to 1.1]; T: 0.9 [95% CI, 0.8 to 1.0]; J: 1.0 [95% CI, 0.9 to 1.1]; K: 1.0 [95% CI, 0.9 to 1.2]; V: 1.0 [95% CI, 0.9 to 1.2]; W/I: 0.8 [95% CI, 0.7 to 1.0]; Z: 1.0 [95% CI, 0.8 to 1.2]), ischemic heart disease (U: 0.9 [95% CI, 0.8 to 1.1]; T: 0.9 [95% CI, 0.7 to 1.0]; J: 1.1 [95% CI, 0.9 to 1.2]; K: 1.1 [95% CI, 0.9 to 1.3]; V: 1.1 [95% CI, 0.9 to 1.4]; W/I: 1.1 [95% CI, 0.8 to 1.4]; Z: 1.1 [95% CI, 0.8 to 1.4]), and ischemic cerebrovascular disease (U: 1.1 [95% CI, 0.9 to 1.4]; T: 0.9 [95% CI, 0.7 to 1.2]; J: 1.1 [95% CI, 0.9 to 1.4]; K: 1.0 [95% CI, 0.8 to 1.4]; V: 1.1 [95% CI, 0.8 to 1.5]; W/I: 0.8 [95% CI, 0.5 to 1.3]; Z: 0.9 [95% CI, 0.6 to 1.4]) did not differ from 1.0 for any haplogroup versus the most common haplogroup H. Results were similar for hospitalization due to infectious and parasitic diseases, respiratory infections, respiratory disorders, malignant neoplasms, digestive disorders, musculoskeletal disorders, neuropsychiatric disorders, and miscarriages. Likewise, hazard ratios for death from all causes were not different from 1.0 for any haplogroup versus haplogroup H (U: 1.0 [95% CI, 0.8 to 1.1]; T: 0.9 [95% CI, 0.8 to 1.1]; J: 0.9 [95% CI, 0.8 to 1.1]; K: 1.0 [95% CI, 0.8 to 1.2]; V: 1.1 [95% CI, 0.9 to 1.3]; W/I: 0.8 [95% CI, 0.7 to 1.1]; Z: 0.9 [95% CI, 0.7 to 1.2]). Finally, after stratification by major causes of death, hazard ratios remained insignificant. CONCLUSIONS: Our results do not support an association of mitochondrial haplogroups with risk of ischemic cardiovascular disease, morbidity from other causes, mortality, or longevity in a large general population of European descent.

High-resolution melting facilitates mutation screening of PYGM in patients with McArdle disease.

Mutations in PYGM, encoding the muscle-specific glycogen phosphorylase (myophosphorylase), are responsible for McArdle disease. Among Caucasians, a large proportion of patients are homozygous for the R50X mutation, but other mutations can affect all the 20 exons of PYGM, making mutation detection laborious. We have developed a high-resolution melting (HRM) assay for mutation detection in PYGM. Twelve McArdle patients were investigated, in whom pre-screening had ruled out homozygosity or compound heterozygosity for the two common G205S and R50X mutations. In total, we identified 16 different variations. Thirteen of these are pathogenic, and three were classified as polymorphisms. Nine variations had not previously been described. One of the novel mutations, c.2430C > T, was initially predicted to result in a silent G810G change, but cDNA analysis demonstrated that the mutation led to abnormal mRNA processing. The HRM protocol reduced the need for direct sequencing by approximately 85%, and is a good approach to search for new mutations in PYGM.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy resulting in stroke in an 11-year-old male.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is caused by mutations in the Notch3 gene on chromosome 19. The condition manifests itself clinically typically in the third to fifth decade with migraine and recurrent episodes of stroke or transient ischaemic attacks. We report the case of an 11-year-old male with CADASIL resulting in stroke with right hemiparesis and dysphasia. Acute magnetic resonance imaging suggested infarction in the left hemisphere; magnetic resonance angiography revealed calibre variation of the intracerebral arteries. The patient suffered from common migraine with five to six attacks per month for 3 years 6 months before the stroke. Attacks occurred early in the morning with severe one-sided headache, photophobia, nausea, and vomiting. Antimigraine medications had no effect. The family history revealed more cases of CADASIL, with an autosomal dominant pattern. The diagnosis of CADASIL was confirmed by the finding of the known mutation of the Notch3 gene running in the family. With treatment in a neurorehabilitation centre the patient recovered most of his functions with only discrete fine-motor and cognitive sequelae. Our case report highlights the need for paediatricians to consider CADASIL in childhood stroke as well as in migraine patients.

