[Phosphofructokinase (PFK)]

This review is aimed to provide the up-to-date knowledge on phosphofructokinase (PFK), a key enzyme of glycolysis, with special references to the recent advances of molecular biology of the enzyme. In human, three isozymes named M (muscle), L (liver) and P (platelet) are identified. Recently, mRNA and gene structures of these isozymes have been clarified. Deficiency of PFK-M is characterized by muscle weakness due to fuel crisis in exercising muscles. Up to now, gene defects of PFK deficient patients have been sought in 38 alleles from Japanese, Ashkenazi Jewish, Non-Ashkenazi Italian, French Canadian and Swiss patients and molecular heterogeneity has been elucidated. Down syndrome, in which trisomy of chromosome 21 is known provides us an interesting gene-dosage effect on PFK-L isozyme. Other various pathologic states affecting PFK activity in vivo are also reviewed briefly.

Phosphofructokinase deficiency: recent advances in molecular biology

Phosphofructokinase (PFK) plays a major role in glycolysis. Deficiency of PFK-M is characterized by muscle weakness due to fuel crisis in exercising muscles. To elucidate the gene defect of PFK-deficient patients, we have cloned and determined the complete structure and transcription mechanism of human PFK-M mRNA and gene. Molecular defects were investigated in three unrelated Japanese family cases. The first case was characterized by a point mutation at the donor site of intron 15 of the PFK-M gene. Cryptic splicing resulted in a 25 amino acid truncation in the patient's PFK-M. The second case possessed a point mutation at the donor site of intron 19, resulting in the skipping of exon 19 and the truncation of 55 amino acids. In the third case, a missense mutation was identified in the coding region. The review of an updated mutation repertoire indicates the heterogeneity of the molecular mechanism of the disease.

Functional expression of human mutant phosphofructokinase in yeast: genetic defects in French Canadian and Swiss patients with phosphofructokinase deficiency

Human phosphofructokinase (PFK) is a tetrameric enzyme, encoded by muscle, liver, and platelet genes. Deficiency of muscle PFK (PFK-M), glycogenosis type VII (Tarui disease), is an autosomal recessive disorder characterized by an exertional myopathy and hemolytic syndrome. Several disease-causing mutations have been identified in the PFK-M gene in Japanese, Ashkenazi Jewish, and Italian patients. We describe the genetic defects in French Canadian and Swiss patients with the disease, and we use a genetically well-defined yeast system devoid of endogenous PFK for structure-function studies of the mutant PFKs. A G-to-A transition at codon 209-in exon 8 of the PFK-M gene, changing an encoded Gly to Asp, is responsible for the disease in a homozygous French Canadian patient. Gly-209-mutated protein is completely inactive in the yeast system. The Swiss patient is a genetic compound, carrying a G-to-A transition at codon 100 in exon 6 (Arg to Gln) and a G-to-A transition at codon 696 in exon 22 (Arg to His). The mutants expressed in yeast generate functional enzyme with modest changes in thermal stability. The advantages and limitations of the yeast system for expression of human mutant PFKs are discussed.

Glycolytic defects in muscle: aspects of collaboration between basic science and clinical medicine

The molecular heterogeneities of enzyme abnormality have been identified successfully since 1990 for major clinical entities of glycogenolytic and glycolytic defects in skeletal muscle. The interchange between clinical medicine and basic science, which enabled these achievements, has a long history. This review introduces several important examples of this interchange, which has borne much fruit in the comprehensive understanding of glycogenolysis-glycolysis in skeletal muscle and the related defects that cause various metabolic diseases. For instance, the presence of "glycogen synthase" was mainly suggested by the pathophysiology of McArdle's disease. Clinical manifestations of muscle phosphofructokinase (PFK) deficiency have indicated that there could be PFK isozymes under separate genetic control. Although glycolysis is a unidirectional pathway, enzyme defects at each step do not necessarily cause similar manifestations. Glycogen accumulation is mostly associated with enzyme defects in glycogenolysis and in the first stage of glycolysis. Since the original report of phosphoglycerate mutase deficiency in 1981, no newly recognized glycolytic defects have been presented. Glycolytic steps for which no enzyme deficiency has been identified seem to provide another important impetus for further study of "fail-safe" mechanisms in regard to monogenic disorders.

