Biomedical subjects
Publications and source records attributed to Kathryn N North.
A common variant of the ACTN3 gene, R577X, results in complete deficiency of the alpha-actinin-3 protein in the fast skeletal muscle fibers of more than a billion humans worldwide. We review the evidence that this genetic variant is strongly associated with elite athlete status and with normal variation in human muscle strength and sprinting speed.
Myosin storage myopathy/hyaline body myopathy is a rare congenital myopathy, with less than 30 cases reported in the literature. It is characterised by the presence of subsarcolemmal hyaline bodies in type 1 muscle fibres and predominantly proximal muscle weakness. Recently, a single mutation (Arg1845Trp) in the slow/beta-cardiac myosin heavy chain gene (MYH7) was identified in four unrelated probands from Sweden and Belgium. The clinical severity and age of onset was variable, despite the same disease-causing mutation and similar histological findings. Here, we report the clinical and morphological findings of two brothers of English/Scottish background with the Arg1845Trp mutation in MYH7. This case report adds to the clinical description of this rare disorder and confirms that Arg1845Trp is a common mutation associated with this phenotype, at least in the White European population.
BACKGROUND: T2 hyperintensities (T2H) on MRI are the most common CNS lesions in individuals with neurofibromatosis type 1 (NF1). OBJECTIVES: The aim was to determine the frequency, signal characteristics and localization of T2H at different ages. In addition, we examined the sensitivity of different MR imaging sequences in detecting these lesions. MATERIALS AND METHODS: We studied prospectively a cohort of children, adolescents and young adults with NF1 using T2-volume (T2-V) and conventional MRI sequences. Lesions were designated as either discrete or diffuse, and the region of signal abnormality was recorded. A total of 103 patients were studied (age range 8.0-25.4 years, mean 13.9 years). RESULTS: The frequency, size, and intensity of T2H decreased with age in the basal ganglia (BG) and the cerebellum/brainstem (CB/BS). The majority of thalamic and CB/BS lesions were diffuse. Of the total cohort, 80% had diffuse bilateral hippocampal hyperintensities and 18.4% had hemispheric lesions best demonstrated on FLAIR; there was no significant difference in the frequency or signal intensity of hemispheric lesions with age. CONCLUSION: Lesions in the cerebral hemispheres and hippocampus imaged by MR do not change in prevalence over time, suggesting a different pathological basis from the lesions in the in BG and CB/BS that resolve with age. FLAIR and T2-V sequences are more sensitive in detecting lesions than standard T2-weighted sequences.
Cardiac surgery is performed in approximately 770,000 adults and 30,000 children in the United States of America annually. In this review we outline the mechanistic links between post-operative myocardial stunning and the development of myocardial edema. These interrelated processes cause a decline in myocardial performance that account for significant morbidity and mortality after cardiac surgery. Factors leading to myocardial edema include hemodilution, ischemia and reperfusion as well as osmotic gradients arising from pathological change. Several members of the aquaporin family of water transport proteins have been described in the myocardium although their role in the pathogenesis and resolution of cardiac edema is not established. This review examines evidence for the involvement of aquaporins in myocardial water handling during normal and pathological conditions.
Water accumulation in the heart is important in ischemia-reperfusion injury and operations performed by using cardiopulmonary bypass, with cardiac dysfunction associated with myocardial edema being the principal determinant of clinical outcome. As an initial step in determining the role of aquaporin (AQP) water channels in myocardial edema, we have assessed the myocardial expression of AQPs in humans, rats, and mice. RT-PCR revealed expression of AQP-1, -4, -6, -7, -8, and -11 transcripts in the mouse heart. AQP-1, -6, -7, and -11 mRNAs were found in the rat heart as well as low levels of AQP-4 and -9. Human hearts contained AQP-1, -3, -4, -5, -7, -9, -10, and -11 mRNAs. AQP-1 protein expression was confirmed by Western blot analysis in all three species. AQP-4 protein was detected in the mouse heart but not in the rat or human heart. To determine the potential functional consequences of myocardial AQP expression, water permeability was measured in plasma membrane vesicles from myocardial cells of wild-type versus various AQP knockout mice. Water permeability was reduced by AQP-1 knockout but not by AQP-4 or AQP-8 knockout. With the use of a model of isolated rat heart perfusion, it was found that osmotic and ischemic stresses are not associated with changes in AQP-1 or AQP-4 expression. These studies support a possible functional role of AQP-1 in myocardium but indicate that early adaptations to osmotic and ischemic stress do not involve transcriptional or posttranslational AQP-1 regulation.