cDNA analyses of CAPN3 enhance mutation detection and reveal a low prevalence of LGMD2A patients in Denmark.

Calpainopathy or limb-girdle muscular dystrophy type 2A (LGMD2A) is generally recognized as the most prevalent form of recessive LGMD and is caused by mutations in the CAPN3 gene. Out of a cohort of 119 patients fulfilling clinical criteria for LGMD2, referred to our neuromuscular clinic, 46 were suspected to have LGMD2A, based on western blot results. Four of these patients were shown to have LGMD2I upon molecular analysis, whereas 16 of the remaining 42 patients harbored mutations in CAPN3 by both direct genomic sequencing and cDNA analyses. In 10 patients, we identified both mutant alleles. In three other, only one heterozygous mutation could be identified on the genomic level; however, CAPN3 cDNA analyses demonstrated homozygosity for the mutant allele, indicating the presence of an unidentified allele that somehow compromise correct CAPN3 RNA processing. In the three remaining patients, only a single heterozygous mutation could be identified both at the genomic level and on full-length CAPN3 cDNA. All three patients exhibited a highly abnormal western blot for calpain-3 and clinical characteristics of LGMD2A. Only three of the genetically confirmed LGMD2A patients were of Danish origin, indicating a five- to sixfold lower prevalence in Denmark compared to other European countries. A total of 16 different CAPN3 mutations were identified, of which 5 were novel. The present study demonstrates the value of cDNA analysis for CAPN3 in LGMD2A patients and indicates that calpainopathy is an uncommon cause of LGMD in the Denmark.

Phenotype and clinical course in a family with a new de novo Twinkle gene mutation.

The Twinkle gene product is important for mtDNA replication. Only a few reports have investigated the clinically effect of mutations in this gene. We describe a new de novo mutation (1110C>A) in the PEO1 gene in a mother and her two sons. The mother had progressive ophthalmoplegia, limb weakness, sensory neuropathy, elevated resting plasma lactate, glucose intolerance and impaired VO2max while her sons only had mild ptosis. In accordance with the clinical presentation, abnormal morphological findings in muscle and multiple deletions and depletion of mtDNA in muscle were more pronounced in the proband than in her sons.

Atypical early-onset Alzheimer's disease caused by the Iranian APP mutation.

BACKGROUND: Approximately 1% of all cases of Alzheimer's disease are inherited autosomal dominantly, and to date, three causative genes have been found, the Presenilin 1 (PSEN1) gene, the Presenilin 2 (PSEN2) gene and the Amyloid precursor protein (APP) gene. We describe atypical phenotypic features in a family with a pathogenic APP gene mutation and discuss possible explanations for these atypical features. METHODS AND RESULTS: We report a family with a history of dementia compatible with autosomal dominant transmission. The disease course in the proband was not typical for Alzheimer's disease as the diagnosis was preceded by 8 years of an isolated amnesia. Further, the proband had epilepsy with complex partial seizures and central degenerative autonomic failure as determined by clinical physiology. Sequencing the three known causative Alzheimer genes revealed a pathogenic missense mutation, APP Thr714Ala (the Iranian mutation). CONCLUSIONS: The atypical clinical phenotype with long prodromal phase, autonomic failure and seizures in this new proband with the APP Thr714Ala mutation illustrates the clinical heterogeneity in families with identical pathogenic mutations.