A new variant case of muscle phosphofructokinase deficiency, coexisting with gastric ulcer, gouty arthritis, and increased hemolysis

Muscle phosphofructokinase (PFK) deficiency includes both clinically and genetically heterogeneous conditions. A 22-year-old man with muscle PFK deficiency due to previously unrecognized mutation was admitted because of gastric ulcer. He had noticed mild fatigability on vigorous exercise, but had never experienced painful cramps and myoglobinuria. His history included five time relapses of gastric ulcer and gouty arthritis at ages 19 and 21 years. His laboratory data showing impaired muscle glycolysis, increased hemolysis, and myogenic hyperuricemia had aspects in common with those reported for the classic form of this disease, except that lactate concentrations in his blood increased considerably after exercise. The mutant PFK enzyme of this patient, who was demonstrated to have a missense mutation, could exert some catalytic activity that permitted glycolytic flux in vivo, thus leading to the absence of typical myopathic symptoms. The association of relapsing gastric ulcer with muscle PFK deficiency was detected for the first time. There is a possibility that oxygen radical-induced tissue damage resulting from increased hypoxanthine on exertion plays a role in the pathogenesis of ulceration, since the patient is more tolerant to exercise than reported cases with the classic form of muscle PFK deficiency.

Mutations in muscle phosphofructokinase gene

Mutations in the muscle phosphofructokinase gene (PFK-M) result in a metabolic myopathy characterized by exercise intolerance and compensated hemolysis. PFK deficiency, glycogenosis type VII (Tarui disease) is a rare, autosomal, recessively inherited disorder. Multiple mutations, including splicing defects, frameshifts, and missense mutations, have recently been identified in patients from six different ethnic backgrounds establishing genetic heterogeneity of the disease. There is no obvious correlation between the genotype and phenotypic expression of the disease. PFK-M deficiency appears to be prevalent among people of Ashkenazi Jewish descent. Molecular diagnosis is now feasible for Ashkenazi patients who share two common mutations in the gene; the more frequent is an exon 5 splicing defect, which accounts for approximately 68% of mutant alleles in this population.

Various classes of mutations in patients with phosphofructokinase deficiency (Tarui's disease)

Muscle phosphofructokinase (PFK-M) deficiency (glycogenosis type VII, Tarui's disease) is characterized by intolerance to vigorous exercise, often accompanied by myoglobinuria. The disease is inherited as an autosomal recessive trait. The clinical manifestations are similar to those in myophosphorylase deficiency (McArdle's disease), and the diagnosis required demonstration of the enzyme defect in muscle biopsy. In the Western hemisphere PFK deficiency appears to be prevalent among people of Ashkenazi Jewish descent. To define the molecular basis of this myopathy, we have studied 11 Ashkenazi and 2 non-Ashkenazi families with the disease. Ashkenazi patients share two common pathogenic mutations, a splicing defect and a nucleotide deletion, which account for approximately 95% of mutant alleles. The molecular diagnosis is now possible in this population by using simple PCR-based tests to screen for these mutations.

An oxidative defect in metabolic myopathies: diagnosis by noninvasive tissue oximetry

Metabolic myopathies due to a variety of enzymatic deficiencies are well recognized. The dynamics of oxygen delivery and utilization during exercise have not been observed previously in these disorders. We used a noninvasive optical technique to measure oxygen consumption in the exercising limb in normal subjects and patients with metabolic myopathies. We measured near-infrared spectra of hemoglobin in the gastrocnemius muscle during treadmill exercise in 10 normal subjects, 1 patient with cytochrome c oxidase deficiency, 2 patients with myophosphorylase deficiency, 3 patients with phosphofructokinase deficiency, and 2 patients with carnitine palmityl transferase deficiency. All normal subjects demonstrated a sustained deoxygenation during exercise, indicating an efficient utilization of delivered oxygen. The patient with cytochrome c oxidase deficiency demonstrated consistent oxygenation during exercise, indicating an underutilization of delivered oxygen. In the patients with myophosphorylase or phosphofructokinase deficiency, abnormal oxygenation during exercise indicated an oxidative defect due to a lack of pyruvate production. In the patients with myophosphorylase deficiency, changes in oxidation coincident with glucose utilization and "the second wind phenomenon" were observed. Patients with carnitine palmityl transferase deficiency demonstrated a normal deoxygenation during exercise. Noninvasive tissue oximetry during exercise demonstrates specific abnormalities in a variety of metabolic myopathies, indicating abnormal oxygen utilization, and will be a useful addition to the clinical investigation of exercise intolerance.

