Thiamine-deficient encephalopathy in the rat is characterized by ataxie gait, loss of righting reflex and curvature of the spine. N euro chemie ai changes include a diminished activity of cerebral pyruvate decarboxylase leading to abnormal pyruvate oxidation. The present study shows that this defective pyruvate oxidation produces a significant depletion of three important amino acid neurotransmitters, namely gamma aminobuiyrie acid (GABA), glutamic acid, andaspartic acid. Such changes could lead to severe neuronal dysfunction and the observed neurological symptoms of thiamine deficiency. Some implications for the pathogenesis of Friedreich's ataxia are discussed.

In light of the available information on the cardiomyopathy of Friedreich's ataxia, the cardiomyopathic Syrian hamster may be an appropriate laboratory model. Cardiomyopathy in these animals is a result of calcium accumulation. We analyzed the atria and right and left ventricles from cardiomyopathic (CM) and random bred (RB) animals for calcium, magnesium, and iron concentrations at 30-40 and 60-70 days of age (age of maximum lesioning). There are no significant differences in the concentration of iron or magnesium among age-matched groups. The concentration of calcium in the left ventricles of the CM animals at 60 days old is 14 fold higher than that of R B animals. Although there is a significant difference in the concentration of calcium in the left ventricles of younger animals, it is not as pronounced as the difference in older animals. Analysis of the taurine concentration in 30-40 day old animals revealed that the CM animals show slightly higher taurine concentrations than RB in the whole heart. In 60 day old CM hamsters the ß-adrenergic receptor density of the ventricles is unchanged. This indicates that calcium overload is not due to a drene rg i c super sensitivity.

Of 13 patients with Fried-reich's ataxia (Type la) and 17 with type Ila recessive ataxias, all were found to have levels of “free erythrocyte protoporphyrin “ (FEP) above the normal range. The rise in FEP in Friedreich's ataxia correlated well with the age of the individual and thus appears to be related to the course of the disease. Subjects with olivo-ponto-cerebel-lar atrophy, Charlevoix syndrome, Duchenne muscular dystrophy, and Parkinson's disease were also found to have significantly elevated FEP, although the distribution overlapped with the normal range.

The finding of elevated FEP may indicate a relative heme deficiency in ataxia due to inhibition offerrochelatase leading to a state of ineffective, persistent erythropoiesis. The possibility of a prosta-glandin abnormality being related to this defect and to the pathogenesis of ataxia is considered.

Detailed in vivo and in vitro studies of glucose and insulin metabolism in Friedreich's ataxia patients and unaffected family members have further defined the extent of the abnormalities in carbohydrate metabolism. The high incidence of glucose intolerance and a hyper-insulinemic response to a glucose challenge in a high percentage of Friedreich's ataxia patients has been confirmed. An increased incidence of glucose intolerance among hétérozygotes is suggested, while the siblings show a more normal distribution of diabetes and a nearly normal insulin response to the glucose tolerance test. Human growth hormone patterns are normal for all groups.

Preliminary studies of insulin binding to erythrocytes suggest a difference in the binding characteristics among diabetic Friedreich's ataxia patients, while the binding in the non-diabetic Friedreich's ataxia group is similar to that of non-diabetic controls. Results from a small group of non-diabetic siblings suggest a normal insulin binding, while a tendency toward increased binding at low insulin concentrations among diabetic family members is noted.

The activity of lipoamide dehydrogenase and two closely related enzymes was studied simultaneously in early, mid, and late passage fibroblast cultures. Friedreich's ataxia fibroblasts tended to lose pyruvate dehydrogenase and citrate synthase activities, while lipoamide dehydrogenase activity remained constant with aging of the cells. Mean pyruvate dehydrogenase activity was lower over-all in fibroblasts from ataxies. Mean citrate synthase activity was higher in ataxie fibroblasts. Present tissue culture media do not represent the best conditions in which to reproduce cofactor binding defects such as those found in other genetic diseases with structural enzyme mutations.

Taurine resembles G ABA in its synaptic effects in the cockroach cereal nerve giantfiber synapse where it exerts a depressant action upon synaptic transmission. Both taurine and G A B A produce an increased conductance of pre- and postsynaptic membranes through changes in the permeability of chloride ions.