Mitochondrial encephalomyopathy with elevated methylmalonic acid is caused by SUCLA2 mutations.

We have identified 12 patients with autosomal recessive mitochondrial encephalomyopathy with elevated methylmalonic acid. The disorder has a high incidence of 1 in 1700 in the Faroe Islands due to a founder effect, and a carrier frequency of 1 in 33. The symptoms comprise hypotonia, muscle atrophy, hyperkinesia, severe hearing impairment and postnatal growth retardation. Neuroimaging showed demyelination and central and cortical atrophy, including atrophy of the basal ganglia, and some of the patients fulfilled the criteria for Leigh syndrome. Urine and plasma methylmalonic acid were elevated. Homozygosity mapping with the Affymetrix 10 K array revealed a homozygous region on chromosome 13q14 harbouring the SUCLA2 gene. Mutations in SUCLA2 were recently shown to cause a similar disorder in a small Israeli family. Mutation analysis identified a novel splice site mutation in SUCLA2, IVS4 + 1G --> A, leading to skipping of exon 4. The SUCLA2 gene encodes the ATP-forming beta subunit of the Krebs cycle enzyme succinyl-CoA ligase. The hallmark of the condition, elevated methylmalonic acid, can be explained by an accumulation of the substrate of the enzyme, succinyl-CoA, which in turn leads to elevated methylmalonic acid, because the conversion of methylmalonyl-CoA to succinyl-CoA is inhibited.

Deletion of exon 16 of the dystrophin gene is not associated with disease.

The DNA of a male harbored a deletion of exon 16 as well as most of introns 15 and 16 of the dystrophin gene. The person was completely healthy, with universal normal muscle strength, and normal muscle histology and creatine kinase levels. The deletion was also present in DNA from a muscle biopsy, excluding mosaicism as an explanation for the phenotype. We conclude that the protein segment encoded by exon 16 of the dystrophin gene is of no importance for the essential function of dystrophin. The findings suggest that even large gene re-arrangements of the dystrophin gene may not always be disease-causing, and caution a diagnosis of dystrophinopathy in sporadic cases of single exon in-frame deletions.

Treatment of mitochondrial neurogastrointestinal encephalomyopathy with dialysis.

OBJECTIVE: To study the effect of continuous ambulatory peritoneal dialysis on nucleoside levels and clinical course in a patient with mitochondrial neurogastrointestinal encephalomyopathy (MNGIE). Patient We studied a patient with genetically verified MNGIE, who prior to treatment had lost weight progressively, developed amenorrhea, vomited multiple times daily, and had abdominal pain. Intervention The patient was treated with peritoneal dialysis for 3 years, and the effect on symptoms and plasma concentrations of thymidine and deoxyuridine were monitored. RESULTS: Dialysis stopped vomiting and reduced abdominal pain, and the patient gained 5 kg in weight and started to menstruate again. Symptoms returned if dialysis was paused. Dialysis did not affect plasma nucleoside levels. CONCLUSIONS: This study shows an unambiguous clinical benefit of peritoneal dialysis on gastrointestinal symptoms in MNGIE. Dialysis did not affect nucleoside levels, indicating elevated thymidine and deoxyuridine levels are not solely responsible for the pathogenesis of MNGIE.

N965S is a common ABCA4 variant in Stargardt-related retinopathies in the Danish population.