Nemaline Myopathy with Intranuclear Rods is a rare variant of nemaline myopathy, due in almost all instances to mutation of ACTA1, the gene encoding skeletal muscle alpha-actin. We describe the novel autosomal dominant occurrence in a three-generation kindred, and review previously reported cases. Onset of myopathic symptoms in our kindred was in infancy or early childhood. Beyond infancy, limb muscle weakness was non-disabling and minimally progressive. A tall thin face and facial myopathy were prominent features in the affected adults. By light microscopy, muscle biopsies ranged from almost normal, to chronic myopathy with sarcoplasmic and intranuclear rods. A heterozygous GTG-ATG mutation (Val163Met) was found in exon 4 of ACTA1 in affected individuals. Actin is normally present within the nucleus in only trace amounts. Mutation at postion 163 may result in intranuclear rods by virtue of its close proximity to a nuclear export signal within the actin molecule.
OBJECTIVE: Our first objective was to determine whether SEPN1 gene mutations are a cause of congenital fiber-type disproportion (CFTD), a rare form of congenital myopathy in which relative hypotrophy of type 1 (slow twitch) muscle fibers is the principal abnormality on histology. Second, we investigated an association between SEPN1-related myopathy and insulin resistance. METHODS: We sequenced SEPN1 in five unrelated CFTD patients with scoliosis and respiratory muscle weakness and screened an additional 22 CFTD patients for abnormalities in SEPN1 by Western blotting and restriction digest for the 943G-->A mutation. We performed oral glucose tolerance tests (OGTTs) in eight SEPN1-related myopathy patients. RESULTS: Two sisters with CFTD were homozygous for the 943G-->A SEPN1 mutation and had clinical features typical of previously reported patients with SEPN1-related myopathy. Five of eight SEPN1-related myopathy patients had abnormalities on OGTT suggestive of insulin resistance. INTERPRETATION: SEPN1 is the second genetic cause of CFTD and the first cause of autosomal recessive CFTD to be identified to our knowledge. CFTD is the fourth clinicopathological presentation that can be associated with mutations in SEPN1. Insulin resistance may be a specific, previously unrecognized aspect of SEPN1-related myopathy.
Cognitive deficits are the most common complication in children with neurofibromatosis type 1 (NF1), and academic achievement is broadly affected. There is a lack of consensus in the literature regarding the frequency of general and specific learning disabilities, which seems to be related to the lack of a consensus on diagnostic criteria. The present study examined the frequency of specific learning disabilities (SLDs) in NF1, using an intellect-achievement discrepancy for diagnosis, as well as general learning difficulty associated with a lowering of general intellectual ability. The cohort consisted of 81 children with NF1 (43 males, 38 females; mean age 11y 6mo [SD 2y 4mo]; range 8y-16y 9mo) and 49 comparison children (20 males, 29 females; mean age 12y [SD 2y 6mo]; range 8y 2mo-16y 8mo). Problems with academic achievement were present in 52% of children with NF1; however, only 20% of the children with NF1 were diagnosed with an SLD (32% had more general learning problems). Only males with NF1 were at significant risk for SLD, and Verbal IQ<Performance IQ discrepancies were predictive of the presence of an SLD. There was a significant comorbidity of literacy-based learning disabilities and attention-deficit-hyperactivity disorder. We were able to define three subtypes of children with NF1 having distinct cognitive profiles, each with important implications for assessment and remediation.
