Presence of Methylthioadenosine Phosphorylase (MTAP) in ... · sensitivity of MTAP (+) and MTAP (-) T-ALL cell lines to L-alanoSine. We found that there is a pronounced difference

Vol. 3, 433-438, March 1997 Clinical Cancer Research 433

Presence of Methylthioadenosine Phosphorylase (MTAP) in

Hematopoietic Stem/Progenitor Cells: Its Therapeutic

Implication for MTAP (-) Malignancies1

John Yu,2 Ayse Batova, Li-en Shao,

Carlos J. Carrera, and Alice L. Yu

Department of Molecular and Experimental Medicine, The ScrippsResearch Institute, La Jolla, California 92037 [J. Y., L-e. S.];Department of Pediatrics, University of California San Diego MedicalCenter, San Diego, California 92103 [A. B., A. L. Y.]; andDepartment of Medicine and Cancer Center, University of California,San Diego, La Jolla, California 92093 [C. J. C.]

ABSTRACT

Methylthioadenosine phosphorylase (MTAP) is important

for the salvage of adenine and methionine. Recently, we foundfrequent deletion of MTAP in T-cell acute lymphoblastic leu-kemia (T-ALL) patients both at diagnosis and at relapse (A.

Batova et aL, Blood, 88: 3083-3090, 1996). In addition, MTAP

deficiency has been reported in other cancers. Thus, MTAPdeficiency in cancer may offer opportunities for developing

selective therapy, which would spare normal cells. It is there-

fore important to document the presence of MTAP activity inhematopoietic stem/progenitor cells. Our approach was to in-vestigate whether hematopoietic stem/progenitor cells can be

rescued from the cytothxicity of an AMP synthesis inhibitor,

L-alanosifle, by 5’-deoxyadenosine, a process that requires

MTAP. Erythroid burst-forming unit, granulocyte/monocyte

colony-forming unit, or granulocyte/erythrocyte/macrophage/

megakaryocyte colony-forming unit progenitors and the prim-itive high proliferative potential colony-forming cells in the

purified CD34� cells were cultured in horse serum-containing

medium, and their colony growth was found to be suppressed

by incubation with 5 p.M or greater concentrations ofL-alaflOSine. However, in the presence of 5-10 ,LM of 5’-deoxya-

denosine, colony formation of hematopoietic stem/primitive

progenitors was restored. On the other hand, 5’-deoxy-5’-

methylthioadenosine, the endogenous substrate of MTAP, wastoxic to hematopoietic stem/progenitors (ffl� < 1 �LM), pre-

sumably due to inhibition of methylation reactions or poly-amine synthesis. We also compared the effects of L-alanosine

Received 9/16/96; revised 12/2/96; accepted 12/13/96.The costs of publication of this article were defrayed in part by the

payment of page charges. This article must therefore be hereby marked

advertisement in accordance with 18 U.S.C. Section 1734 solely to

indicate this fact.I Supported by Grants U10CA28439, MOb RR00827, and FDA 001 129(to A. L. Y.); CA54892 (to C. J. C.); and DK40218 (to J. Y.) and MOIRR00833 from the NIH. A. B. is supported by NIH Training Grant2T32-HLO71O7. This is publication 10160-MEM from The Scripps

Research Institute.2 To whom requests for reprints should be addressed, at Department ofMolecular and Experimental Medicine, NX 3, The Scripps ResearchInstitute, 10550 North Torrey Pines Road, La Jolla, CA 92037.

and 5’-deoxyadenosine on MTAP (+) and MTAP (-) T-ALLcell lines. Treatment of MTAP (+) Molt 4 and MTAP (-)

CEM cell lines with L-alanosine in the presence of S’-deoxya-denosine resulted in killing of MTAP (-), but not MTAP (+)cells. Therefore, our findings demonstrate the presence ofMTAP in human hematopoietic stem/progenitor cells and sup-port the possibffity of targeting MTAP in the design of anenzyme-selective therapy for T-ALL and other MTAP-defi-

cient malignancies.

INTRODUCTION

MTAP3 is an important salvage enzyme for both adenine

and methionine. Specifically, it cleaves MTA, which is gener-

ated during the synthesis of polyamines, into adenine and

5-rnethylthionbose-l-phosphate. Adenine and 5-methylthiori-

bose- I -phosphate are efficiently salvaged to form adenine nu-

cleotides and methionine, respectively (Fig. I).