PURPOSE: The study was conducted to resolve the spectrum of ABCA4 mutations in a cohort of unrelated Danish residents with early-onset macular dystrophy. METHODS: A microarray technique was used to analyze known ABCA4 mutations in genomic DNA from a selected group of 161 unrelated individuals referred to the national low vision clinic. The clinical observation time varied from a single examination to follow-up over 35 years. RESULTS: Fifty-nine allegedly disease-associated ABCA4 variants were found in 197 alleles (61.2%) from 124 (77.0%) patients. Two or three mutations were present in 73 (45.3%) patients, and only one mutation was found in 51 (31.7%) patients. The mutation spectrum included 45 missense mutations, five nonsense mutations, two frame shift deletions, and seven splice site mutations. The relative frequency among the mutations varied considerably. Twenty-eight mutations occurred only once among 197 alleles, while the five most abundant mutations were encountered in 50% of the mutation-carrying alleles. The rate of mutation detection, assessed as the fraction of individuals carrying at least one ABCA4 mutation, varied from 27% to 90% among seven phenotypic groups, and a single mutation, p.N965S (c.2894A>G) in the first nucleotide-binding domain accounted for 16.2% of 197 disease-associated alleles. The mutation causes moderate to serious phenotypes and eventually blindness. CONCLUSIONS: Our study is the first mutation analysis of Stargardt-related retinopathies in a large cohort of patients from a Scandinavian population. The mutation detection rate, performed with an array-based technique, was comparable to that of other microarray-based ABCA4 studies as well as studies using more laborious techniques involving screening methods followed by sequencing. Four out of five of the most prevalent ABCA4 mutations are reported to be frequent in other Western European populations as well. However, the prevalence of the most common Danish mutation, N965S, significantly deviates from the one found in other studies. This underscores that the ABCA4 mutation spectrum within relatively stable populations might be skewed due to founder effects. The clinical spectrum of patients, who are either homozygous or compound heterozygous for the N965S mutation, indicates that this mutation has an early and profound effect on retinal function.

A 17q21.31 microduplication, reciprocal to the newly described 17q21.31 microdeletion, in a girl with severe psychomotor developmental delay and dysmorphic craniofacial features.

Array-CGH analysis using 244k Agilent oligoarray revealed a de novo 17q21.31 microduplication in a 10-year-old girl with severe psychomotor developmental delay, facial dysmorphism, microcephaly, abnormal digits and hirsutism. The duplication encompassed the MAPT and CRHR1 genes and was reciprocal to the recently described 17q21.31 microdeletion, associated with a recognizable clinical phenotype. Genotyping showed that the duplication was derived from non-allelic homologous recombination of paternal H1 and H2 haplotypes. To our knowledge this is the first report of a patient with a 17q21.31 microduplication.

Aerobic training is safe and improves exercise capacity in patients with mitochondrial myopathy.

Exercise intolerance is a prominent symptom in patients with mitochondrial myopathy (MM), but it is still unsettled whether exercise training is safe and beneficial for patients with MM. To address this, we studied the effect of 12 weeks cycle training on exercise capacity, quality of life and underlying molecular and cellular events in five patients with single large-scale deletions, one with a microdeletion and 14 with point mutations of mitochondrial DNA (mtDNA), and 13 healthy subjects. Each training session lasted 30 min, and was performed at an intensity of 70% of VO2max (maximal oxygen uptake). Each subject performed 50 training sessions in 12 weeks. All subjects were evaluated before and after training, and 13 MM patients were studied after 8 weeks of deconditioning. Evaluation included VO2max and mutation load and mtDNA quantity, mitochondrial enzymatic activity, and number of centrally nucleated, apoptotic, ragged red and cytochrome oxidase (COX)-negative fibres in muscle biopsies from the quadriceps muscle. After 12 weeks of training, VO2max and muscle citrate synthase increased in MM (26 and 67%) and healthy (17 and 65%) subjects, while mtDNA quantity in muscle only increased in the MM patients (81%). In the MM patients, training did not change mtDNA mutation load in muscle, mitochondrial enzyme complex activities, muscle morphology and plasma creatine kinase. After deconditioning, VO2max and citrate synthase activity returned to values before training, while muscle mtDNA mutation load decreased. These findings show that aerobic training efficiently improves oxidative capacity in MM patients. Based on unchanged levels of mutant load in muscle, morphological findings on muscle biopsy and plasma creatine kinase levels during training, the treatment appears to be safe. Regular, supervised aerobic exercise is therefore recommended in MM patients with the studied mutations.

