(Source: 2012 Oct 18; Cell Stem Cell)
Results and Discussion
Human trisomies are a major health problem, accounting for 23% of spontaneous abortions. Down syndrome (DS) is the most frequent autosomal trisomy (chromosome 21), with numerous clinical manifestations including heart defects, impaired cognition, premature aging, and a high frequency of specific leukemias. Trisomies produce diverse effects on cultured cells. Fibroblasts from trisomic mice proliferate slowly and have increased sensitivity to specific drugs, while human pluripotent stem cells with trisomy 12 or 17 have an apparent growth advantage in vitro. In DS, the effects of an extra copy of chromosome 21 have been studied by comparing normal and trisomic cells from distinct individuals, where differences in age, medical history, genetic polymorphisms, and cell passaging may have influenced results. Ideally, the effects of human trisomy should be compared in otherwise isogenic, disomic cell lines, as done previously in yeast and mice.
Induced pluripotent stem cells (iPSCs) offer a promising way to study human trisomy because they can be derived from the somatic cells of trisomic individuals or fetuses, cultured indefinitely, and differentiated into diverse cell types. We generated six independent iPSC lines from DS fibroblasts as described and validated their trisomy 21 karyotypes. These iPSCs were used to derive disomic subclones. A TKNEO fusion gene encoding both thymidine kinase and neomycin resistance was introduced into chromosome 21 of DS iPSCs for positive-negative selection. The Amyloid Precursor Protein (APP) gene was chosen as the target locus due to its location on the long arm of chromosome 21 and high expression in iPSCs. The adeno-associated virus vector AAV-APPe3ITKNA was used to introduce an internal ribosome entry site (IRES)-TKNEO-polyadenylation site cassette into exon 3 of the APP gene by homologous recombination. Five transduced, G418-resistant clones were isolated from each of three DS iPSC lines and analyzed by Southern blots. Twelve of these clones were targeted correctly at one APP allele and lacked random integrants. Based on these results, 0.14% of all iPSC colony-forming units (CFUs) underwent gene targeting, comparable to previous reports.
APP-targeted clones were cultured in the absence of G418 selection for three passages to allow chromosome loss to take place, and then were cultured in the presence of gancyclovir (GCV) to select against TKNEO expression. A total of 33 GCV-resistant clones derived from four different DS iPSCs were screened for disomy by quantitative Southern blot analysis using three probes from the long arm of chromosome 21 (centromeric, APP target locus, and telomeric) and a reference probe on chromosome 6. By comparison to trisomic and disomic controls, we determined that 23 of the 33 GCV-resistant clones had lost one copy of chromosome 21. Seven of these clones were karyotyped and all had two copies of chromosome 21, validating the Southern blot method and demonstrating that disomic iPSCs had been derived from trisomic DS iPSCs. We also showed that different copies of chromosome 21 had been targeted and lost, based on SNP analysis of the disomic clones.
Because the targeted copy of chromosome 21 was not required for cell growth, we were able to measure the frequencies of all causes of GCV resistance without bias. Overall, loss of an entire copy of chromosome 21 was the most common cause in five out of five independent experiments with an average frequency of 1.4 × 10−4. The remaining 10 clones were characterized further to determine the basis for their GCV resistance. Southern blots showed that eight clones were still trisomic at chromosome 21. Six of these contained the TKNEO gene, which was either mutated in the coding region (five clones) or silenced based on the lack of mRNA expression (one clone) . The frequencies of TKNEO coding mutations and silencing in DS iPSCs were similar to those observed previously in mouse ESCs (Chan. Two other trisomy 21 clones (C1-5-13 and C3-5-10) had lost the TKNEO gene. These TKNEO deletions were analyzed further to see if loss of heterozygosity (LOH) extending from a recombination crossover point to the telomere had occurred. However, three distinct chromosome 21 haplotypes were present based on SNP analysis, suggesting that more localized gene conversion events were responsible. The absence of clones with extended LOH was unexpected because this is a major cause of gene inactivation/loss in mouse ESCs and human lymphocytes. The two remaining GCV-resistant clones contained three copies of a portion of chromosome 21 (Figure 1C), but only two copies of more telomeric sequences. One of these clones contained a silenced TKNEO gene (C1-5-12), while the other lacked TKNEO and was disomic at chromosome 21 by karyotyping (C3-1-2), suggesting that there may have been a localized duplication of pericentromeric sequences not detected by cytogenetic analysis. Based on this classification, the different causes of GCV resistance were chromosome loss > TKNEO coding mutation > TKNEO transgene deletion > TKNEO transgene silencing
