Serial bull cloning by somatic cell nuclear transfer

Evaluation of postnatal growth, hematology, telomere length and semen attributes of multiple clones and re-clone of superior buffalo breeding bulls

The present study comprehensively evaluates the postnatal growth, hematology, telomere length, and semen attributes of multiple clones and re-clone derived from superior buffalo breeding bulls. To the best of our knowledge, we successfully produced multiple clones and a re-clone of an earlier cloned buffalo bull from an embryo developed from an adult bull's skin-derived cell for the first time. The cloned bulls' growth, blood hematology, plasma biochemistry, and telomere length were all shown to be normal at various stages of development. The bulls were used for semen production after being screened for testicular growth and training. Semen characteristics such as volume, concentration, and initial motility of fresh sperm as well as motility and kinetics characteristics such as straightness (STR), average lateral head displacement (ALH), and beat cross frequency (BCF) of frozen-thawed sperms of the cloned bulls were found to be similar to those of non-cloned bulls, including the donor bulls. Additionally, it was found that cloned bulls' functional sperm attributes, including acrosome intactness, mitochondrial membrane potential, and superoxide anion status, were comparable to those of non-cloned bulls. These characteristics are necessary for sperm to pass through the female reproductive system, penetrate the oocyte, and efficiently fertilize. Finally, this study adds to our understanding of the postnatal development, hematology, telomere length, and sperm characteristics of superior buffalo breeding bulls that have been cloned and re-cloned. The findings provide the groundwork for improving cloning practices, refining reproductive procedures, and optimizing the use of cloned genetic material in animal breeding and conservation.

Effects of Recloning on the Telomere Lengths of Mouse Terc+/- Nuclear Transfer-Derived Embryonic Stem Cells

Haploinsufficiency of genes that participate in the telomere elongation and maintenance processes, such as Terc and Tert, often lead to premature aging related diseases such as dyskeratosis congenita and aplastic anemia. Previously we reported that when mouse Terc+/- tail tip fibroblasts (TTFs) were used as the donor cells for somatic cell nuclear transfer (SCNT, also known as "cloning"), the derivative embryonic stem cells (ntESCs) had elongated telomeres. In the present work, we are interested to know if an additional round of SCNT, or recloning, could bring further elongation of the telomeres. Terc+/- TTFs were used to derive the first generation (G1) ntESCs, followed by a second round SCNT using G1-Terc+/- ntESCs as donor cells to derive G2-Tert+/- ntESCs. Multiple lines of G1- and G2-Terc+/- ntESCs were efficiently established, and all expressed major pluripotent markers and supported efficient chondrocyte differentiation in vitro. Comparing to the donor TTFs, telomere lengths of G1-ntESCs were elongated to the level comparable to that in wildtype ntESCs. Interestingly, recloning did not further elongate telomere lengths of the Terc+/- ntESCs. Together, our work demonstrates that while a single round of SCNT is a viable means to reprogram Terc haploinsufficient cells to the ESC state, and to elongate these cells' telomere lengths, a second round of SCNT does not necessarily further elongate the telomeres.

Telomere length in dromedary camels (Camelus dromedarius) produced by somatic cell nuclear transfer (SCNT) and their age-matched naturally produced counterparts