Cleavage by MTAP is the sole catabolic pathway for MTA.

MTAP is abundant in many tissues, including most bone mar-

row and peripheral blood nucleated cells (1). On the other hand,

MTAP has been shown to be deficient in some rnurine and

human tumor cells (2, 3). Previously, we reported MTAP en-

zyrne deficiency in one T-ALL and one common ALL patient

among 20 leukemia patients (4). Recently, using quantitative

PCR and Southern blot analyses, we have found frequent dde-

tion of MTAP in T-ALL samples. Deletions that included exon

8 of MTAP occurred in 33.3% of patients at diagnosis and in

39.4% patients at relapse (5). In addition to T-ALL, MTAP

deficiency has been reported at a high frequency in non-small

cell lung cancer (6), glioma (3), isolated cases of rectal adeno-

carcinoma (1), and acute nonlymphoid leukemia (I, 7).

Kamatani et a!. (3) have suggested that MTAP deficiency

in cancer may offer opportunities for developing an enzyme-

selective chemotherapy that would spare normal cells. In MTAP

(-) cancer cells, the salvage of methionine or adenine from

MTA would be blocked, resulting in an increased dependency

on an exogenous supply of these two nutrients. It is conceivable

that the MTAP (-) cancer cells will be more sensitive than

normal cells to the cytotoxicity of either inhibitors of de novo

purine synthesis or of methionine depletion (3, 6, 8, 9). Al-

though MTAP activity has been demonstrated in more than 98%

of nucleated cells in human marrow (3), it remains unclear

3 The abbreviations used are: MTAP, methylthioadenosine phosphoryl-ase; MTA, 5’-deoxy-S’-methylthioadenosine; I-ALL, I-cell acute lym-phoblastic leukemia; ALL, acute lymphoblastic leukemia; IL, interleu-

kin; HPP-CFC, high proliferative potential colony-forming cell; BFU-E,erythroid burst-forming unit; CFU-GM, granubocyte/monocyte colony-forming unit; CFU-GEMM, granulocyte/erythrocyte/macrophage/megakaryocyte colony-forming unit.

Research. on March 1, 2020. © 1997 American Association for Cancerclincancerres.aacrjournals.org Downloaded from

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1005’-deoxy-5’-methylthioadefloSifle

(5’- deoxyadenosine)

Methylthioadenosine

Phosphorylase (MTAP)

V�d.

5-methylthiorlbose-1-phosphate enine

,�, (5-deoxyribose-1-phosphate)

03”00

U‘4-

0

C,)

0

U3)

80

60

40

20

0

T

� � U

-4

0 10 20 30 40 50 60

Concentrations of 5’-deoxyadenosine QiM)

n Fig. 2 Differential sensitivity of MTAP (+) and MTAP (-) I-cellI lines to L-alanosine in the presence of various concentrations of 5’-

deoxyadenosine. Molt-4 (0) and CEM (S) were plated at a density ofAMP 2.5 x l0� cells/mi in complete RPMI. Cells were treated first withn increasing concentrations of 5 ‘-deoxyadenosine and 2 h later with 30t p.M alanosine. Viable cell number was determined by trypan bluen exclusion 4 days after plating. Cell numbers of control culture with

I complete RPMI alone are 17.5 X l0� ± 0.7 X l0� and 16.1 X l0� ±0.9 X l0� cells/ml for Molt-4 and CEM, respectively.

434 Presence of MTAP in Early Hematopoietic Cells

MethionineATP

Fig. I Role of MTAP in the salvage of adenine nucleotides from MTAand 5’-deoxyadenosine. 5’-deoxyadenosine, although not a natural sub-

strate, is cleaved by MTAP, as is the natural substrate MTA, to generateadenine.

whether the hematopoietic stem/progenitor cells contain func-

tional MTAP, because these early stem/progenitor cells repre-

sent a very minor subpopulation of bone marrow nucleated cells

(less than 1 in l0� cells). To address this issue, we investigated

whether hematopoietic stem/progenitor cells can be rescued

from the cytotoxicity of a de novo AMP synthesis inhibitor,

L-alanosine, by 5’-deoxyadenosine, an alternative MTAP sub-

strate. The present study indicates the presence of MTAP activ-

ity in both early primitive hernatopoietic stem and progenitor

cells in human CD34� preparations. We also compared the

sensitivity of MTAP (+) and MTAP (-) T-ALL cell lines to

L-alanoSine. We found that there is a pronounced difference in

the sensitivity of MTAP (-) and MTAP (+) cell lines to

alanosine cytotoxicity in the presence of 5’-deoxyadenosine.