Mutations in the dysferlin (DYSF) and caveolin-3 (CAV3) genes are associated with muscle disease. Dysferlin is mislocalized, by an unknown mechanism, in muscle from patients with mutations in caveolin-3 (Cav-3). To examine the link between Cav-3 mutations and dysferlin mistargeting, we studied their localization at high resolution in muscle fibers, in a model muscle cell line, and upon heterologous expression of dysferlin in muscle cell lines and in wild-type or caveolin-null fibroblasts. Dysferlin shows only partial overlap with Cav-3 on the surface of isolated muscle fibers but co-localizes with Cav-3 in developing transverse (T)-tubules in muscle cell lines. Heterologously expressed dystrophy-associated mutant Cav3R26Q accumulates in the Golgi complex of muscle cell lines or fibroblasts. Cav3R26Q and other Golgi-associated mutants of both Cav-3 (Cav3P104L) and Cav-1 (Cav1P132L) caused a dramatic redistribution of dysferlin to the Golgi complex. Heterologously expressed epitope-tagged dysferlin associates with the plasma membrane in primary fibroblasts and muscle cells. Transport to the cell surface is impaired in the absence of Cav-1 or Cav-3 showing that caveolins are essential for dysferlin association with the PM. These results suggest a functional role for caveolins in a novel post-Golgi trafficking pathway followed by dysferlin.
Mutations in alpha-skeletal actin (ACTA1) underlie several congenital muscle disorders including nemaline myopathy (NM). Almost all ACTA1-NM patients have normal cardiac function, and, even lethally affected congenital NM patients exhibit an unremarkable gestation with decreased foetal movement just prior to birth. Although alpha-skeletal actin is thought to be the predominant sarcomeric actin in human heart (Boheler KR, Carrier L, de la Bastie D, et al. Skeletal actin mRNA increases in the human heart during ontogenic development and is the major isoform of control and failing adult hearts. J Clin Invest 1991;88:323-30 ), ACTA1-NM patients almost never exhibit a cardiac phenotype. In this study, we define the relative expression of skeletal and cardiac actin proteins in human heart and skeletal muscle. We show that alpha-cardiac actin is the predominant sarcomeric isoform in human donor hearts and in early foetal skeletal muscle development. Skeletal actin is the predominant isoform from 25 to 27 weeks gestation and is the exclusive isoform expressed in muscle from infancy through to adulthood. These findings are consistent with clinical observations of NM patients and assist us to better understand the pathogenesis of inherited myopathies and cardiomyopathies with mutations in actin.
OBJECTIVE: To assess the frequency and severity of specific cognitive deficits in children with neurofibromatosis type 1 (NF1) in a large unbiased cohort. METHODS: Extensive cognitive assessments were performed in 81 children with NF1 ages 8 to 16 years and their performance was compared with that of 49 unaffected sibling controls. RESULTS: Eighty-one percent of the children with NF1 had moderate to severe impairment in one or more areas of cognitive functioning. Although 51% of children with NF1 performed poorly on tasks of reading, spelling, and mathematics, specific learning disabilities (as defined by IQ-achievement discrepancies) were present in only 20% of the children. Sustained attention difficulties were present in 63% of children with NF1, with 38% of children with NF1 fulfilling the diagnostic criteria for attention deficit-hyperactivity disorder. The NF1 neuropsychological profile is characterized by deficits in perceptual skills (visuospatial and visuoperceptual), executive functioning (planning and abstract concept formation), and attention (sustained and switching). Interestingly, both verbal and visual memory was unaffected in NF1 children, and their memory skills were in general stronger than their level of general intellectual function. Although both expressive and receptive language skills were significantly impaired in NF1 children, they appeared to be relatively better preserved than visuospatial abilities once IQ is taken into account. CONCLUSION: There is an extremely high frequency of cognitive problems in children with neurofibromatosis type 1, making cognitive dysfunction the most common complication to affect quality of life in these children.
Physical fitness is a complex phenotype influenced by a myriad of environmental and genetic factors, and variation in human physical performance and athletic ability has long been recognised as having a strong heritable component. Recently, the development of technology for rapid DNA sequencing and genotyping has allowed the identification of some of the individual genetic variations that contribute to athletic performance. This review will examine the evidence that has accumulated over the last three decades for a strong genetic influence on human physical performance, with an emphasis on two sets of physical traits, viz. cardiorespiratory and skeletal muscle function, which are particularly important for performance in a variety of sports. We will then review recent studies that have identified individual genetic variants associated with variation in these traits and the polymorphisms that have been directly associated with elite athlete status. Finally, we explore the scientific implications of our rapidly growing understanding of the genetic basis of variation in performance.