There are controversial reports on the restoration of eroded telomere length in offspring produced by somatic cell nuclear transfer (SCNT) in different animal species. To the best of our knowledge, no earlier studies report the telomere length in naturally produced or cloned animals in any of the camelid species. Therefore, the present study was conducted to estimate the telomere length in dromedary camels produced by SCNT, the donor cells, and their age-matched naturally produced counterparts by Terminal Restriction Fragment (TRF) length analysis and real-time Q PCR T/S ratio methods. Genomic DNA was extracted from venous blood collected from 6 cloned animals and their age-matched counterparts. Using the southern blot technique, digested DNA was blotted onto a positively charged nylon membrane, and its hybridization was carried out using telomere (TTAGGG)n specific, DIG-labeled hybridization probe (Roche Diagnostics, Germany) at 42 °C for 4 h. Stringent washes were carried out at the same temperature, followed by a chemiluminescence reaction. The signals were captured using the Azure Biosystems C600 gel documentation system. A TeloTool program from MATLAB software with a built-in probe intensity correction algorithm was used for TRF analysis. The experiment was replicated three times, and the data, presented as mean ± SEM, were analyzed using a two-sample t-test (MINITAB statistical software, Minitab ltd, CV3 2 TE, UK). No difference was found in the mean telomere length of cloned camels when compared to their naturally produced age-matched counterparts. However, the telomere length was more (P < 0.05) than that of the somatic cells used for producing the SCNT embryos. A moderate positive Pearson correlation coefficient (r = 0.6446) was observed between the telomere lengths estimated by TRF and Q PCR T/S ratio method. In conclusion, this is the first study wherein we are reporting telomere length in naturally produced and cloned dromedary camels produced by somatic cell nuclear transfer. We found that telomere lengths in cloned camels were similar to their age-matched naturally produced counterparts, suggesting that the camel cytoplast reprograms the somatic cell nucleus and restores the telomere length to its totipotency stage.

Semen parameters and fertility potency of a cloned water buffalo (Bubalus bubalis) bull produced from a semen-derived epithelial cell

Semen contains epithelial cells that can be cultured in vitro. For somatic cell nuclear transfer applications, it is essential to know whether clone(s) produced from semen-derived epithelial cells (SedECs) are healthy and reproductively competent. In this study, the semen and fertility profile of a cloned bull (C1) that was produced from a SedEC were compared with its donor (D1) and with two cloned bulls (C2, C3) that were produced from commonly used skin-derived fibroblast cells (SkdFCs). We observed variations in some fresh semen parameters (ejaculated volume and mass motility), frozen-thawed sperm parameters (plasma membrane integrity, and computer-assisted semen analysis (CASA) indices), but values are within the normal expected range. There was no difference in sperm concentration of ejaculated semen and frozen-thawed semen parameters which include sperm motility, percentage of live and normal morphology sperm, and distance traveled through oestrus mucus. Following in vitro fertilization (IVF) experiments, zygotes from C1 had higher (P < 0.05) cleavage rates (81%) than C2, C3, and D1 (71%, 67%, and 75%, respectively); however, blastocyst development per cleaved embryo and quality of produced blastocysts did not differ. The conception rate of C1 was 46% (7/15) and C2 was 50% (8/15) following artificial insemination with frozen-thawed semen. Established pregnancies resulted in births of 7 and 6 progenies sired by C1 and C2, respectively, and all calves show no signs of phenotypical abnormalities. These results showed that semen from a cloned bull derived from SedECs is equivalent to semen from its donor bull and bulls cloned from SkdFCs.

Increased pregnancy losses following serial somatic cell nuclear transfer in goats

Serial cloning by somatic cell nuclear transfer (SCNT) is a critical tool for the expansion of precious transgenic lines or resetting the lifespan of primary transgenic cells for multiple genetic modifications. We successfully produced second-generation cloned goats using donor neonatal fibroblasts from first-generation clones. However, our attempts to produce any third-generation clones failed. SCNT efficiency decreased progressively with the clonal generations. The rate of pregnancy loss was significantly greater in recloning groups (P<0.05). While no pregnancy loss was observed during the first round of SCNT, 14 out of 21 pregnancies aborted in the second round of SCNT and all pregnancies aborted in the third round of SCNT. In this retrospective study, we also investigated the expression of 21 developmentally important genes in muscle tissue of cloned (G1) and recloned (G2) offspring. The expression of most of these genes in live clones was found to be largely comparable to naturally reproduced control goats, but fibroblast growth factor 10 (FGF10), methyl CpG binding protein 2 (MECP2) and growth factor receptor bound protein 10 (GRB10) were differentially expressed (P<0.05) in G2 goats compared with G1 and controls. To study the effects of serial cloning on DNA methylation, the methylation pattern of differentially methylated regions in imprinted genes H19 and insulin like growth factor 2 receptor (IGF2R) were also analysed. Aberrant H19 DNA methylation patterns were detected in G1 and G2 clones.