Metabolic and work capacity of skeletal muscle of PFK-deficient dogs studied in situ

Mechanical and metabolic relationships of muscle lacking phosphofructokinase (PFKD) activity were compared with muscle having normal phosphofructokinase (NORM) activity by using the gastrocnemius-plantaris muscle group with isolated circulation in situ. Muscle contractile properties were similar in both groups. Initial power output (W) during repetitive tetanic (200 ms, 50 impulses/s) isotonic contractions was similar in both groups; however, W declined significantly more (30-80%) in PFKD than in NORM muscle over time, with a constant O2 uptake (VO2)/W. Despite similar O2 and substrate delivery, PFKD muscle had a lower VO2 (42-55%), less glucose uptake, similar free fatty acid uptake, and lactic acid uptake rather than output, during contractions. Muscle venous H+ concentration, strong ion difference, and PCO2 increased during contractions, the magnitude of change being smaller in PFKD muscle. Elevating arterial lactate concentration before contractions in PFKD muscle resulted in significant improvements in W and VO2 without altering the acid-base exchange at the muscle. Increasing O2 delivery by increasing arterial O2 concentration in PFKD dogs did not improve W or VO2. We conclude that, despite no inherent mechanical or contractile differences, PFKD muscle has a severely limited oxidative capacity and exaggerated fatigue and blood flow responses to contractions due to limited substrate metabolism resulting from the inability to utilize glycogen and/or glucose.

Common mutations in the phosphofructokinase-M gene in Ashkenazi Jewish patients with glycogenesis VII--and their population frequency

Phosphofructokinase (PFK) catalyzes the rate-limiting step of glycolysis. Deficiency of the muscle enzyme is manifested by exercise intolerance and a compensated hemolytic anemia. Case reports of this autosomal recessive disease suggest a predominance in Ashkenazi Jews in the United States. We have explored the genetic basis for this illness in nine affected families and surveyed the normal Ashkenazi population for the mutations we have found. Genomic DNA was amplified using PCR, and denaturing gradient-gel electrophoresis was used to localize exons with possible mutations. The polymorphic exons were sequenced or digested with restriction enzymes. A previously described splicing mutation, delta 5, accounted for 11 (61%) of 18 abnormal alleles in the nine families. A single base deletion leading to a frameshift mutation in exon 22 (delta C-22) was found in six of seven alleles. A third mutation, resulting in a nonconservative amino acid substitution in exon 4, accounted for the remaining allele. Thus, three mutations could account for all illness in this group, and two mutations could account for 17 of 18 alleles. In screening 250 normal Ashkenazi individuals for all three mutations, we found only one delta 5 allele. Clinical data revealed no correlation between the particular mutations and symptoms, but male patients were more symptomatic than females, and only males had frank hemolysis and hyperuricemia. Because PFK deficiency in Ashkenazi Jews is caused by a limited number of mutations, screening genomic DNA from peripheral blood for the described mutations in this population should enable rapid diagnosis without muscle biopsy.

A new variant of muscle phosphofructokinase deficiency in a Japanese case with abnormal RNA splicing

A genetic defect was investigated in a newly diagnosed Japanese case with muscle type phosphofructokinase (PFK-M) deficiency. Polymerase chain reaction (PCR) amplification of patient cDNA revealed an in-frame truncation of 165 bases. This was compatible to the complete deletion of exon 19. The rest of the sequence was identical to that of the normal PFK-M cDNA. Sequencing of PCR amplified genomic DNA of the patient revealed a point mutation from G to A at the 5' donor site of intron 19. This mutation resulted in the skipping of exon 19 in the patient mRNA. Homozygosity of this patient was confirmed by allele specific amplification of the genomic DNA. Donor mutations in intron 15 and intron 5 associated with different splicing errors were previously reported to cause this disease. Thus, the human PFK-M gene mutations are heterogeneous, however, the donor mutations and splicing errors would represent one of the frequent causes of this disease.

Late-onset muscular weakness in phosphofructokinase deficiency due to exon 5/intron 5 junction point mutation: a unique disorder or the natural course of this glycolytic disorder?