Taurine and ß-alanine uptake kineiics were studied in cultured skin fibroblasts from 9 patients with Friedreich's Ataxia and 8 controls. No significant difference was observed. The data support the presence of normal ß-amino acid carrier protein in Friedreich's Ataxia cell membrane.

The distribution of amino acids in two Friedreich's ataxia brains is compared to 4 control brains. Glutamate and G A BA were decreased in the ce r ebe l-lar hemispheres and/or in the vermis. Taurine concentrations were uniformly elevated throughout the areas studied. Taurine /glutamate and taurine/ GABA ratios were consistently elevated in Friedreich's ataxia brains, whereas glutamate/ glutaminę ratios were consistently decreased (with normal glutaminę concentrations).

The binding of ‘H-quinu-clidinyl henzilate. a muscarinic cholinergic antagonist, of xH-dihydroalprenolol, a beta adrenergic antagonist, and of ﹜H-flunitrazepam, a ligand which labels benzodiazepine receptors, was examined in several regions of control and Fried-retch's ataxia (FA) brains. yH-Quinuclidi-nyl henzilate binding appeared to increase in the inferior olivory nucleus, anterior and posterior cerebellar vermi but was unaltered m the dentate nucleus and cerebellar hemisphere of FA brain. The binding of 1 H-dihydroalprenolol seemed to increase in the inferior olivary nucleus yet was not different from controls in the dentate nucleus, cerebellar hemisphere, anterior and posterior cerebellar vermi of FA brains. * H-Flunitrazepam binding was slightly lowered in the inferior olivary and dentate nuclei but was unchanged in the other FA brain regions examined. The present study suggests possible trends in neurotransmitter receptor alterations in post-mortem brain tissue of FA patients.

Glutamic and aspartic acid uptake was measured in skin fibroblasts from patients with Friedreich's Ataxia, dicarboxylic aminoaciduria, and normal individuals. The results showed no difference in uptake kinetics of either dicarboxylic amino acids between Friedreich's Ataxia and normal cells, but reduced uptake velocities in dicarboxylic aminoaciduria fibroblasts. Friedreich's Ataxia fibroblasts were, however, less calcium-dependant and more magnesium and phosphate-dependent than controls in glucose-free incubation mixture. This difference might be related to some degree of glucose intolerance by Friedreich's Ataxia fibroblasts in culture.

Studies were undertaken to further characterize plasma lipids and lipoprotein abnormalities in Friedreich's ataxia. The high density lipoprotein (HDL) apo AI/All ratio was quantitated by densitometry and found to be normal. The free to esterified cholesterol ratio in HDL was lower in Friedreich's ataxia because of a reduction in the amount of free cholesterol in this lipoprotein class. The fatty acid profile of the cholesteryl-ester (CE) fraction was markedly deficient in linoleic acid (18:2) in both total plasma and HDL· There was a compensatory increase in saturated acids. The HDL phospholipid (PL) fraction also showed a reduction in the proportion of 18:2 with a concomitant increase in stearic (18:0) and oleic acid (18:1) while the HDL triglycéride (TG) fraction showed only an increase in palmi t oleic (16:1) and oleic acids. Feeding of soya lecithin rich in 18:2 failed to increase significantly the 18:2 content of HDL-CE and HDL-PL but lowered the percentage of 16:1 and 18:1 in all 3 lipid classes of HDL. Although the total plasma CE fatty acid profile was perturbed in Friedreich's Ataxia, total plasma PL and TG fatty acid patterns were unaffected. Among the plasma lipoprotein fatty acid profiles, that of the low density lipoprotein (LDL) was most affected, then that of the HDL. The very low density lipoprotein (VLDL) fatty acid composition showed an increase in 16:1 and a decrease in 18:2 which were entirely corrected by lecithin feeding. These results suggest the existence of a metabolic defect in the incorporation of 18:2 into chylomicron phospholipids within the intestinal mucosa.