Generation of CMAHKO/GTKO/shTNFRI-Fc/HO-1 quadruple gene modified pigs

As an alternative source of organs for transplantation into humans, attention has been directed to pigs due to their similarities in biological features and organ size. However, severe immune rejection has prevented successful xenotransplantation using pig organs and tissues. To overcome immune rejection, recently developed genetic engineering systems such as TALEN coupled with somatic cell nuclear transfer (SCNT) to make embryos could be used to produce pigs compatible with xenotransplantation. We used the TALEN system to target the non-Gal antigen cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) gene in pigs that is naturally deleted in humans. Gal-deleted cells expressing both soluble human tumor necrosis factor receptor I IgG₁-Fc (shTNFRI-Fc) and human hemagglutinin -tagged-human heme oxygenase-1 (hHO-1) were transfected with a TALEN target for CMAH. Cells lacking CMAH were negatively selected using N-glyconeuraminic acid (Neu5Gc)/magnetic beads and the level of Neu5Gc expression of isolated cells were analyzed by FACS and DNA sequencing. Cloned embryos using 3 different genetically modified cell clones were respectively transferred into 3 recipients, with 55.6% (5/9) becoming pregnant and three cloned pigs were produced. Successful genetic disruption of the CMAH gene was confirmed by sequencing, showing lack of expression of CMAH in tail-derived fibroblasts of the cloned piglets. Besides decreased expression of Neu5Gc in piglets produced by SCNT, antibody-mediated complement-dependent cytotoxicity assays and natural antibody binding for examining immuno-reactivity of the quadruple gene modified pigs derived from endothelial cells and fibroblasts were reduced significantly compared to those of wild type animals. We conclude that by combining the TALEN system and transgenic cells, targeting of multiple genes could be useful for generating organs for xenotransplantation. We produced miniature pigs with quadruple modified genes CMAHKO/GTKO/shTNFRI-Fc/hHO-1 that will be suitable for xenotransplantation by overcoming hyperacute, acute and anti-inflammatory rejection.

The growth and reproduction performance of TALEN-mediated β-lactoglobulin-knockout bucks

With the technological development of several engineered endonucleases (EENs), such as zinc-finger nucleases, transcription activator-like effector nucleases (TALENs) and CRISPR/Cas9, gene targeting by homologous recombination has been efficiently improved to generate site-specifically genetically modified livestock. However, few studies have been done to investigate the health and fertility of these animals. The purpose of the present study is to investigate if gene targeting events and a recloning procedure would affect the production traits of EEN-mediated gene targeted bucks. TALEN-mediated β-lactoglobulin (BLG) gene mono-allelic knockout (BLG (+/-)) goats and bi-allelic knockout (BLG (-/-)) buck produced by using sequential gene targeting combined with recloning in fibroblasts from BLG (+/-) buck were used to evaluate their health and fertility. Birth weight and postnatal growth of BLG (+/-) bucks were similar to the wild-type goats. None of the parameters for both fresh and frozen-thawed semen quality were significantly different in BLG (+/-) or BLG (-/-) bucks compared to their corresponding comparators. In vitro fertilization (IVF) test revealed that the proportion of IVF oocytes developing to the blastocyst stage was identical among BLG (+/-), BLG (-/-) and wild-type bucks. Conception rates of artificial insemination were respectively 42.3, 38.0 and 42.6 % for frozen-thawed semen from the BLG (+/-), BLG (-/-) and wild-type bucks. In addition, germline transmission of the targeted BLG modification was in accordance with Mendelian rules. These results demonstrated that the analyzed growth and reproductive traits were not impacted by targeting BLG gene and recloning, implicating the potential for dairy goat breeding of BLG (+/-) and BLG (-/-) bucks.