Late-onset muscle weakness is rare in glycolytic disorders. There are two reports in the literature of phosphofructokinase (PFK)-deficient Ashkenazi Jews with severe vacuolar myopathy manifesting in late adulthood. The genetic abnormality in these patients is unknown. We report a third patient with a similar syndrome: early-onset exercise intolerance in young childhood and progressive weakness in a limb-girdle distribution appearing at 57 years of age, leading to severe incapacity. Muscle histology showed diffuse vacuolar changes, and muscle fibers contained excess glycogen-like material. Muscle biochemistry was diagnostic for PFK deficiency. DNA analysis from the patient and his family showed that he was homozygous for a recently identified point mutation at the exon 5/intron 5 junction (a G-to-A change); two other family members were heterozygous for this mutation. It is not clear whether late-onset weakness is the natural course for all PFK-deficient patients or whether the exon 5 mutation carries increased risk for this severe myopathy.

Identification of three novel mutations in non-Ashkenazi Italian patients with muscle phosphofructokinase deficiency

We have identified three novel mutations in four non-Ashkenazi Italian patients with muscle phosphofructokinase (PFK-M) deficiency (Tarui disease). Patient 1 was homozygous for an A-to-C substitution at the 3' end of intron 6 of the PFK-M gene, changing the consensus splice-junction sequence AG to CG. The mutation leads to activation of two cryptic splice sites in exon 7, resulting in one 5 bp- and one 12 bp-deleted transcript. An affected brother was also homozygous, and both parents were heterozygous, for the splice-junction mutation. Patient 2 was homozygous for a G-to-C substitution at codon 39, changing an encoded arginine (CGA) to proline (CCA). Patient 3 was heterozygous for an A-to-C substitution at codon 543, changing an encoded aspartate (GAC) to alanine (GCC); the PFK-M gene on the other allele was not expressed, but sequencing of the reported regulatory region of the gene did not reveal any mutation.

A 5' splice junction mutation leading to exon deletion in an Ashkenazic Jewish family with phosphofructokinase deficiency (Tarui disease)

A deficiency of the muscle isoform of the enzyme, phosphofructokinase (PFK, EC 2.7.1.11), leads to an illness (glycogenosis, Type VII) characterized by myopathy and hemolysis. A patient with this disease and an affected sister were found to have a G to A substitution at the 5' donor site of intron 5 of the PFK-M gene. This mutation led to a splicing defect: a complete deletion of the preceding exon in the patient's mRNA. The patient, an affected sister, and related and unrelated family members, who were of Ashkenazic Jewish background, were screened for the mutation by denaturing gradient gel electrophoresis and by allele specific hybridization of genomic DNA. The affected sisters are homozygous for the mutation, and their children, who are unaffected, are heterozygous. The only previously characterized genetic defect in this disease, found in a Japanese patient, was a G to T mutation at the beginning of intron 15 with splicing to a cryptic site within exon 15 (1). Both mutations lead to inframe deletions, but of different parts of the protein. The differences between the two aberrant proteins may account for clinical differences between our patients and the Japanese patient.

Muscle metabolism during lactate infusion in human phosphofructokinase deficiency

We studied the effect of intravenous infusion of sodium lactate (La) on muscle high-energy phosphate metabolism, pH, and venous effluent [NH3] in three patients with muscle phosphofructokinase (PFK) deficiency and three healthy subjects during maximal-effort rhythmic handgrip exercise (5 s of contraction alternated with 5 s of rest) performed for 3 min. Healthy subjects were matched to PFK-deficient patients for gender and maximal handgrip strength. Force production was recorded and during lactate infusion was matched to that without lactate. 31P-magnetic resonance spectroscopy was used to measure intracellular phosphocreatine (PCr), orthophosphate (Pi), ATP, and pH in the flexor digitorum profundus of the exercising forearm. La infusion had no effect on healthy subjects or patients during rest. In healthy subjects, La infusion had no effect on depletion of PCr; accumulation of Pi, ADP, or venous effluent NH3; or decline in pH in exercising muscle. In contrast, during exercise in PFK-deficient patients, [PCr] was higher (17.9 +/- 1.9 vs. 13.1 +/- 1.4 mmol/kg) and [phosphomonoester] (11.4 +/- 1.3 vs. 6.5 +/- 1.4 mmol/kg), [Pi] (9.2 +/- 1.8 vs. 5.0 +/- 0.7 mmol/kg), [ADP] (60.4 +/- 7.0 vs. 37.9 +/- 9.9 mumol/kg), and venous effluent [NH3] (335 +/- 136 vs. 176 +/- 61 mM) were lower (P < 0.05) during La infusion than in control conditions. The effects of La infusion on intracellular [PCr], [Pi], [phosphomonoester], [ADP], and [NH3] in PFK-deficient patients are consistent with the hypothesis that exogenous La augments the rate of oxidative phosphorylation in active muscle by bypassing the enzymatic block at PFK.