These findings lend further support for targeting MTAP in the

design of an enzyme selective therapy for T-ALL and other

MTAP-deficient malignancies, as suggested previously by oth-

ers (3, 6, 8, 9).

MATERIALS AND METHODS

Materials. L-Alanosine was a generous gift from the

Drug Synthesis and Chemistry Branch, Developmental Thera-

peutics Program, National Cancer Institute. 5’-deoxyadenosine

was purchased from Sigma Chemical Co. (St. Louis, MO.).

Cell Lines. The T-ALL-derived cell lines Molt-4 and

CEM were obtained from ATCC (Rockville, MD) and were

maintained in RPM! supplemented with 10% dialyzed horse

serum, 2 mr�i glutamine, and 1% penicillin/streptornycin. Unlike

bovine serum, horse serum is devoid of MTAP activity (10).

Cells were plated at a density of 2.5 X 10� cells/mi in complete

RPMI. For experiments, cells were treated with increasing con-

centrations of 5’-deoxyadenosine and, 2 h later, with 30 �i.M

L-alanosine. Viable cell number was determined by trypan blue

exclusion 4 days after plating.

CD34� Cells. CD34� cells were obtained from the pe-

ripheral blood of normal donors after mobilization with G-CSF

or GM-CSF (Dr. A. D. Ho at University of California, San

Diego) and were purified using the Isolex irnmunornagnetic

rnicrosphere separation system after being tagged with mono-

cbonal antibody HPCA-l (Baxter, Santa Ana, CA; Ref. 1 1).

Based on analysis by flow cytornetry, these isolated cell prep-

arations consisted of more than 90% CD34� cells.

Clonogenic Assays. Culture of erythroid progenitor cells

was performed as described by Iscove et al. ( 12) with slight

modifications. Briefly, 1 X l0� CD34� cells were plated in

35-mm Petri dishes in a 1-mb mixture containing Iscove’s mod-

ified Dulbecco’s medium, 1 .0% rnethylcellubose, 30% (v/v)

dialyzed horse serum, 2 units of erythropoietin, 15 ng of IL-3

(Arngen, Thousand Oaks, CA), 50 units of GM-CSF (Genetics

Institute, Cambridge, MA), 0. 1 �i.M a-thioglycerol, 1% (w/v)

BSA, 50 lU/mI penicillin, and 50 p.g/ml streptomycin. The

amount of test reagents in each culture is specified in the Fig.

legends. The dishes were incubated at 37#{176}Cin a humidified

incubator flushed with 5.0% CO2 for a total of 14 days.

To enumerate CFU-E-derived colonies, the dishes were

examined on an inverted microscope for hemogbobinized and

compact colonies with eight or more cells on day 7 of culture.

BFU-E were scored on day 14 and identified as large aggregates

of 64 or more hemogbobinized cells or as clusters of three or

more subcolonies consisting of 8 or more hernogiobinized cells

per subcolony. The CFU-GM-derived colonies were enumerated

as a group of more than 50 granulo-rnonocytic translucent cells.

The CFU-GEMM mixed colony was defined as a group of more

than 64 cells containing more than 20% hemogbobinized cells



100-

80-

0

I-

00U4-

0

,0

0I-

0

0

0U

40-

20- -I U

0

Concentrations of Alanosine (pM)

Fig. 3 Effects of various concentrations of L-alanosine on colony for-

mation of BFU-E or CFU-GM and HPP-CFC. Approximately 1 X l0�CD34’ cells were cultured in the absence or presence of increasingamounts of L-alanosine in BFU-E (I), CFU-GM (0), CFU-GEMM( . ), or HPP-CFC (D) cultures as indicated. Values shown are the mean

(bars, SD) in which individual BFU-E, CFU-GM, CFU-GEMM, andHPP-CFC colony numbers were normalized as a percentage of thecolony counts obtained in control cultures to which no L-alanosine was

added. A total of three experiments have been performed.

and more than 20% translucent cells after 14 days of culture.

Colonies with more than 80% hernogbobinized cells were re-

ferred to as erythroid colonies, and those with more than 80%

granubo-monocytic cells were considered CFU-GM.