Impaired cognitive function in women with congenital adrenal hyperplasia.

CONTEXT: Congenital adrenal hyperplasia (CAH) is a disorder with a wide spectrum of severity. OBJECTIVE: The objective of this study was to investigate cognitive function in CAH women. DESIGN: This was a case-control study. SETTING: This study was conducted at a tertiary center for pediatric endocrinology at the University Hospital of Copenhagen. PARTICIPANTS: Thirty-five Danish CAH women (age 17-51 yr) were included, and participation rate was 84%. Control women were recruited through the Danish Civil Registration System and matched on age and education. MAIN OUTCOME MEASURES: An abbreviated form of the Wechsler Adult Intelligence Scale was used, i.e. full-scale intelligence quotient (IQ; five of 11 subtests), which included three of six verbal IQ subtests and two of five performance IQ subtests. RESULTS: A significantly lower IQ was found in CAH patients compared with controls with respect to mean full-scale IQ (84.5 vs. 99.1; P < 0.001), mean verbal IQ (86.6 vs. 97.3; P < 0.001), and mean performance IQ (85.7 vs. 101.3; P < 0.001). The salt-wasting CAH group had lower IQ scores than the simple-virilizing CAH group, which reached significance for mean total IQ (81.2 vs. 92.8, P = 0.04) and mean verbal IQ (84.7 vs. 95.5, P = 0.05), and additionally, lower scores than the late-onset CAH group, which reached significance for performance IQ (mean 81.5 vs. 96.2, P = 0.02). CONCLUSIONS: Impaired cognitive function was observed in patients with CAH, especially in salt-wasting CAH. These intriguing findings may reflect adverse effects of hyponatremic episodes, suboptimal postnatal hormone replacement therapy or prenatal adrenal androgen excess, and the potential psychosocial consequences of the disorder.

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

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

Muscle phenotype and mutation load in 51 persons with the 3243A>G mitochondrial DNA mutation.

BACKGROUND: Mitochondrial disorders are generally not associated with a clear phenotype-genotype relationship, which complicates the understanding of the disease and genetic counseling. OBJECTIVE: To investigate the relationship between the muscle and blood mitochondrial DNA mutation load and phenotype. DESIGN: Survey. SETTING: The Neuromuscular Research Unit, Rigshospitalet, Copenhagen, Denmark. PARTICIPANTS: Fifty-one persons with the 3243A>G point mutation of mitochondrial DNA, and 20 healthy control subjects. METHODS: We recorded the maximal oxygen uptake (Vo(2)max), maximal workload, resting and peak-exercise plasma lactate levels, muscle and blood mutation load, muscle morphology, and presence of diabetes mellitus and hearing impairment in all subjects. RESULTS: Muscle mutation load (mean +/- SE, 50% +/- 5%; range, 2%-95%) correlated with Vo(2)max and resting plasma lactate level (P<.001; R>/=0.64). All persons except 5 with a muscle mutation load above 50% had abnormal Vo(2)max and morphology on muscle biopsy findings. Persons with hearing impairment and diabetes mellitus had a muscle mutation load above 65%. The mutation load in blood (mean +/- SE, 18% +/- 3%; range, 0%-61%) did not correlate with Vo(2)max, resting plasma lactate levels, or presence of hearing impairment or diabetes mellitus. CONCLUSIONS: This study demonstrates a close relationship between the muscle mutation load and phenotype in persons carrying the 3243A>G mutation. The lack of correlation between the mutation load in blood and symptoms from other tissues emphasizes the importance of assessing phenotype-genotype correlations in the same tissue in mitochondrial disease. The results indicate that the threshold of muscle mutation load at which oxidative impairment occurs can be as low as 50%, which is as much as 40% lower than that suggested by in vitro studies.