Nemaline myopathy is a human neuromuscular disorder associated with muscle weakness, Z-line accumulations (rods), and myofibrillar disorganization. Disease-causing mutations have been identified in genes encoding muscle thin filament proteins: actin, nebulin, slow troponin T, betaTropomyosin, and alphaTropomyosin(slow). Skeletal muscle expresses three tropomyosin (Tm) isoforms from separate genes: alphaTm(fast)(alphaTm, TPM1), betaTm (TPM2), and alphaTm(slow) (gammaTm, TPM3). In this article, we show that the level of betaTm, but not alphaTm(fast) protein, is reduced in human patients with mutations in alphaTm(slow) and in a transgenic mouse model of alphaTm(slow)(Met9Arg) nemaline myopathy. A postnatal time course of Tm expression in muscles of the mice indicated that the onset of alphaTm(slow)(Met9Arg) expression coincides with the decline of betaTm. Reduction of betaTm levels is independent of the degree of pathology (rods) within a muscle and is detected before the onset of muscle weakness. Thus, reduction in the level of betaTm represents an early clinical diagnostic marker for alphaTm(slow)-based mutations. Examinations of tropomyosin dimer formation using either recombinant proteins or sarcomeric extracts show that the mutation reduces the formation of the preferred alpha/beta heterodimer. We suggest this perturbation of tropomyosin isoform levels and dimer preference alters sarcomeric thin filament dynamics and contributes to muscle weakness in nemaline myopathy.
We describe a four-generation family with a previously unreported form of congenital fiber-type disproportion that follows an X-linked inheritance pattern. Affected male family members have a striking pattern of weakness. From birth there is marked ptosis, facial weakness, poor sucking, hypotonia, respiratory weakness, and relatively preserved limb strength. Most affected male individuals die of respiratory failure within the first months of life. A mild dilated cardiomyopathy developed in infancy in the sole surviving affected male member of this family. Some carrier female individuals manifest milder signs. We have demonstrated linkage to two regions of the X chromosome, Xp22.13 to Xp11.4 and Xq13.1 to Xq22.1, with a maximum logarithm of odds score of 3.25 in the latter region. We propose that clinical clues can differentiate this disorder from other forms of congenital fiber-type disproportion so that affected families can receive appropriate genetic counseling.
BACKGROUND: Carnitine palmitoyltransferase-II deficiency (CPT-II deficiency) is a rare disorder of lipid metabolism, in which the accumulation of long-chain acylcarnitines is a diagnostic marker. HPLC with fluorescence detection is an attractive analysis method due to its favorable combination of sensitivity, specificity, ease of analysis and minimal capital equipment costs. METHODS: Long-chain acylcarnitines were isolated from tissue homogenates (0.5-2 mg wet weight) or plasma (50 microl) using silica gel columns and derivatized with 2-(2,3-naphthalimino)ethyl trifluoromethanesulfonate. Quantitation was by HPLC and fluorescence detection with standard curves (0.0-5.0 nmol/ml) for myristoyl-, palmitoleoyl-, palmitoyl-, oleoyl- and stearoylcarnitine using heptadecanoylcarnitine as the internal standard. RESULTS: Significantly greater amounts of long-chain acylcarnitines were quantified in patients with CPT-II deficiency when compared to controls; e.g. (nmol/ml in patient plasma, controls mean+/-standard deviation): myristoylcarnitine (0.3, not detectable), palmitoleoylcarnitine (0.5, 0.1+/-0.1), palmitoylcarnitine (0.9, 0.1+/-0.0), oleoylcarnitine (3.0, 0.2+/-0.1), stearoylcarnitine (0.4, not detectable). CONCLUSIONS: This method can be used to quantitate long-chain acylcarnitines, illustrating their accumulation in CPT-II deficiency. The analysis was accomplished using inexpensive and widely available instrumentation and is appropriate for research investigators who require precise quantitation of long-chain acylcarnitines in complex biological samples.