HPP-CFC. Approximately 1 X l0� highly purified

CD34� cells were cultured in I ml of I .2% methylcellulose in

Iscove’s modified Dulbecco’s medium containing 30% dialyzed

horse serum, 1% BSA, 0.1 �iM a-thioglycerol, 15 ng of IL-3,

100 units of GM-CSF, 100 ng of stern cell factor (Amgen), and

20 ng of IL-la (Genetics Institute). The cultures were incubated

for 21-28 days at 37#{176}Cat 5% 02 and 10% CO2. after which time

HPP-CFC colonies were scored using an inverted microscope as

colonies greater than 0.5 mm in diameter (13).

DISCUSSION

The MTAP gene has been mapped to chromosome 9p2l in

close proximity to the tumor suppressor genes p15 and p16,

which encode inhibitors for cyclin-dependent kinases 4 and 6

(15). A high frequency ofp/6 gene deletion and mutations have

been found in a wide variety oftumor cell lines (16) and primary

Clinical Cancer Research 435

protected when 5’-deoxyadenosine is provided as a source of

purines.

Effects of L-Alanosine on Human Hematopoietic Stem/

Progenitor Cells in the Absence or Presence of 5’-Deoxya-

denosine. To develop MTAP-targeted therapy, it is important

to demonstrate that normal cells can be spared, especially he-

matopoietic cells, because the dose-limiting toxicity for a ma-

jority of anticancer agents is rnyebosuppression. We therefore

analyzed the effects of L-alanosine on human hematopoietic

stem/progenitor cells. We also investigated whether hernatopoi-

etic stem/progenitor cells in the purified CD34� preparations

0 2 4 6 8 10 can be rescued from L-alanosine toxicity by 5’-deoxyadenosine,a process that requires MTAP.

BFU-E, CFU-GM, or CFU-GEMM progenitors from the pu-

rifled CD34� cells were cultured in horse serum-containing me-

diurn with increasing concentrations of L-alanosine. As shown in

Fig. 3, their colony growth was suppressed by 5 p.M (or greater)

concentrations of L-alanOsine. However, with the addition of 5-20

�LM of 5’-deoxyadenosine, colony formation of BFU-E or

CFU-GM progenitors was restored to control levels (Fig. 4B),

although concentrations greater than 20 p.M of 5’-deoxy-adenosine

alone also suppressed growth of these progenitors (Fig. 4A). Sim-

ilarly, the colony formation of multipotential CFU-GEMM as de-

fined in “Materials and Methods” was also inhibited by L-alanosine

and rescued with the addition of 5’-deoxyadenosine (data not

shown). Furthermore, exogenous addition of adenine in cultures

salvaged BFU-E or CFU-GM colony formation from suppression

by L-alanosifle, as expected (Fig. 4C).

To determine the presence of MTAP activity in hemato-

poietic stem cells, we examined the ability of early primitive

hematopoietic cells to be rescued from L-alanosine toxicity by

5’-deoxyadenosine using HPP-CFC culture (13). These HPP-

CFCs represent a very primitive stem/progenitor cell population,

although perhaps more mature than the long-term culture-initi-

ating cells (14). It was found that colony formation by HPP-CFC

from regenerated bone marrow was profoundly suppressed by

concentrations of L-alanosine as low as I p.M (Fig. 3), but they

were effectively rescued with the addition of 5-20 jiM 5’-

deoxyadenosine or adenine (Fig. 4, B and C, respectively).

These experiments indicate that the salvage pathway mediatedRESULTS through MTAP is apparently intact in early primitive hemato-

Growth Inhibition of MTAP (-) T-ALL Cell Lines by poietic stem/progenitor cells such as HPP-CFCs.

L-Alanosine in the Presence of 5’-Deoxyadenosine. Malig- On the other hand, MTA, the endogenous substrate of MTAP,

nant cells lacking MTAP are not able to recycle adenine from was toxic to hematopoietic stem/progenitors (ID50 < 1 p.M), pre-

MTA and are thus more dependent on de novo synthesis. By surnably due to feedback inhibition of polyamine synthesis, and

blocking de novo synthesis of AMP with L-alanosine and pro- thus could not be used in vitro to demonstrate the protection of

viding 5 ‘-deoxyadenosine as the only source of adenine, it normal cells from L-alanOsine toxicity (data not shown). Our find-

should be possible to selectively kill MTAP (-) cells. ings thus indicate the presence of MTAP in both hematopoietic

