(Received March 14; accepted for publication April 30, 1973.)
Supported in part by research grants CA-07357 and CA-03161, and institutional cancer training grant CA-08017, all from the National Cancer Institute, National Institutes of Health.
ADDRESS FOR REPRINTS: (J.M.F.) Texas Children’s Hospital, 6621 Fannin Street, Houston,
Texas 77025.
PEDIATRICS, Vol. 52, No. 6, December 1973
846
LEUKEMIA
IN TWINS
John M. Falletta, M.D., Kenneth
A. Starling, M.D.,
and Donald J. Fernbach, M.D.
From the Department of Pediatrics, Baylor College of Medicine, and the Research Hematology
Laboratory, Texas Children’s Hospital
ABSTRACT. A disorder for which identical twins are not completely concordant must be caused, at
least in part, by environmental factors. Acute
childhood leukemia is such a disorder, with con-cordance ratios as follows : approximately 1:5 for monozygotic twins, approximately 1:80 for dizygo-tic twins, and more than 1:500 for ordinary siblings.
Of all malignancies, only acute childhood leu-kemia carries such an extraordinar risk of twin
concordance. It is the only childhood malignancy
in which a large number of circulating malignant
cells are found. In most instances, twins with
concordant leukemia have their onset of illness within weeks or months of each other, suggesting a similar inciting event. Since placenta! cross-circulation is found commonly in monozygotic
twins and occasionally in dizygotic twins, cross-infusion of malignant cells has been postulated to account for the high twin concordance ratios. Leukemia in most twins might then represent only one occurrence of leukemia and not two.
This hypothesis, while consistent with most ob-servations, does not explain the pattern of disease in the twins described in this paper. Our proband became ill at nearly 6 years of age, and her twin’s illness occurred more than 6 years later. These twins probably remained at risk of concordant
disease because of postnatal factors affecting
pre-disposed siblings. Whether the predisposition was
due to genetic or to prenatal environmental factors
remains unknown. Pediatrics, 52:846, 1973,
LEU-KEMIA, TWINS.
The relative contribution of heredity and environment in the genesis of acute leu-kemia is unclear. But the issue may be
evaluated by studying the occurrence of
acute leukemia in twins. A high
concord-ance rate in monozygotic
(
MZ)
twinsmight suggest that inherited factors govern
the onset of the disease. However, the effect of a heritable factor cannot with cer-tainty be distinguished from the response
to an intra-uterine insult,’ unless
varia-tion between twin siblings in the type of leukemia or the age at disease onset is
noted. This variation must then be at-tributed to pre- or postnatal environmental factors.2 Twin studies in general do not
permit further definition of the timing or nature of such environmental factors, al-though the disease occurrence in a
particu-lar twin pair might suggest the timing of the leukemogenic event(s).
The following report describes a MZ twin
pair with acute leukemia. The proband was
nearly 6 years old when her diagnosis was
made, and her twin developed leukemia
6 years, 3 months later. The long interval
be-tween their illnesses suggests that separate
environmental insults led to their disease.
Although a prenatal origin of their disease
cannot be excluded, their ages suggest that
separate postnatal leukemogenic events
occurred.
Genetic and environmental interaction may also be evaluated by comparing the
occurrence of acute leukemia in dizygotic
(
DZ) twins versus ordinary siblings.2 DZ twins are as genetically dissimilar as areordinary siblings, but they have a common
prenatal environment.2’ 3 A major excess in
their leukemia concordance rate compared
with ordinary siblings would tend to reflect
environmental factors, probably prenatal in origin. As will be shown, DZ twins
847
of concordance compared with observed
rates for ordinary siblings.
CASE REPORTS
The second pregnancy of the 24-year-old mother was uneventful, except for intermittent,
spontane-ous vaginal bleeding during the first 6 weeks.
Obstetrical x-ray studies were not done. Spontane-ous labor of 6 hours’ duration occurred in Decem-ber 1959, after approximately 34 weeks’ gestation.
The interval between the delivery of the twins
was 8 minutes. A single placenta with two
um-bilical cords was delivered intact. A membrane which could be divided into two layers separated the two cords, leading the obstetrician to note that two amnions and one chorion were probably present; histologic confirmation was not obtained.