DS fibroblasts and lymphocytes have been reported to have either increased or decreased cell growth in comparison to nonisogenic, disomic cells. We measured the proliferation of four passage-matched, isogenic pairs of disomic and trisomic iPSCs and two control pairs with GCV-resistant trisomic clones that had TKNEO mutations, using mixed cultures in which we tracked the percentage of trisomic cells by Southern blots for the TKNEO gene. In all cases the disomic cells expanded preferentially during coculture, with an average population doubling time of 37 ± 0.7 hr as compared to 45 ± 0.9 hr for trisomic cells. Pure cultures of disomic clones also expanded more quickly than their trisomic counterparts, with 3–7 hr shorter doubling times during the proliferative period. Our results are consistent with those obtained from isogenic mouse cell lines showing that trisomies inhibit proliferation and with the observation that disomy 21 mosaicism increases over time in DS. Based on the profiling of cell cycle phases with bromodeoxyuridine (BrdU) and propidium iodide (PI), a greater percentage of trisomic cells were in the G1 phase, which was not found in mouse trisomy models. These results were not due to altered differentiation kinetics or apoptosis, since expression of the pluripotency surface marker SSEA3 and the apoptosis marker Annexin V were similar in disomic and trisomic cells
The impaired proliferation of trisomic cells has been attributed to gene dosage effects and compensatory changes in protein degradation and energy consumption that result in increased sensitivity to compounds affecting these processes. We performed mixed culture proliferation experiments in the presence of three of these compounds (17-allylamino-17-demethoxygeldanamycin [17-AAG], 5-aminoimidazole-4-carboxamide riboside [AICAR], and chloroquine), but even at the highest tolerated doses, none of these drugs selectively inhibited the proliferation of trisomy 21 iPSCs. Thus, the impaired proliferation of trisomic DS iPSCs is not due to the same alterations in protein degradation and energy consumption observed in trisomic mouse fibroblasts. We also measured proliferation in the presence of cytarabine because DS leukemia cells are particularly sensitive to this drug however, no differences were observed.
We compared the global gene expression profiles of two parental trisomic iPSCs, four derived disomic iPSCs, and human embryonic stem cells (H1 ESCs). Pearson correlation analysis showed that both trisomic and disomic iPSCs clustered closely together (coefficients >0.98), similar to replicate samples of H1 ESCs. A direct comparison of isogenic trisomic and disomic pairs showed the expected upregulation of chromosome 21 genes with trisomy, and remarkably little additional variation. Only three other genes were >2-fold dysregulated in trisomic cells: ACTA2, PTRF, and RPL39L were overexpressed. It is tempting to speculate that altered expression of ACTA2 (Alpha 2 actin) may play a role in DS cardiovascular malformations because ACTA2 mutations are known to cause vascular abnormalities.
Trisomy 21 has been proposed to influence hematopoietic and endothelial differentiation. We analyzed cells from day 14 embryoid bodies (EBs) for the presence of hematopoietic and endothelial surface markers and progenitor cells, but did not detect consistent differences between two isogenic pairs of trisomic and disomic iPSCs, arguing that this in vitro model poorly recapitulates the known effects of trisomy 21 on hematopoiesis during human development. In vivo, all four clones formed trilineage teratomas after implantation into immunodeficient mice (example in Figure S2D), and the trisomic lines produced fewer endothelia within these teratomas. Impaired endothelia formation is consistent with prior observations of DS cells, suggesting that in vivo modeling may be more accurate.
In summary, we have generated DS iPSCs and removed the extra copy of chromosome 21 by a combination of gene targeting and negative selection. Our study provides a measurement of spontaneous mutation frequencies in human pluripotent stem cells, with chromosome loss being the most common cause of GCV resistance. Previously, an alternative strategy for chromosome loss was described in mouse cells that consisted of Cre-mediated recombination at engineered, inverted loxP sites. The advantages of our approach are that recombinase protein is not required, abnormal chromosomal forms are not produced, and the frequency of spontaneous chromosome loss can be measured.
Trisomy removal has practical applications. DS patients have a 10- to 20-fold increased risk of leukemia, so trisomy removal might eventually be used to treat DS leukemia by autologous transplantation of patient-derived, disomic cells that lack the proleukemic effects of the extra copy of chromosome 21. Our approach could also be used to eliminate the unwanted trisomies that frequently arise during stem cell culture, thereby allowing one to revert a valuable, trisomic clone to a normal karyotype.Acknowledgments:
This work was supported by a grant to D.R. from Horizon Discovery and NIH grants DK55759, HL53750, and GM086497 to D.R.; DK077864 to K.C.; and HL46557 to T.P. The authors thank Yi Jiang, Gaoying Ren, Raisa Stolitenko, Andrew Herman, and Sam Lam for technical assistance.