When MTAP (+) Molt-4 and MTAP (-) CEM cells were stern and progenitor cells from human CD34� preparations and the

cultured in medium containing L-alanosine, both cell lines were possibility of targeting MTAP with the use of L-alanosine for the

growth inhibited with IC50 ranging from 6 to 8 p.M (data not selective therapy of T-ALL and other MTAP-deficient cancers.

shown). However, as shown in Fig. 2, in the presence of 5’-

deoxy-adenosine, the MTAP (+) Molt-4 cells were able to

proliferate despite the presence of 30 �LM L-alanosine to a cell

number that was 80% of the control. In contrast, MTAP (-)

CEM cells were not rescued from L-alanOsine toxicity with the

addition of 5’-deoxyadenosine. These data confirm that de novo

purine synthesis inhibitors, such as L-alanosine, can selectively

kill MTAP (-) T-ALL cells, whereas MTAP (+) cells will be



(A)

0 10 20 30 40

Concentrations of 5’-deoxyadenoslno (pM)

0 0.1 1 5 10 20Conceatritiofts of 5’-d.oxysdonoslne (pM)

I

I

120

100

$0

60

40

20

020

CFU-GM (El)’ or HPP-CFC (U) cultures. C, CD34� cells were culturedin the absence or presence of 5 p.M L-alanosine with various amounts ofadenine in BFU-E (0), CFU-GM (EJ)’ or HPP-CFC (U) cultures. In allexperiments, values shown are the mean (bars, SD) in which individualBFU-E or CFU-GM colony numbers were normalized as a percentage of

the colony counts obtained in control cultures to which no L-alanosineand 5 ‘-deoxyadenosine was added. A total of three experiments wereperformed.


(B)

I 120

�100

� $0

160

0

40

20

0

(C)

0 1 5 10Concentration.. of Ad.n1�* (pM)

Fig. 4 Rescue of colony formation of BFU-E, CFU-GM, or HPP-CFCfrom L-alanosine toxicity by various concentrations of 5’-deoxyade-nosine or adenine. A, approximately 1 X l0� CD34’ cells were culturedin the absence or presence of increasing amounts of 5’-deoxyadenosinein BFU-E (s), CFU-GM (0), or HPP-CFC (V) cultures as indicated. B,CD34� cells were cultured in the absence or presence of 5 p.M L-

alanosine with various amounts of 5’-deoxyadenosine in BFU-E (0),

malignancies (17-20). We found that over 60% of T-ALL

patients have a deletion in chromosome 9p2l involving the pitS

and/or piS gene(s) (21). We also found that the MTAP gene,

which is mapped adjacent to the pi6 gene, is frequently co-

deleted with the pi6 gene in close to 40% of cases of primary

T-ALL (21). Frequent MTAP deficiency was also found in other

primary tumors, including gliorna (83%), non-small cell lung

cancer (33%), ALL (1 1 - 14%), and acute nonlymphoid leu-

kemia (7 - 12%; Refs. 1, 6, 7, 22, and 23).

Because MTAP plays an important role in the salvage of

adenine and methionine during polyamine synthesis, malignant

cells deficient in MTAP would be more dependent on an exog-

enous source of adenine and methionine than their normal

counterparts. Previously, we reported that incubation of the

MTAP (-) CEM cells in methionine-deficient medium resulted

in an initial growth inhibition followed by gradual cell death

(24). An alternative approach to taking advantage of MTAP

deficiency in cancer cells is to use drugs that inhibit de novo

punne synthesis, such as L-alanosine (NSC 153, 353; Ref. 25).

L-Alanosine is a potent inhibitor of the conversion of IMP to

AMP, and its active metabolite inhibits adenylosuccinate syn-

thetase (26). Because no other enzyme is inhibited by L-al-

anosine or its metabolites at p.� concentrations; it specifically

blocks the synthesis of ATP, but not the synthesis of GTP.

Moreover, it was reported that the growth-inhibitory effect of

alanosine was completely reversed with the addition of adenine

in vitro, in concert with a restoration of depleted ATP pools (27,

28).

We have recently found that the MTAP (-) CEM cells

appear to be as sensitive to L-alanosine as the MTAP (+) Molt-4

cells. However, addition of MTA, the naturally occurring sub-

strate of MTAP, partially protected the MTAP (+) Molt-4 cells

but not the MTAP (-) CEM cells from L-alanosine cytotoxicity

(5). Complete protection was not achieved in vitro because of

the cytotoxic effects of the exogenous MTA at high concentra-

tions. Using the less toxic 5’-deoxyadenosine as the substrate of

MTAP in the present study, we were able to demonstrate a

complete rescue of the Molt-4 cells from alanosine cytotoxicity.

