The present study systematically registered a recurrent gain of chromosome 1q in all NSC derivatives of pluripotent stem cell lines maintained in long-term culture. This 1q duplication was most often isolated when first observed; cells acquired at later passages demonstrated additional chromosomal abnormalities, including aneuploidy and polyploidy. In NSC lines exhibiting the 1q duplication, cell behavior may be heavily biased toward self renewal, as the potential for neuronal differentiation may be altered. NSCs may also fail to survive and/or differentiate after transplantation. The relatively small number of cell lines assayed did not permit us to determine whether these functional consequences relate directly to the 1q duplication or to its specific association with a particular chromosome. Indeed, both the size of the duplicated fragment and the recipient chromosomes differed between the 2 batches of mutant VUB05-HD NSCs that exhibited different differentiation capacities. We cannot exclude that the different ages of the control and mutant cells in the graft experiment could account for the failure of the mutant cells to engraft: control cells never had more than 15–20 passages, whereas homogeneous populations of mutant cells were always older, a technicality that precluded firm conclusion. It is worth noting, however, that 1q translocation did not systematically hamper the differentiation of NSCs in neurons in vitro (Table ). Whether the 1q translocation observed in NSCs derived from pluripotent stem cells may be consequential for tumorigenesis is another unsolved issue in the absence of affected cell survival in our transplantation assay. Nevertheless, chromosomal rearrangements — both the duplication of a segment and its translocation onto different recipient chromosomes — have already been observed and termed jumping translocations
(JTs; ref. 13
and Supplemental Table 2). Acquired JT aberrations are seen mostly in hematological neoplasias, in which they are associated with poor prognosis. The majority of published data concern hematological malignancies (14
), which can explain why JTs have mostly been observed in hematological neoplasias. However, duplication of the 1q arm has been also observed in solid tumors, such as breast cancer, chordomas, hepatocellular carcinoma, retinoblastoma, and pediatric brain tumors (Supplemental Table 2), which suggests that this defect may affect many cell phenotypes. In the case of pediatric brain tumors, the 1q gain correlates with poor clinical outcome independent of tumor grade and histological type (15
). JTs recurrently involve chromosome 1q in these cases. This common specific alteration in NSCs derived from pluripotent stem cells that escaped senescence in the present study, and in all these malignancies, suggests that causal mechanisms and functional consequences may also be similar, and this calls for caution. 1q JT randomly occurred in the present study onto recipient chromosomes 1, 4, 5, 8, 10, 13, 15, 17, 18, 21, 22, and Y, which strongly suggests that the abnormality primarily concerns the 1q region itself, rather than the recipient chromosome.
How 1q JT is related to loss of evolution toward senescence, and whether this is a direct or indirect connection, is beyond the scope of the present study. It is interesting to note, however, that chromosome 1q is the longest human chromosome arm, containing more than 1,700 genes and 40 miRNAs (NCBI Map Viewer database;
). Some of these genes may be deregulated after duplication and translocation, for example, as seen with the BCR-Abelson translocation in chronic myeloid leukemia. Accordingly, a study by Fournier et al. identified 1q12 chromosome translocations in B cell lymphoma and proposed that 1q12 rearrangements represent a new paradigm for long-range epigenetic deregulations in cancer (16
In contrast to prior studies using undifferentiated pluripotent stem cells themselves (17
), there have been few investigations of chromosomal abnormalities in their differentiated derivatives, although analyses of mouse ESC derivatives have identified chromosomal changes (20
) in cultured mouse neurospheres and NSC lines (Supplemental Table 3). Quite interestingly, the human 1q chromosome arm corresponds to mouse chromosomes 1 and 3, and chromosome 1 is also the most affected site in the mouse neurospheres and NSC lines analyzed (Supplemental Table 3). Long-term cell culture of human NSCs derived from fetal brain also revealed chromosomal abnormalities (ref. 21
and Supplemental Table 3). At odds with our results, however, are the results of 2 studies that reported a stable karyotype in neural derivatives of pluripotent stem cells that were extensively propagated (refs. 22, 23, and Supplemental Table 3). This discrepancy may be attributable to the capacity of some ES or iPS cell–derived neural cell lines to continue dividing over many more passages than other lineages; it would be of major interest to examine the phenotypic differences between NSCs analyzed by those teams compared with those of our group, as slight technical differences in differentiation protocols may be the cause. For instance, high levels of antioxidants may increase aneuploidy in stem cell cultures (24
), and high levels of B27 were used in our study that may have exacerbated the genomic instability of the NSCs. However, antioxidants induce a variety of chromosomal abnormalities (24
), whereas in our study, the chromosomal changes were clearly nonrandom. It would also be necessary to use common approaches for screening for chromosomal defects, as the other studies relied on G-banding karyotyping, whereas some abnormalities — albeit not all — required more detailed techniques to be revealed, namely, mFISH and aCGH.
Why the 1q region is especially prone to JT is a matter of speculation. Chromosome stability is regulated by constitutive heterochromatin (25
). The presence of fragile sites FRA1J
(1q12) and FRA1F
(1q21) in the heterochromatic region may predispose the 1q arm to genomic instability. The other chromatin feature that may predispose to instability is condensation state. It has been suggested that the first step in the chromosome 1q JT may be caused by heterochromatic decondensation, leading to centromeric destabilization (26
). Partial endoreduplication would then occur when heterochromatin is decondensed, facilitating the formation of a JT. This heterochromatin decondensation of chromosome 1q12, the largest heterochromatin site in the human genome, is characteristic of immunodeficiency, centromeric region instability, and facial anomalies (ICF) syndrome (27
). ICF syndrome, a rare autosomal-recessive disease caused primarily by a mutated DNA methyltransferase gene (DNMT3B
; ref. 27
), is characterized by decondensation of the juxtacentromeric heterochromatin of chromosomes 1 and 16, which are then prone to breakage. This chromosomal instability is associated with demethylation of the juxtacentromeric regions of these chromosomes (27
). The similarity of the centromeric instability observed in neural derivatives compared with that in patients with JT or with ICF syndrome suggests a similar etiologic mechanism.
Our present results advocate regular monitoring, not only of human pluripotent stem cells (as explicitly requested by international guidelines; ref. 28
), but also of their progenies, in particular when these seem to avoid senescence. We propose checking chromosome 1q status as a regular control for genomic integrity in neural derivatives of pluripotent stem cells. Such controls will be particularly needed when neural derivatives are considered for cell therapy, as this may compromise safety, as well as for in vitro drug discovery and toxicology, because JT would likely bias results.