TWIN A: (TCH #251,308) weighed 1.87 kg at birth. Her neonatal period was uneventful and she was discharged from the hospital at 35 days of age weighing 2.56 kg. Her growth and
develop-ment proceeded normally. Immunization to
diptheria-pertussis-tetanus, polio, and smallpox
occurred without sequelae. She recovered from
rubeola, rubella, and varicella infections without
complication. She was identical to her sister in
physical appearance. At 5 years, 10 months of age she developed fever, pharyngitis, and epistaxis.
She was pale and her liver was palpable 4 cm
below the right costal margin. Lymph nodes and
spleen were not palpably enlarged. Peripheral
blood studies were as follows: hemoglobin, 5.6
gm/100 ml; hematocrit, 18%; leukocyte count,
1,500/cu mm with 47% segmented neutrophils and
53% lymphocytes; reticulocyte count, 0.3%; platelet
count, 36,000/cu mm. Smears from the bone
marrow aspirate were diagnostic of acute stem
cell leukemia. The red blood cell type was A Rh
(
D) negative. Therapy with 6-mercaptopurine and prednisone led to clinical and bone marrow re-mission. Subsequent therapy included methotrexate and cyclophosphamide. Her final hospitalizationoccurred while in bone marrow relapse when she
developed persistent epistaxis, fever, and cellulitis
of the vulva. Pseudomonas aeruginosa grew from
a culture of her blood. She did not respond to
antibiotic therapy and died 5 days after admission, 28 months after diagnosis. Postmortem examination revealed widespread agranulocytic cellulitis of the perineal area, agranulocytic pneumonia, and diffuse infiltration of undifferentiated mononuclear cells in the liver, spleen, lymph nodes, kidneys, lungs, colon, and skin.
TWIN B: (TCH #339,990) weighed 1.85 kg
at birth. Spontaneous respiration was not estab-lished for 2 minutes. She then had tachypnea, grunting respiration, chest retractions, and a poor Moro reflex. She received intravenous fluids for 2 days, then was gavage fed for 4 days. Her
respira-tory function improved, and she subsequently tolerated oral feedings well. She was discharged from the hospital at 37 days of age weighing 2.50
kg. Her subsequent growth and development were
normal. Immunization to diptheria-pertussis-tetanus,
polio, rubeola, and smallpox occurred without
sequelae. She recovered from varicella and rubella
infections without complication. At 12 years, 1
month of age she became pale and febrile, bruised easily, and had a 1 x 1 cm right postauricular mass thought to be a lymph node. Her liver and spleen
were not palpably enlarged. Peripheral blood
studies were as follows: hemoglobin, 5.5 gm/100
ml; hematocrit, 15%; leukocyte count, 4,300/cu mm with 26% segmented neutrophils and 74%
lympho-cytes; reticulocyte count, 0.1%; platelet count,
3,000/cu mm. Smears from the bone marrow
aspirate were diagnostic of acute stem cell leukemia and were identical with the initial bone marrow
smears of her twin. The red blood cell type was
also A Rh (D ) negative. Initial therapy with yin-cristine and prednisone led to clinical and bone marrow remission. She received intrathecal pro-phylaxis using methotrexate, cytosine arabinoside, and hydrocortisone. Presently she remains in
din-ical and bone marrow remission, receiving
6-mercaptopurine as continuous maintenance with a
month of prednisone reinforcement every 3 months.
FAMILY HISTORY: The father (age 26 when
the twins were born ) has multiple sclerosis; his symptoms began in 1958. Male siblings born in 1958 and 1962 are normal. A paternal granduncle died with an unspecified malignancy. The family history is negative for other twinning, congenital
malformations, or blood disorders. The mother had
one spontaneous abortion in 1961.
DISCUSSION
Acute leukemia occurs in both members
of a twin set more frequently than does
any other malignancy. Twenty-seven
con-cordant twin pairs have been reported.