In contrast, the MTAP (-) CEM cells were not rescued by

5’-deoxyadenosine. These findings suggest the possibility of

targeting MTAP for selective therapy of T-ALL.

It was reported that more than 98% of nucleated cells in

human bone marrow exhibited MTAP activity (3). Because the

early primitive hematopoietic stem/progenitor cells represent

less than 0. 1 % of nucleated cells in bone marrow and peripheral

blood (29), it remains to be shown whether these early hema-

topoietic cells contain functional MTAP activity. This is an

important issue in the design of MTAP-selective chemotherapy.

Human hematopoietic stem/progenitor cells express the CD34



Clinical Cancer Research 437

antigen, but the population of CD34� cells is heterogeneous.

The purified CD34� preparations contain cells that are func-

tionally primitive stem cells (< 1 in 102 CD34’ cells), as well as

more mature, lineage-specific progenitors. In the present study,

we found that marrow cultures of a variety of hematopoietic

lineages were completely protected from alanosine toxicity by

5’-deoxyadenosine. Such findings confirmed the presence of

functional MTAP activity in the committed progenitors. like

erythroid BFU-E, granubocytic/monocytic CFU-GM, or multi-

potent CFU-GEMM (29). In addition, culture of HPP-CFCs,

which requires at least three or more hematopoietic factors for

proliferation and generates colonies as large as 3000-8000 cells

(13), provided specific information on the effect of L-alanosine

on the proliferation and differentiation of the early primitive

HPP-CFCs. The fact that the addition of 5’-deoxyadenosine

rescues these cells from L-alanosine toxicity suggests the pres-

ence of MTAP activity in these early primitive hematopoietic

cells. In the murine model, HPP-CFC has been shown to have a

highly significant correlation with cells capable of repopulating

the bone marrow of lethally irradiated mice and therefore rep-

resents a primitive cell population, closely related to stem cells.

One recent study suggests that there is MTAP “deficien-

cy” in human hematopoietic committed progenitors because

MTA failed to reverse the suppression of the colony forma-

tion of BFU-E, CFU-GM, and CFU-GEMM by methionine

depletion (30). Our observation of a great reduction in the

colony growth of hematopoietic progenitor cells in the pres-

ence of p.� concentrations of MTA is consistent with this

study, as well as previous reports on MTA toxicity in murine

and human hernatopoietic progenitors (3 1 , 32). However, our

study also demonstrated the rescue of alanosine-induced in-

hibition of colony formation of these hernatopoietic progen-

itors by 5’-deoxyadenosine, a process that requires MTAP

activity in these cells. There are two possible explanations for

the back of protection from methionine starvation by MTA.

First, hematopoietic stem/progenitor cells are extremely sen-

sitive to the known MTA toxicity in in vitro marrow culture,

presumably due to inhibition of methylation reactions, as

well as to polyamine synthesis (33-36), which may be im-

portant in proliferation/differentiation of hematopoietic cells.

During normal hematopoiesis, the endogenous MTA pro-

duced under physiological conditions must be rapidly sal-

vaged into adenine and methionine by MTAP to avoid accu-

mulation of toxic levels of MTA. Second, salvage of

methionine from MTA requires the conversion of 5-methyl-

thioribose-i-phosphate into methionine by a series of corn-

plex oxidations via the intermediate 2-keto-4-methylthiobu-

tyrate (37). The apparent lack of MTA rescue in vitro may

therefore be a consequence of defects in the other complex

enzymatic reactions following MTAP reaction, instead of

MTAP deficiency per Se.

Taken together, our results, along with those of previous

studies, suggest that use of specific de novo purine synthesis

inhibitors can selectively kill MTAP (-) cancer cells under

conditions in which MTAP (+) normal cells, including hema-

topoietic stem/progenitor cells, will be protected by using the

endogenous MTA as a source of adenine. Thus, the presence of

a common metabolic defect such as MTAP deficiency can

facilitate development of selective chemotherapy for several

types of cancer, including T-ALL.

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1997;3:433-438. Clin Cancer Res J Yu, A Batova, L Shao, et al. for MTAP (-) malignancies.hematopoietic stem/progenitor cells: its therapeutic implication Presence of methylthioadenosine phosphorylase (MTAP) in

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