Correlation between the effects of rT3 and IMP dehydrogenase inhibitors on normal and trisomic 21 lymphocyte cultures

Marie PEETERS1, Marie-Odile RÉTHORÉ1, Sylvie de KERMADEC2, J. LEJEUNE1

PEETERS Marie, RÉTHORÉ Marie-Odile, KERMADEC Sylvie (de), LEJEUNE J. - Correlation between the effects of rT3 and IMP dehydrogenase inhibitors on normal and trisomic 21 lymphocyte cultures. Ann Génét, 1989, 32, n° 4, 211-213.

Résumé :

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As reported earlier, children with Down syndrome have significantly lower levels of serum rT3 as compared to age-matched controls (Lejeune et al., 1988). Using an in vitro assay previously described (Lejeune et al., 1986), we studied the variations of mitotic index in 72 hours lymphocyte cultures to which either rT3 (5 mg/L), mycophenolic acid (0.6 x 10-6M), 6-mercaptopurine (2.9 x 10-8M) or 2,3 DPG tris salt (3.1 mg/L) (Sigma products) were added during the last 16-20 hours of culture. The reason for studying these products simultaneously was that in a previous studv we showed that low doses of 6-mercaptopurine cytes of patients with trisomy 21, indicating that anomalies of purine metabolism may play an important role in Down syndrome (Peeters M.A. and Lejeune J., 1989).

Fig. 1. - % change in mitotic index : Triangle : 6-mercaptopurine ; Losange : 2-3 DPG; Rond : mycophenolic acid.

Peripheral blood lymphocytes from 33 patients with trisomy 21 (age range : 5-41, mean: 17.3 ± 9.8 ; 14 females, 19 males) and from 35 controls (age range: 7-76, mean: 34.8 ± 13.3, 29 females and 6 males) were cultured according to standard methods in TC 199 medium supplemented with 25 % human AB serum, PHA-C and antibiotics.

Analysis was made by comparing the mitotic index of each experiment to the patient's own control culture and results were expressed as the percentage.

For each person the mitotic index shift is plotted in figure 1. The rT3 effect is on abscisses ; on ordinates are the effects of mycophenolic acid (circles), of 2-3-DPG (lozenges) and of 6-mercaptopurines (triangles).

In table I the mitotic index shift is reported as the mean and its standard error for n individuals.

As is shown in figure 1, results showed a highly significant correlation between the in vitro responses to rT3, mycophenolic acid (a specific inhibitor of inosine monophosphate (IMP) deshydrogenase) 2-3-DPG inhibitor of IMP dehydrogenase) and 6-mercapto-purine (which inhibits several enzymes of the de novo purine synthesis pathway, amongst which IMP dehydrogenase).

No age effect was observed and no correlation was demonstrated between the rT3 effect and the effect of other additives tested such as inosine and alanosine which inhibits the first step in the conversion of inosine monophosphate to adenosine monophosphate.

These date suggest that the effect ofmycophenolic acid, 2-3-DPG and 6-mercaptopurine are very strongly correlated with rT3 effect, in the trisomic 21 as well as in controls.

The simplest explanation would be that rT3 acts upon the same biochemical step as the other three inhibitors of IMP dehydrogenase.

IMP dehydrogenase is a key enzyme in the IMP branchpoint and its activity has been shown to be closely linked to cellular proliferation and malignancy (Jackson and Weber, 1975). Inhibitors of IMP dehydrogenase have been found to induce HL60 cells to mature morphologically and functionally (Lucas et al., 1983). Its activity has been reported to be low in neonatal rat livers (Jackson et al., 1975), perhaps in correlation with the high levels of rT3 during this period.

Reverse T3 has been known since 1956 but interest in its metabolic action is only fairly recent and information about its biochemical effects remains scanty. Serum rT3 concentrations are high in the foetus and newborn as well as in a variety of nonthyroidal disorders (Choppa, 1986).

Given the importance of IMP dehydrogenase in the mechanisms of cellular proliferation and malignancy, rT3 could well be a naturally occurring hormone capable of controlling myeloproliferative processes.

Transient myeloproliferative and congenital leukemoid reactions are frequent in trisomy 21; whether these maturational haemotological defects are due to low circulating levels of rT3 and whether they would respond to rT3 therapy remains to be investigated.

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References

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