Factors Determining the Efficiency of Porcine Somatic Cell Nuclear Transfer: Data Analysis with Over 200,000 Reconstructed Embryos

Data analysis in somatic cell nuclear transfer (SCNT) research is usually limited to several hundreds or thousands of reconstructed embryos. Here, we report mass results obtained with an established and consistent porcine SCNT system (handmade cloning [HMC]). During the experimental period, 228,230 reconstructed embryos and 82,969 blastocysts were produced. After being transferred into 656 recipients, 1070 piglets were obtained. First, the effects of different types of donor cells, including fetal fibroblasts (FFs), adult fibroblasts (AFs), adult preadipocytes (APs), and adult blood mesenchymal (BM) cells, were investigated on the further in vitro and in vivo development. Compared to adult donor cells (AFs, APs, BM cells, respectively), FF cells resulted in a lower blastocyst/reconstructed embryo rate (30.38% vs. 37.94%, 34.65%, and 34.87%, respectively), but a higher overall efficiency on the number of piglets born alive per total blastocysts transferred (1.50% vs. 0.86%, 1.03%, and 0.91%, respectively) and a lower rate of developmental abnormalities (10.87% vs. 56.57%, 24.39%, and 51.85%, respectively). Second, recloning was performed with cloned adult fibroblasts (CAFs) and cloned fetal fibroblasts (CFFs). When CAFs were used as the nuclear donor, fewer developmental abnormalities and higher overall efficiency were observed compared to AFs (56.57% vs. 28.13% and 0.86% vs. 1.59%, respectively). However, CFFs had an opposite effect on these parameters when compared with CAFs (94.12% vs. 10.87% and 0.31% vs. 1.50%, respectively). Third, effects of genetic modification on the efficiency of SCNT were investigated with transgenic fetal fibroblasts (TFFs) and gene knockout fetal fibroblasts (KOFFs). Genetic modification of FFs increased developmental abnormalities (38.96% and 25.24% vs. 10.87% for KOFFs, TFFs, and FFs, respectively). KOFFs resulted in lower overall efficiency compared to TFFs and FFs (0.68% vs. 1.62% and 1.50%, respectively). In conclusion, this is the first report of large-scale analysis of porcine cell nuclear transfer that provides important data for potential industrialization of HMC technology.

Telomere elongation facilitated by trichostatin a in cloned embryos and pigs by somatic cell nuclear transfer

Telomere attrition and genomic instability are associated with organism aging. Concerns still exist regarding telomere length resetting in cloned embryos and ntES cells, and possibilities of premature aging of cloned animals achieved by somatic cell nuclear transfer (SCNT). Trichostatin A (TSA), a histone deacetylase inhibitor, effectively improves the developmental competence of cloned embryos and animals, and recently contributes to successful generation of human ntES cells by SCNT. To test the function of TSA on resetting telomere length, we analyzed telomeres in cloned blastocysts and pigs following treatment of SCNT embryos with TSA. Here, we show that telomeres of cloned pigs generated by standard SCNT methods are not effectively restored, compared with those of donor cells, however TSA significantly increases telomere lengths in cloned pigs. Telomeres elongate in cloned porcine embryos during early cleavage from one-cell to four-cell stages. Notably, TSA facilitates telomere lengthening of cloned embryos mainly at morula-blastocyst stages. Knockdown of pTert by shRNA in donor cells reduces telomerase activity in cloned blastocysts but does not abrogate telomere elongation in the TSA-treated embryos (p > 0.05). However, genes associated with recombination or telomerase-independent mechanism of alternative lengthening of telomeres (ALT) Rad50 and BLM show increased expression in TSA-treated embryos. These data suggest that TSA may promote telomere elongation of cloned porcine embryos by ALT. Together, TSA can elongate telomeres in cloned embryos and piglets, and this could be one of the mechanisms underlying improved development of cloned embryos and animals treated with TSA.

