Acute lymphoblastic leukemia in adult identical twins

Leukemia in twins: lessons in natural history

Identical infant twins with concordant leukemia were first described in 1882, and since that time many such pairs of infants and older children have been described. It has long been recognized that this situation offers a unique opportunity to identify aspects of the developmental timing, natural history, and molecular genetics of pediatric leukemia in general. We reviewed both the older literature and more recent molecular biologic studies that have uncovered the basis of concordance of leukemia. Molecular markers of clonality, including unique, genomic fusion gene sequences, have provided unequivocal evidence that twin pairs of leukemia have a common clonal origin. The only plausible basis for this, first suggested more than 40 years ago, is that following initiation of leukemia in one twin fetus, clonal progeny spread to the co-twin via vascular anastomoses within a single, monochorionic placenta. This explanation has been endorsed by the identification of clonotypic gene fusion sequences in archived neonatal blood spots of individuals who subsequently developed leukemia. These analyses of twin leukemias have thrown considerable light on the natural history of disease. They reveal a frequent prenatal origin and an early or initiating role for chromosome translocations. Further, they provide evidence for a variable and often protracted latency and the need, in childhood acute lymphoblastic leukemia (ALL)/acute myeloblastic leukemia (AML), for further postnatal exposures and/or genetic events to produce clinical disease. We argue that these insights provide a very useful framework for attempts to understand etiologic mechanisms.

Incidence of childhood cancer in twins

The incidence of childhood cancer in twins was evaluated by linking a roster of 30,925 twins born in Connecticut (United States) between 1930 and 1969 with the Connecticut Tumor Registry. Cancer, exclusive of nonmelanoma skin cancer, was identified in 19 females and 12 males under 15 years of age. The incidence rate among twins was 7.9 cancers per 100,000 person-years (PY) overall, and 9.7 and 6.1 per 100,000 PYs for females and males, respectively. Four of 13 leukemias occurred in two female twin pairs, representing concordance rates of 18 percent overall and 29 percent for like-sex pairs, which are somewhat higher than values reported previously. The number of cancers expected was computed on the assumption that twins experienced the same sex-, age-, and calendar time-specific cancer rates as recorded for all Connecticut-born children. Because active follow-up of individuals was not conducted, an adjustment to person-years of observation was made to account for childhood mortality, including the high perinatal mortality characteristic of twins. Childhood cancer was 30 percent less frequent than expected (standardized incidence ratio [SIR] = 0.7; 95 percent confidence interval [CI] = 0.5-0.9), a deficit that is marginally greater than those found in previous studies. Both leukemia (SIR = 0.8; CI = 0.4-1.4), and all other cancers combined (SIR = 0.6; CI = 0.3-0.9) occurred less often than expected. The deficit was greater among males (SIR = 0.5; CI = 0.2-0.8) than among females (SIR = 0.9; CI = 0.5-1.4) and was especially pronounced among males younger than five years (SIR = 0.2; CI = 0.0-0.7). The data support the view that twins, particularly male twins, have a lower risk of childhood cancer than single-born children. Any added risk for twins associated with their greater frequency of exposure to prenatal X-rays appears to have been insufficient to offset an 'effect' of twinning per se. Possible explanations for this finding include (i) the low birthweight distribution of twins, or (ii) selective early mortality of twin fetuses or neonates who would otherwise have developed a clinical cancer.