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James Larkin: PhD Research Summary

My PhD was spent in the Protein Damage and Systems Biology Research group under the supervision of Paul Thornalley. The group is based at the Clinical Sciences Research Institute, part of the University of Warwick Medical School. My research investigated the link between thiamine deficiency and diabetes. It is known that diabetic patients are markedly thiamine deficient despite an adequate dietary intake and there is potential for thiamine to help prevent or reverse early stage symptoms of diabetic kidney disease. I have been investigating why diabetic patients are thiamine deficient and what the mechanism is behind the deficiency.

What follows are excerpts from my thesis edited for a more general audience and explaining the background to my work.

Thiamine deficiency in diabetes

Chronic thiamine deficiency is associated with the classical diseases beri-beri [Tanphaichitr 1970] and Wernicke-Korsakoff syndrome (which comprises Wernicke's encephalopathy and Korsakoff's associated psychosis) [Butterworth 2003, Pannunzio 2000]. These are only observed in patients who have a severe, chronic deficiency of thiamine which could be due to a diet lacking the vitamin (i.e. malnutrition), chronic alcoholism (the most common reason for presentation with severe thiamine deficiency in the developed world) or increased clearance of thaimine in renal dialysis or HIV/AIDS (reviewed by [Thornalley 2005]). Thiamine deficiency is conventionally assessed by means of the thiamine effect, or percentage unsaturation of transketolase with its cofactor TPP. None of the likely reasons for a severe thiamine deficiency are are associated with diabetes mellitus but a mild thiamine deficiency may well be present in the diseased state. This has been considered for a long time [Janes 1948] but only recently has more detailed study been undertaken.

The evidence for a mild thiamine deficiency in diabetic patients has mounted since the 1960s from a series of small observation trials. Most recently, our laboratory studied the thiamine status of 26 type 1 and 48 type 2 diabetic patients with and without microalbuminuria in the Colchester region along with 20 normal healthy control volunteers [Thornalley 2007]. We found that the plasma thiamine concentration, as measured by HPLC, was decreased 76% in type 1 diabetic patients and 75% in type 2 diabetic patients (median concentrations (95% confidence intervals): normal volunteers 64.1nM (58.5-69.7nM), type 1 diabetic patients 15.3nM (11.5-19.1nM; p < 0.001) and type 2 diabetic patients 16.3nM (13.0-19.6nM; p < 0.001); [Figure 1]).

Figure 1 - Diabetic thiamine deficiency. This frequency distribution shows that diabetic patients have marked plasma thiamine deficiency. Data in the inset represent median values for 20, 26 and 48 patients for control, type 1 and type 2 diabetes respectively.

This decrease in plasma thiamine concentration was linked to a marked increase in renal clearance and fractional excretion of thiamine in the diabetic state. Renal clearance rates were increased 24-fold in type 1 diabetes and 16-fold in type 2 diabetes (median clearance rates were 3.7, 86.5 and 59.8mL per minute in control, type 1 and type 2 diabetic patients respectively) and fractional excretion was increased 25-fold in type 1 diabetic patients and 15-fold in type 2 diabetic patients (median fractional excretion was 2.8%, 71.2% and 41.6% in control, type 1 and type 2 diabetic patients respectively). There was no evidence of a nutritional deficiency of thiamine as judged by urinary thiamine in the normal range (> 0.20μmol per day [Finglas 1993]). Rather, plasma thiamine concentration negatively correlated with both renal clearance and fractional excretion of thiamine suggesting that the plasma thiamine deficiency was linked to renal mis-handling of filtered thiamine i.e. a substantial decline in tubular re-uptake of thiamine in diabetes. Additionally, plasma thiamine concentration and urinary excretion of thiamine negatively correlated with the concentration of soluble vascular cell adhesion molecule 1 (sVCAM-1), a marker of vascular dysfunction.

Interestingly the same study revealed why clinical thiamine deficiency in diabetes has gone unreported: erythrocyte TK activity, the primary clinical measure of thiamine deficiency [Haas 1988], was not significantly different between control patients and diabetic patients, that is all patient values were below 15%. Further investigation showed that the plasma deficiency was masked by increased abundance of THTR-1 and the TMP-transporter RFC-1 in erythrocyte membranes which maintained intracellular thiamine concentrations despite the low plasma abundance. These transporters are likely up-regulated in erythrocyte precursors in reponse to the low thiamine concentration in plasma. In confirmation, direct analysis of intra-erythrocyte thiamine concentrations by HPLC showed no difference between diabetic patients and control volunteers, confirming that the higher abundance of transporter protein was masking plasma thiamine deficiency.

My work has been building on this background and has been investigating the potential mechanisms behind thiamine deficiency in diabetes as well as investigating whether therapy with thiamine is beneficial to diabetic patients.

References