Successful serial recloning in the mouse over multiple generations

Previous studies of serial cloning in animals showed a decrease in efficiency over repeated iterations and a failure in all species after a few generations. This limitation led to the suggestion that repeated recloning might be inherently impossible because of the accumulation of lethal genetic or epigenetic abnormalities. However, we have now succeeded in carrying out repeated recloning in the mouse through a somatic cell nuclear transfer method that includes a histone deacetylase inhibitor. The cloning efficiency did not decrease over 25 generations, and, to date, we have obtained more than 500 viable offspring from a single original donor mouse. The reprogramming efficiency also did not increase over repeated rounds of nuclear transfer, and we did not see the accumulation of reprogramming errors or clone-specific abnormalities. Therefore, our results show that repeated iterative recloning is possible and suggest that, with adequately efficient techniques, it may be possible to reclone animals indefinitely.

Using somatic-cell nuclear transfer to study aging

In mammals, a diploid genome following fertilization of haploid cells, an egg, and a spermatozoon is unique and irreproducible. This implies that the generated unique diploid genome is doomed with the individual's inevitable demise. Since it was first reported in 1997 that Dolly the sheep had been cloned, many mammalian species have been cloned successfully using somatic-cell nuclear transfer (SCNT). The success of SCNT in mammals enables us not only to reproduce offspring without germ cells, that is, to "passage" a unique diploid genome, but also to address valuable biological questions on development, nuclear reprogramming, and epigenetic memory. Successful cloning can also support epigenetic reprogramming where the aging clock is reset or reversed. Recent work using iPS cell technology has explored the practicality and led to the recapitulation of premature aging with iPSCs from progeroid laminopathies. As a result, reprogramming tools are also expected to contribute to studying biological age. However, the efficiency of animal cloning is still low in most cases and the mechanism of reprogramming in cloned embryos is still largely unclear. Here, based on recent advances, we describe an improved, more efficient mouse cloning protocol using histone deacetylase inhibitors (HDACis) and latrunculin A, which increases the success rates of producing cloned mice or establishing ES cells fivefold. This improved method of cloning will provide a strong tool to address many issues including biological aging more easily and with lower cost.

Factors influencing the efficiency of generating genetically engineered pigs by nuclear transfer: multi-factorial analysis of a large data set

BACKGROUND

Somatic cell nuclear transfer (SCNT) using genetically engineered donor cells is currently the most widely used strategy to generate tailored pig models for biomedical research. Although this approach facilitates a similar spectrum of genetic modifications as in rodent models, the outcome in terms of live cloned piglets is quite variable. In this study, we aimed at a comprehensive analysis of environmental and experimental factors that are substantially influencing the efficiency of generating genetically engineered pigs. Based on a considerably large data set from 274 SCNT experiments (in total 18,649 reconstructed embryos transferred into 193 recipients), performed over a period of three years, we assessed the relative contribution of season, type of genetic modification, donor cell source, number of cloning rounds, and pre-selection of cloned embryos for early development to the cloning efficiency.

RESULTS

109 (56%) recipients became pregnant and 85 (78%) of them gave birth to offspring. Out of 318 cloned piglets, 243 (76%) were alive, but only 97 (40%) were clinically healthy and showed normal development. The proportion of stillborn piglets was 24% (75/318), and another 31% (100/318) of the cloned piglets died soon after birth. The overall cloning efficiency, defined as the number of offspring born per SCNT embryos transferred, including only recipients that delivered, was 3.95%. SCNT experiments performed during winter using fetal fibroblasts or kidney cells after additive gene transfer resulted in the highest number of live and healthy offspring, while two or more rounds of cloning and nuclear transfer experiments performed during summer decreased the number of healthy offspring.