Inherited phosphofructokinase deficiency in an American cocker spaniel

A 3-year-old female American Cocker Spaniel with a chronic hemolytic disorder and hemolytic crises was found to have M-type phosphofructokinase deficiency. This inherited erythroenzymopathy and myopathy is commonly diagnosed in English Springer Spaniels, but the family study of this Cocker Spaniel, although supporting an autosomal recessive mode of inheritance, did not reveal any English Springer Spaniel ancestors. Molecular genetic studies did, however, identify the same mutation in this dog as we previously reported in the English Springer Spaniel breed, suggesting that this mutation originated prior to the separation of these 2 breeds.

Decrease of phosphofructokinase activity in relation to the pathogenesis of triorthocresyl-phosphate-induced delayed neuropathy

The in vivo effect of a single dose of the neuropathic compound triorthocresyl-phosphate (TOCP) on phosphofructokinase (PFC, E.C. 2.7.1.11) and its relation with the initiation step (inhibition and aging of neuropathy target esterase, NTE) in the TOCP-induced delayed neuropathy have been studied. Hens were treated with a neurotoxic dose of TOCP (500 mg/kg, p.o.) and with a protective compound (Phenylmethanesulfonyl fluoride, PMSF, 30 mg/kg s.c.) in different combinations: TOCP, TOCP + PMSF, PMSF + TOCP and PMSF. PFK activity was determined in brain and sciatic nerve 1, 3, 7 and 15 days after treatment. PFK activity decreased in sciatic nerve 15 days after dosing with TOCP or TOCP + PMSF. When animals were dosed with the protective agent (PMSF) alone or before administering the neurotoxic compound, PFK activity was unaltered and clinical signs of neuropathy were absent. The data presented here suggest that phosphofructokinase is involved in the pathogenesis of the neuropathy induced by TOCP.

Muscle phosphofructokinase deficiency in a myopathic child with severe mental retardation and aplasia of cerebellar vermis

Muscle phosphofructokinase (PFK) deficiency in man is responsible for at least two forms of myopathy; one is characterized by painful contractures of muscles and typically occurs in adults, whereas the other is often disabling and typically occurs in childhood, with psychomotor and growth retardation. In this investigation, a young myopathic patient with severe mental retardation and aplasia of the cerebellar vermis presented with muscular hypotrophy of the limbs, generalized hypotonia, convergent strabismus and marked pain during passive movement. Biopsy of quadriceps femoris muscle showed variation in the fiber size with sarcoplasmic areas positive for periodic acid-Schiff stain. Histochemical qualitative reaction for PFK showed no staining of muscle fibers; ultrastructural studies showed abnormal accumulation of glycogen granules in both intermyofibrillar and subsarcolemmal areas. While some enzyme activities in the muscular crude extract were significantly lower than in controls, direct assay of PFK revealed no activity, thus demonstrating that the child's myopathy was due to the lack of PFK activity.

Fatal familial infantile glycogen storage disease: multisystem phosphofructokinase deficiency

An infant girl of consanguinous Bedouin parents suffered from fatal early onset of progressive generalized muscle weakness. Her older brother suffered from similar weakness and cardiomyopathy, which led to his death at the age of 21 months. A muscle biopsy performed on the propositus at the age of 9 months was PAS-negative, and showed nonspecific myopathic changes. A second muscle biopsy, performed at 21 months of age, a few days before her death, and postmortem study of heart and liver, disclosed excessive extralysosomal glycogen storage and reduced phosphofructokinase-1 (PFK-1) activity. Because the genes encoded for PFK-1 in liver and muscle are located on separate chromosomes, the reduced enzyme activity in both tissues could not be related to a single mutation for this enzyme. Activity of 6-phosphofructose-2-kinase (PFK-2), a recently discovered physiological activator to all PFK-1 isozymes, was normal in the liver. The possibility that this multisystem PFK-1 deficiency may be related to the absence of a yet unknown activator, common to all PFK-1 isozymes, is discussed.