These include 21 pairs listed by Zuelzer
and Cox in their comprehensive review,’
five previously unreported pairs
ascer-tamed by Miller from a death certfficate survey,4 and the present pair. (The third and sixth twin sets from Miller’s Table I are identical in identifying features with cases previously reported.5’6
)
Fourteenpairs were male and 12 were female, with
only 1 male-female set. This proportion
differs strikingly from the distribution of
twins in the general population, in which
about one third are male-female pairs.’ The
LEUKEMIA IN TWINS
the like-sex pairs were MZ. While zygosity
was often not clearly established, three pairs proven to be DZ have been reported.1
If one considers all other sets to be MZ, the
MZ to DZ ratio is 24:3 or 8:1.
All cases of concordant acute leukemia
in twins have occurred in At
least 14 of the 27 probands were under 1
year of age at diagnosis, in contrast to the peak incidence of childhood leukemia
which occurs in 3-year-old children.r The
leukemia risk for MZ twins (the pair taken
as a single unit) is the same or slightly below the overall leukemia risk for any child.8 However, once a MZ twin develops leukemia, the co-twin has an exceptionally
high risk (about 1 :5) of also developing
the In addition, the co-twin
usu-ally becomes ill within weeks or months of his twin.’ Lack of concordance in about
80% of MZ twins indicates that genetic fac-tors alone do not account for the excess
6 Pre- and postnatal environmental
factors must play an important role.
In some instances a MZ pair may have
leukemia even when a single pre- or post-natal insult would not likely account for their concordance. Our proband was nearly
6 years old when her illness began, and her
sister became ill more than 6 years later. Their common intra-uterine experience and
genetic identity may have predisposed to extra-uterine susceptibility. But a single
insult would not likely have led to their
disease because they became ill 75 months apart. Although their disease may have had
a prenatal origin, their ages suggest that
one and possibly both of them experienced
a signfficant postnatal leukemogenic insult.
This twin pair differs strikingly from the
other 23 presumably MZ concordant pairs.
None of the other co-twins became ill or
died more than 3 years after their twin;
only three were separated by more than 1
year.”4 Only one other proband became ill
at an older age (7 years ).1
The effect of genetic and environmental
factors in leukemogenesis may also be
evaluated by comparing leukemia
con-cordance in DZ twins and ordinary siblings.
DZ twins are as genetically dissimilar as are
ordinary 23 A major variation in
their rate of concordant leukemia,
com-pared to ordinary siblings, would tend to
reflect environmental factors. The following
calculations, based on a review of leukemia in twins and siblings, suggest that DZ twins
rank between MZ twins and ordinary siblings in concordance risk.
Since the reported ratio of MZ to DZ
concordant pairs is 8: 1, and DZ twinning
occurs about twice as often as MZ
twin-3 the differential risk of leukemia
concordance in the general twin population
(
MZ: DZ) is about 16: 1. The observed MZconcordance risk is about 1 :5,’ giving a
DZ risk of about 1 :80
(
1/16 of the MZ risk)
. Estimates of the concordance rate ofacute leukemia in ordinary siblings range
from a rate equal to the risk in the general pediatric population to nearly four times
that rate.9 The overall incidence of acute
leukemia in children is about 3.7 new
cases/100,000 children/year. About one in
2,000 children will develop acute leukemia
between birth and 15 years of age. Even
the maximum estimated sibling
concor-dance risk (nearly 1:500) is considerably
less than the risk of about 1 :80 suggested for DZ twins.
These calculations are based on all re-ports of concordant acute leukemia in twins. Our observed MZ:DZ ratio probably
represents a valid approximation of the
true concordance ratio, for it is doubtful
that any bias exists in the reporting of MZ
or DZ pairs. However, at least one pair presumed to be MZ4 was actually DZ.5 Since the zygosity of other presumed MZ
pairs is not definitely established, the
cal-culated DZ risk is a minimum estimate.
A common prenatal experience is the major feature distinguishing DZ twins from ordinary siblings. Intra-uterine exposure to leukemogens probably contributes to their
excess leukemia. However, twins also have
a similar postnatal environment. Unlike
ordinary siblings, DZ twins would likely be exposed to a particular postnatal leuke-mogen at the same age and at a similar
point in their immunologic development.