CONCLUSION

Although the effects of individual factors may be different between various laboratories, our results and analysis strategy will help to identify and optimize the factors, which are most critical to cloning success in programs aiming at the generation of genetically engineered pig models.

Advancing pig cloning technologies towards application in regenerative medicine

Regenerative medicine is expected to make a significant contribution by development of novel therapeutic treatments for intractable diseases and for improving the quality of life of patients. Many advances in regenerative medicine, including basic and translational research, have been developed and tested in experimental animals; pigs have played an important role in various aspects of this work. The value of pigs as a model species is being enhanced by the generation of specially designed animals through cloning and genetic modifications, enabling more sophisticated research to be performed and thus accelerating the clinical application of regenerative medicine. This article reviews the significant aspects of the creation and application of cloned and genetically modified pigs in regenerative medicine research and considers the possible future directions of the technology. We also discuss the importance of reproductive biology as an interface between basic science and clinical medicine.

Zinc-finger nickase-mediated insertion of the lysostaphin gene into the beta-casein locus in cloned cows

Zinc-finger nickases (ZFNickases) are a type of programmable nuclease that can be engineered from zinc-finger nucleases to induce site-specific single-strand breaks or nicks in genomic DNA, which result in homology-directed repair. Although zinc-finger nuclease-mediated gene disruption has been demonstrated in pigs and cattle, they have not been used to target gene addition into an endogenous gene locus in any large domestic species. Here we show in bovine fetal fibroblasts that targeting ZFNickases to the endogenous β-casein (CSN2) locus stimulates lysostaphin gene addition by homology-directed repair. We find that ZFNickase-treated cells can be successfully used in somatic cell nuclear transfer, resulting in live-born gene-targeted cows. Furthermore, the gene-targeted cows secrete lysostaphin in their milk and in vitro assays demonstrate the milk's ability to kill Staphylococcus aureus. Our success with this strategy will facilitate new transgenic technologies beneficial to both agriculture and biomedicine.

Development of intergeneric and intrageneric somatic cell nuclear transfer (SCNT) cat embryos and the determination of telomere length in cloned offspring

Somatic cell nuclear transfer (SCNT) holds potential as a useful tool for agricultural and biomedical applications. In vitro development of marbled cat intergeneric SCNT reconstructed into domestic cat cytoplast revealed that cloned, marbled cat embryo development was blocked at the morula stage. No pregnancies resulted from the transfer of one- to eight-cell stage embryos into domestic cat surrogate mothers. This suggested that abnormalities occurred in the cloned marbled cat embryos, which may be associated with incomplete reprogramming during early embryo development. Two pregnancies were established in surrogate mothers that received cloned domestic cat embryos, but SCNT offspring developed abnormally. Some specific phenotypes that were observed included incomplete abdominal wall disclosure, improper fetal development. In addition, some of the fetuses were mummified or stillbirths. The two live births died within 5 days. Telomere lengths of cloned kittens as determined by qualtitative polymerase chain reaction (qPCR) were inconclusive: some were found to be shorter, longer, or the same as donor control cells. Our findings support the hypothesis that telomere lengths do not govern the health of these cloned animals. A lack of complete reprogramming may lead to developmental failure and the abnormalities observed in cloned offspring.

Post-mortem re-cloning of a transgenic red fluorescent protein dog

Recently, the world's first transgenic dogs were produced by somatic cell nuclear transfer. However, cellular senescence is a major limiting factor for producing more advanced transgenic dogs. To overcome this obstacle, we rejuvenated transgenic cells using a re-cloning technique. Fibroblasts from post-mortem red fluorescent protein (RFP) dog were reconstructed with in vivo matured oocytes and transferred into 10 surrogate dogs. One puppy was produced and confirmed as a re-cloned dog. Although the puppy was lost during birth, we successfully established a rejuvenated fibroblast cell line from this animal. The cell line was found to stably express RFP and is ready for additional genetic modification.