The syntrophins and alpha-dystrobrevin form a subcomplex with dystrophin at the skeletal muscle membrane, and are also highly concentrated at the neuromuscular synapse. Here we demonstrate that the different syntrophins and alpha-dystrobrevin isoforms have distinct expression patterns during human skeletal muscle development, and are differentially affected by loss of dystrophin anchorage and denervation in human neuromuscular disease. During normal fetal development, and in Duchenne muscular dystrophy and denervation disorders, alpha1-syntrophin and alpha-dystrobrevin are absent or markedly reduced at the sarcolemmal membrane. beta1-Syntrophin is the predominant syntrophin isoform expressed at the muscle membrane during development, and it undergoes upregulation in response to loss of alpha1-syntrophin in Duchenne muscular dystrophy and in denervation. Upregulation of beta1-syntrophin in neuromuscular disorders is associated with re-expression of the fetal nicotinic acetylcholine receptor gamma-subunit, cardiac actin, and neonatal myosin, suggesting reversion of muscle fibers to an immature phenotype. We show that denervation specifically affects expression of the syntrophin-dystrobrevin subcomplex and does not affect levels or localization of other members of the dystrophin-associated protein complex. Our results confirm that dystrophin is required for anchorage of the syntrophin-dystrobrevin subcomplex and suggest that expression of the syntrophin-dystrobrevin complex may be independently regulated through neuromuscular transmission.
Mutations in the three collagen VI genes COL6A1, COL6A2 and COL6A3 cause Bethlem myopathy and Ullrich congenital muscular dystrophy (UCMD). UCMD, a severe disorder characterized by congenital muscle weakness, proximal joint contractures and marked distal joint hyperextensibility, has been considered a recessive condition, and homozygous or compound heterozygous mutations have been defined in COL6A2 and COL6A3. In contrast, the milder disorder Bethlem myopathy shows clear dominant inheritance and is caused by heterozygous mutations in COL6A1, COL6A2 and COL6A3. This model, where dominant mutations cause mild Bethlem myopathy and recessive mutations cause severe UCMD was recently challenged when a patient with UCMD was shown to have a heterozygous in-frame deletion in COL6A1. We have studied five patients with a clinical diagnosis of UCMD. Three patients had heterozygous in-frame deletions in the N-terminal region of the triple helical domain, one in the alpha1(VI) chain, one in alpha2(VI) and one in alpha3(VI). Collagen VI protein biosynthesis and assembly studies showed that these mutations act in a dominant negative fashion and result in severe collagen VI matrix deficiencies. One patient had recessive amino acid changes in the C2 subdomain of alpha2(VI), which prevented collagen VI assembly. No collagen VI mutations were found in the fifth patient. These data demonstrate that rather than being a rare cause of UCMD, dominant mutations are common in UCMD, now accounting for four of the 14 published cases. Mutation detection in this disorder remains critical for accurate genetic counseling of patients and their families.
We have studied a cohort of nemaline myopathy (NM) patients with mutations in the muscle alpha-skeletal actin gene (ACTA1). Immunoblot analysis of patient muscle demonstrates increased gamma-filamin, myotilin, desmin and alpha-actinin in many NM patients, consistent with accumulation of Z line-derived nemaline bodies. We demonstrate that nebulin can appear abnormal secondary to a primary defect in actin, and show by isoelectric focusing that mutant actin isoforms are present within insoluble actin filaments isolated from muscle from two ACTA1 NM patients. Transfection of C2C12 myoblasts with mutant actin(EGFP) constructs resulted in abnormal cytoplasmic and intranuclear actin aggregates. Intranuclear aggregates were observed with V163L-, V163M- and R183G-actin(EGFP) constructs, and modeling shows these residues to be adjacent to the nuclear export signal of actin. V163L and V163M actin mutants are known to cause intranuclear rod myopathy, however, intranuclear bodies were not reported in patient R183G. Transfection studies in C2C12 myoblasts showed significant alterations in the ability of V136L and R183G actin mutants to polymerize and contribute to insoluble actin filaments. Thus, we provide direct evidence for a dominant-negative effect of mutant actin in NM. In vitro studies suggest that abnormal folding, altered polymerization and aggregation of mutant actin isoforms are common properties of NM ACTA1 mutants. Some of these effects are mutation-specific, and likely result in variations in the severity of muscle weakness seen in individual patients. A combination of these effects contributes to the common pathological hallmarks of NM, namely intranuclear and cytoplasmic rod formation, accumulation of thin filaments and myofibrillar disorganization.