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.
TABLE I. - Shift of the mitotic index in trisomic 21 lymphocyte
cultures compared to controls. The mean and standard error are indicate,
together with the numbers of individuals and the correlation coefficient
between the effect of rT3 and other additives.
|Trisomy 21||+ 0.254 ± 0,0560 n = 45||+
0,139 ± 0,525 n = 24 ; r = 0,82***||- 0.004 ± 0,438 n = 14
; r = 0,87***||+ 0,390 ± 0,206 n = 7 ; r = 0.73**||+ 0,017
+ 0,220 n = 7 ; r = 0,18*||-0,156 ± 0,395 n = 9 ; r = 0,22
|Controls||+ 0,347 ± 0,671 n = 33||+ 0,199
± 0,457 n = 23 ; r = 0,79***||+ 0,415 ± 0,886 n = 8 ; r =
0,98***||+ 0,285 ± 0,318 ; n = 2||+ 0,008 ± 0,234 n
= 12 ; r = 0,33||- 0,091 ± 0.199 n = 8 ; r = 0,66
|* P < 0,05; ** P < 0.01 ; *** P <
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
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.
1. CHOPPA I.J. - Nature, sources and relative biologic significance of
thyroid circulating hormones. In : Inqbar S.H., Braverman L.E. A fundamental
and clinical text. Ed Wernefs The thyroid, pp. 140-143, Philadelphia, J.B.
Lippincott Company, 1986.
2. JACKSON R.C., WEBER G. - IMP dehydrogenase, an enzyme linked with
proliferation and malignancy. Nature, 1975, 256, 331-333.
3. LEJEUNE J., PEETERS M., BLOIS M.C. (de), BERGERE M., GRILLOT A.,
RETHORE M.O, VALLÉE G., IZEMBART M., DEVAUX J.P. - Fonction
thyroïdienne et trisomie 21. Excès de TSH et déficit en
rT3. Ann Génét, 1988, 31, 137-143.
4. LEJEUNE J., RETHORE M.O., BLOIS M.C. (de), MAUNOURY-BUROLLA C., MIR
M., NICOLLE L., BOROWY F., BORGHI E., RECAN D. - Métabolisme des
monocarbones et trisomie 21: Sensibilité au méthotrexate. Ann
Génét, 1986, 29, 16-19.
5. LUCAS D.I., WEBSTER H.K., WRIGHT D.G. - Purine metabolism in
myeloid precursor cells during maturation. J Clin Invest, 1983, 72,
6. PEETERS M.A., LEJEUNE J. - Beneficial effect of 6-mercaptopurine on
the mitotic index of trisomy 21 lymphocytes. Implications for future research.
Ann Génét, 1989, 32, 21-25.