Epidemiological evidence has suggested that some pediatric leukemias may be initiated in utero and, for some pairs of identical twins with concordant leukemia, this possibility has been strongly endorsed by molecular studies of clonality. Direct evidence for a prenatal origin can only be derived by prospective or retrospective detection of leukemia-specific molecular abnormalities in fetal or newborn samples. We report a PCR-based method that has been developed to scrutinize neonatal blood spots (Guthrie cards) for the presence of numerically infrequent leukemic cells at birth in individuals who subsequently developed leukemia. We demonstrate that unique or clonotypic MLL-AF4 genomic fusion sequences are present and detectable in neonatal blood spots from individuals who were diagnosed with acute lymphoblastic leukemia at ages 5 months to 2 years and, therefore, have arisen during fetal hematopoiesis in utero. This result provides unequivocal evidence for a prenatal initiation of acute leukemia in young patients. The method should be applicable to other fusion genes in children with common subtypes of leukemia and will be of value in attempts to unravel the natural history and etiology of this major subtype of pediatric cancer.Epidemiological evidence suggests that exposures or events that occur prenatally or in infancy might play a role in the etiology of pediatric acute leukemia, the most common type of childhood cancer in developed countries (1-3). Being able to backtrack leukemic clones to the time of such events would have a considerable impact on our understanding of the natural history of the disease and on the design and interpretation of epidemiological studies. This requires access to both a leukemia-specific marker and, retrospectively, appropriate biological material. The only biological markers that can provide definitive identification of a leukemic clone are clonotypic alterations in DNA that are present in the leukemic cells at diagnosis, e.g., unique nonconstitutive mutations or rearrangements of genes (4-6). These provide specific and sensitive molecular markers for tracking the disease clone with the significant caveat that the mutant DNA sequence identified is not necessarily the initiating or first mutation in the leukemia and, therefore, might be absent at early stages of clonal evolution. The one readily available retrospective source of DNA from leukemic children is the Guthrie card or blood spot taken routinely by heel prick on most newborns (7,8). Normally used to detect evidence of inborn errors of metabolism, DNA from these spots has been used to detect constitutive mutations (9-11) and exogenous viral sequences (12, 13) using PCR but not, as far as we are aware, for acquired molecular abnormalities in leukemia or other cancers. We reasoned that blood spot DNA from individuals who developed leukemia at a young age would enable us to test the idea that the leukemic clone with its acquired molecular marker could have an in utero fetal origin and therefore be present, albeit at a...

Histone modifications are key regulators of chromatin function. However, little is known to what extent histone modifications can directly impact on chromatin. Here, we address how a modification within the globular domain of histones regulates chromatin function. We demonstrate that H3K122ac can be sufficient to stimulate transcription and that mutation of H3K122 impairs transcriptional activation, which we attribute to a direct effect of H3K122ac on histone-DNA binding. In line with this, we find that H3K122ac defines genome-wide genetic elements and chromatin features associated with active transcription. Furthermore, H3K122ac is catalyzed by the coactivators p300/CBP and can be induced by nuclear hormone receptor signaling. Collectively, this suggests that transcriptional regulators elicit their effects not only via signaling to histone tails but also via direct structural perturbation of nucleosomes by directing acetylation to their lateral surface.

HP1 proteins are major components of heterochromatin, which is generally perceived to be an inert and transcriptionally inactive chromatin structure. Yet, HP1 binding to chromatin is highly dynamic and robust silencing of heterochromatic genes can involve RNA processing. Here, we demonstrate by a combination of in vivo and in vitro experiments that the fission yeast HP1(Swi6) protein guarantees tight repression of heterochromatic genes through RNA sequestration and degradation. Stimulated by positively charged residues in the hinge region, RNA competes with methylated histone H3K9 for binding to the chromodomain of HP1(Swi6). Hence, HP1(Swi6) binding to RNA is incompatible with stable heterochromatin association. We propose a model in which an ensemble of HP1(Swi6) proteins functions as a heterochromatin-specific checkpoint, capturing and priming heterochromatic RNAs for the RNA degradation machinery. Sustaining a functional checkpoint requires continuous exchange of HP1(Swi6) within heterochromatin, which explains the dynamic localization of HP1 proteins on heterochromatin.