849
likelihood of the twins having a concordant
response to the postnatal leukemogen. Regardless of zygosity, leukemia occur-ring in both members of a twin pair ap-pears to be due in large part to their shared
prenatal environment. Clarkson and Boyse1#{176}
have suggested that the most important
prenatal factor leading to the shared
en-vironment is placental cross-circulation, which occurs commonly in MZ twins1’ and occasionally in DZ 1 Leukemia in
twins, MZ in particular, might then
repre-sent only one occurrence of leukemia and
not two, with infusion of malignant cells
from the affected twin to the unaffected
co-twin leading to the concordance. The brief
interval separating the onset of disease in many twin pairs would support. this view. Even more convincing. is the observation
that only acute leukemia of childhood
car-ries such an extraordinary risk of twin
con-cordance.
Other childhood malignancies,
particular-ly neuroblastoma, are thought to have a
pre-natal origin but do not have the high twin
concordance rate noted for acute leukemia.
These malignancies differ from leukemia in
part by the absence of large numbers of
circulating malignant cells, making cross-transfusion much less likely for tumor cells than for leukemic cells. While the cross-circulation hypothesis is attractive in ex-plaining many instances of twin leukemia, it does not account for those instances in
which the proband’s illness occurred later
in childhood. Such pairs, as illustrated by
our twins, remain at risk of concordant
disease probably because of postnatal
fac-tors affecting predisposed twins. Whether
the predisposition is genetic or is due to
factors in the prenatal environment remains
unknown.
SUMMARY
Twin girls, who probably were
monozy-gotic, both developed acute leukemia. The
proband’s illness began when she was nearly 6 years old, and her twin’s occurred 75 months later. This long interval suggests
that separate environmental insults were responsible for their disease.
Concordance ratios for acute childhood leukemia in twins and ordinary siblings illustrate a decreasing risk of leukemia
con-cordance with decreasing genetic and environmental similarity. The risk of
concordance for monozygotic twins is ap-proximately 1 :5, for dizygotic twins it is approximately 1 : 80, and for ordinary
sib-lings it is more than 1:500.
REFERENCES
1. Zuelzer, W. W., and Cox, D. E. : Genetic as-pects of leukemia. Semin. Hematol., 8:228, 1969.
2. Thompson, J. S., and Thompson, M. W.:
Genetics in Medicine, ed. 1. Philadelphia and London: W. B. Saunders, 1966, p. 233. 3. McKusick, V. : Human Genetics, ed. 2. In
Suskind, S. R., and Hartman, P. E., ed.:
Foundations of Modern Genetics Series.
Englewood iiffs, New Jersey: Prentice-Hall, Inc., 1969, p. 116.
4. Miller, R. W. :‘iDeaths from childhood leukemia and solid tumors among twins and other sibs in the United States. J. Nat. Cancer Inst., 46:203, 1971.
5. Sandberg, A. A., Cortner, J., Takagi, N.,
Moghadam, M. A., and Crosswhite, L. H.:
Differences in chromosome constitution of
twins with acute leukemia. New Engl. J.
Med., 275:809, 1963.
6. Pearson, E. A., Grello, F. W., and Cone,
T. E., Jr. : Leukemia in identical twins. New
Eng. J.Med. 268:1151, 1963.
7. Fernbach, D. J.: Natural history of acute
leukemia. In Sutow, W. W., Vietti, T. J.,
and Fernbach, D. J., ed. : Clinical Pediatric Oncology. St. Louis, Missouri: C. V. Mosby,
in press.
8. MacMahon, B., and Levy, M. A. : Prenatal origin of childhood leukemia. New Eng. J. Med., 270:1082, 1964.
9. Miller, R. W. : Persons with exceptionally high risk of leukemia. Cancer Res., 27:2420, 1967. 10. Clarkson, B. D., and Boyse, E. A. : Possible
explanation of the high concordance for
acute leukaemia in monozygotic twins. Lan-cet, 1:699, 1971.
11. Strong, S. J., and Corney, G.: The Placenta in
Twin Pregnancy, ed. 1. Oxford: Pergamon
Press, 1967, p. 65.
Acknowledgment
The authors wish to thank Drs. Clark W. Heath, Jr., Dennis O’Connor, and Henry Falk for their
help in developing the ideas presented in this
manuscript, and Drs. George A. Dawson and