Expression and methylation status of imprinted genes in placentas of deceased and live cloned transgenic calves

Placental deficiencies are linked with developmental abnormalities in cattle produced by somatic cell nuclear transfer (SCNT). To investigate whether the aberrant expression of imprinted genes in placenta was responsible for fetal overgrowth and placental hypertrophy, quantitative expression analysis of six imprinted genes (H19, XIST, IGF2R, SNRPN, PEG3, and IGF2) was conducted in placentas of: 1) deceased (died during perinatal period) transgenic calves (D group, n = 4); 2) live transgenic calves (L group, n = 15); and 3) conventionally produced (control) female calves (N group, n = 4). In this study, XIST, PEG3 and IGF2 were significantly over-expressed in the D group, whereas expression of H19 and IGF2R was significantly reduced in the D group compared to controls. The DNA methylation patterns in the differentially methylated region (DMR) from H19, XIST, and IGF2R were compared using Bisulfite Sequencing PCR (BSP) and Combined Bisulfite Restriction Analysis (COBRA). In the D group, H19 DMR was significantly hypermethylated, but XIST DMR and IGF2R ICR were significantly hypomethylated compared to controls. In contrast, there were no noticeable differences in the expression and DNA methylation status of imprinted genes (except DNA methylation level of XIST DMR) in the L group compared to controls. In conclusion, altered DNA methylation levels in the DMRs of imprinted genes in placentas of deceased transgenic calves, presumably due to aberrant epigenetic nuclear reprogramming during SCNT, may have been associated with abnormal expression of these genes; perhaps this caused developmental insufficiencies and ultimately death in cloned transgenic calves.

How to improve the success rate of mouse cloning technology

It has now been 13 years since the first cloned mammal Dolly the sheep was generated from somatic cells using nuclear transfer (SCNT). Since then, this technique has been considered an important tool not only for animal reproduction but also for regenerative medicine. However, the success rate is still very low and the mechanisms involved in genomic reprogramming are not yet clear. Moreover, the NT technique requires donated fresh oocyte, which raises ethical problems for production of human cloned embryo. For this reason, the use of induced pluripotent stem cells for genomic reprogramming and for regenerative medicine is currently a hot topic in this field. However, we believe that the NT approach remains the only valid way for the study of reproduction and basic biology. For example, only the NT approach can reveal dynamic and global modifications in the epigenome without using genetic modification, and it can generate offspring from a single cell or even a frozen dead body. Thanks to much hard work by many groups, cloning success rates are increasing slightly year by year, and NT cloning is now becoming a more applicable method. This review describes how to improve the efficiency of cloning, the establishment of clone-derived embryonic stem cells and further applications.

Telomere dynamics in human cells reprogrammed to pluripotency

Background

Human induced pluripotent stem cells (IPSCs) have enormous potential in the development of cellular models of human disease and represent a potential source of autologous cells and tissues for therapeutic use. A question remains as to the biological age of IPSCs, in particular when isolated from older subjects. Studies of cloned animals indicate that somatic cells reprogrammed to pluripotency variably display telomere elongation, a common indicator of cell “rejuvenation.”

Methodology/Principal Findings

We examined telomere lengths in human skin fibroblasts isolated from younger and older subjects, fibroblasts converted to IPSCs, and IPSCs redifferentiated through teratoma formation and explant culture. In IPSCs analyzed at passage five (P5), telomeres were significantly elongated in 6/7 lines by >40% and approximated telomere lengths in human embryonic stem cells (hESCs). In cell lines derived from three IPSC-teratoma explants cultured to P5, two displayed telomeres shortened to lengths similar to input fibroblasts while the third line retained elongated telomeres.

Conclusions/Significance

While these results reveal some heterogeneity in the reprogramming process with respect to telomere length, human somatic cells reprogrammed to pluripotency generally displayed elongated telomeres that suggest that they will not age prematurely when isolated from subjects of essentially any age.