Characterization of phosphofructokinase-deficient canine erythrocytes

Dogs homozygously affected with muscle-type phosphofructokinase (PFK) deficiency had about 20% of normal erythrocyte PFK activity and exhibited a compensated haemolytic anaemia. Erythrocyte glucose-6-phosphate and fructose-6-phosphate concentrations were increased and dihydroxyacetone phosphate and 2,3-bisphosphoglycerate values were below normal in affected dogs. Other intermediates distal to the PFK step were not significantly below normal and fructose-1,6-bisphosphate was even above normal. Erythrocyte ATP was higher than normal in affected dogs owing to the reticulocytes present. Abnormal adenylate metabolism was demonstrated by low ATP/AMP and ADP/AMP ratios and the inability to maintain ATP content when affected erythrocytes were incubated with cyanide. Glucose-1,6-bisphosphate content was normal, and fructose-2,6-bisphosphate content in affected canine erythrocytes was higher than normal. Studies of erythrocyte PFK isozymes revealed altered enzyme kinetic properties in affected dogs which appeared to be due to the loss of the M-type subunit.

Basal and insulin-mediated carbohydrate metabolism in human muscle deficient in phosphofructokinase 1

Biopsies were obtained from the quadriceps femoris muscle of two male patients deficient in phosphofructokinase (PFK) 1. In the basal state the patients had markedly higher contents of UDP-glucose (approximately 5-fold), hexose monophosphates (approximately 7- to 13-fold), inosine monophosphate (IMP) (approximately 15-fold), and fructose 2,6-bisphosphate (F-2,6-P2; approximately 6-fold) than controls. Fructose 1,6-bisphosphate was not detectable, and phosphocreatine was lower (33 and 54 mmol/kg dry wt) than in controls [72 +/- 4 (SD)]. Patients had normal fasting plasma glucose and insulin levels and basal glucose turnover rates and responded normally to a 75-g oral glucose challenge. Patients were also studied during euglycemic hyperinsulinemia (approximately 95 mg/dl; 40 and 400 mU.m-2.min-1). Whole body glucose disposal rates were normal during both insulin infusion rates. Biopsies taken after the 400 mU insulin infusion showed decreases in acetylcarnitine and citrate and increases in the fractional activity of glycogen synthase. It is suggested that the high basal levels of F-2,6-P2 are, at least partly, a consequence of the high levels of fructose 6-phosphate, which will stimulate flux through PFK-2 and inhibit fructose-2,6-bisphosphatase. The low phosphocreatine and high IMP contents indicate that carbohydrate availability is important for control of high-energy phosphate metabolism, even in the basal state. The insulin-mediated decreases in acetylcarnitine and citrate suggest an activation of the tricarboxylic acid cycle in skeletal muscle but an absence of the normal response to replenish these intermediates.

Phosphorus magnetic resonance spectroscopy (31P MRS) in neuromuscular disorders

Phosphorus magnetic resonance spectroscopy monitors muscle energy metabolism by recording the ratio of phosphocreatine to inorganic phosphate at rest, during exercise, and during recovery from exercise. In mitochondrial diseases, abnormalities may appear during some or all these phases. Low phosphocreatine-inorganic phosphate ratios at rest are not disease-specific, but can be increased by drug therapy in several myopathies. Phosphorus magnetic resonance spectroscopy can also record intracellular pH and thus identify disorders of glycogen metabolism in which the production of lactic acid is blocked during ischemic exercise. The measurements of accumulated sugar phosphate intermediates further delineate glycolytic muscle defects. Myophosphorylase deficiency responds to intravenous glucose administration with improved exercise bioenergetics, but no such response is seen in phosphofructokinase deficiency. The muscular dystrophies show no specific bioenergetic abnormality; however, elevation of phospholipids metabolites and phosphodiesters was detected in some cases. While phosphorus magnetic resonance spectroscopy remains primarily a research tool in metabolic myopathies, it will be clinically useful in identifying new therapies and monitoring their effects in a variety of neuromuscular disorders.