The observed anomalies in high density lipoproteins in Friedreich's ataxia led us to investigate the state of cellular membranes in this illness. As a preliminary screening program, we studied the shape of erythrocytes; the phospho-lipid content of platelets and the transport properties of these membranes as indirectly reflected in the absorption of Vit E and the renal handling of orally injected taurine. All these investigations were normal, except for a tendency towards more echinocytes in Friedreich's ataxia and the significant increase in taurine urinary excretion after an oral load. We concluded that the possible membrane abnormalities are not major and will have to be searched for with more subtle and specific tests.

In a study of the lipid composition of erythrocyte membranes in Friedreich's ataxia, the concentration of the major membrane components (phosr pholipids, cholesterol and protein) in ataxie patients, family members, and control subjects were found to be the same. The total fatty acid distribution was also normal. However, an altered distribution of phospholipid classes in erythrocytes was noted (an increase of PI + PS and a decrease of P E in Friedreich's ataxia patients).

Electron spin resonance, scanning electron microscopic, and SDS-polyacrylamide gel electrophoretic studies of erythrocytes in Friedreich's ataxia have been performed. No alteration in the physical state of membrane lipids, in morphology, or in the staining profile of erythrocytes in Friedreich's ataxia could be demonstrated. An altered conformation and I or organization of proteins in ery-throcyte membranes in this disorder was suggested by spin labeling studies

(P < 0.025), favoring the possibility of a generalized membrane abnormality in Friedreich's ataxia. These findings are discussed in relation to other inherited neurological diseases where similar studies have been performed.

Coded erythrocyte samples from ten individuals with Friedreich*š ataxia, from parents of five of these individuals, and from five unrelated control individuals were subjected to lecţin agglutination tests at three temperatures; before and after trypsinization; and before and after treatment with echinocyte-producing sodium salicylate and stornato^ cyte-pröducing tetracaine followed by shape-fixation with glutaraldehyde. The aġglutinins tested were the polycationic poly-L-lysirie (PLL) and four lectins with different saccharide specificities: soybean agglutinin, wheat germ agglutinin, Ufex europeus agglutinin (VEA) and concana-valin A. Altogether, over 45,000 individual test wells were scored, the status of each blood donor with respect to diagnosis being disclosed to the experimenters only after all results were tabulated.

The majority of these tests revealed no significant difference among the three groups of blood samples. A few tests did reveal statistically valid (p<0.0l) differences between groups, the most significant of which were the following: Trypsin ized control RBC were more sensitive, on average, to agglutination by VEA (fucose-inhibited) than were RBC of ataxies or their parents. Non-trypsinized control “stomatocytes” were less sensitive, on average, to agglutination by PLL than were those of ataxies or their parents. Trypsinization appeared, on average, to sensitive control but not ataxia or parent RBC to PLL-agglutination. Other differences of borderline (p-0.01-0.025) or near borderline (p = 0.025-0.05) significance were also noted. None of the statistically significant, Friedreich's ataxia-telated differences in median agglutination titers were large, the greatest being about threefold, and in every case the ranges of individual titers within the differing groups overlapped. Thus, none of these tests at present offers a method of pre clinic al diagnosis or carrier detection, and only further tests can establish whether even the differences observed in the present series of tests are reproducible.

This overview summarizes the investigations carried out during the second part of Phase Two of the Quebec Cooperative Study of Friedreich's Ataxia. These investigations outline in more details the fundamental role played by an abnormality in the fatty acid composition (deficient linoleic acid, 18:2) of the cholesterol esters of high density lipoproteins (HDL) in the phenotypic expression of the disease. They postulate a defective incorporation of linoleic acid to surface phos-phatidylcholine of chylomicrons and consequent relative and absolute decreases in lipoproteinprotein components because of overpacking with defective cholesteryl esters. Secondarily to these changes, the postulated lack of activation of the lipoamide dehydrogenase (LAD) of the pyruvate dehydrogenase (PDH) complex could result in slow pyruvate oxidation, glucose intolerance, deficient synthesis of acetylcholine, and depletion of glutamic and aspartic acid pools. In parallel, abnormal phosphatidyl-choline molecules could be incorporated to membranes, resulting in specific defects in some functions of these membranes, including transport of calcium and I or taurine and myelinization. The framework of an understanding of Friedreich's ataxia is now available, but much fundamental and clinical work remains to be done to fill in and prove each one of these postulated steps.