Application of genetically modified and cloned pigs in translational research

Pigs are increasingly being recognized as good large-animal models for translational research, linking basic science to clinical applications in order to establish novel therapeutics. This article reviews the current status and future prospects of genetically modified and cloned pigs in translational studies. It also highlights pigs specially designed as disease models, for xenotransplantation or to carry cell marker genes. Finally, use of porcine somatic stem and progenitor cells in preclinical studies of cell transplantation therapy is also discussed.

Gene expression and development of early pig embryos produced by serial nuclear transfer

During nuclear transfer, reprogramming makes the donor nucleus capable of directing development of the reconstructed embryo. In most cases reprogramming is incomplete, which leads to abnormal expression of early embryonic genes and subsequently, to reduced developmental potential. In the present study, we monitored the expression of Oct4, Nanog, and Sox2 in cloned porcine embryos and evaluated whether serial nuclear transfer, the transfer of nuclei of cloned embryos into enucleated oocytes, has the potential to provide a more complete reprogramming of the donor genome. The data suggested that Nanog and Sox2 expression is properly reactivated after nuclear transfer, but the relative abundance of Oct4 transcripts is abnormally low in cloned porcine blastocysts compared to control embryos produced by in vitro fertilization. When the nuclei of 8- to 16-cell stage cloned embryos were introduced into enucleated oocytes to expose the chromosomes repeatedly to the ooplasmic factors, the resulting embryos showed poor developmental potential: a significantly lower percentage of embryos developed to the 4-cell (12.0% vs. 31.8%), 8-cell (3.1% vs. 15.0%) and blastocyst (0% vs. 8.7%) stages compared to those produced following a single round of nuclear transfer (P < 0.05). The additional time for reprogramming also did not improve gene expression. By the late 4-cell stage, Oct4 and Sox2 expression levels were low in serial nuclear transfer embryos compared to those in embryos generated by in vitro fertilization or nuclear transfer. Overall, both developmental and gene expression data indicated that reprogramming of the donor nucleus could not be improved by serial nuclear transfer in the pig.

Transcriptional reprogramming of gene expression in bovine somatic cell chromatin transfer embryos

BACKGROUND

Successful reprogramming of a somatic genome to produce a healthy clone by somatic cells nuclear transfer (SCNT) is a rare event and the mechanisms involved in this process are poorly defined. When serial or successive rounds of cloning are performed, blastocyst and full term development rates decline even further with the increasing rounds of cloning. Identifying the "cumulative errors" could reveal the epigenetic reprogramming blocks in animal cloning.

RESULTS

Bovine clones from up to four generations of successive cloning were produced by chromatin transfer (CT). Using Affymetrix bovine microarrays we determined that the transcriptomes of blastocysts derived from the first and the fourth rounds of cloning (CT1 and CT4 respectively) have undergone an extensive reprogramming and were more similar to blastocysts derived from in vitro fertilization (IVF) than to the donor cells used for the first and the fourth rounds of chromatin transfer (DC1 and DC4 respectively). However a set of transcripts in the cloned embryos showed a misregulated pattern when compared to IVF embryos. Among the genes consistently upregulated in both CT groups compared to the IVF embryos were genes involved in regulation of cytoskeleton and cell shape. Among the genes consistently upregulated in IVF embryos compared to both CT groups were genes involved in chromatin remodelling and stress coping.

CONCLUSION

The present study provides a data set that could contribute in our understanding of epigenetic errors in somatic cell chromatin transfer. Identifying "cumulative errors" after serial cloning could reveal some of the epigenetic reprogramming blocks shedding light on the reprogramming process, important for both basic and applied research.

Differential proteomic analysis of bovine conceptus fluid proteins in pregnancies generated by assisted reproductive technologies

Proteomic analysis of bovine conceptus fluid proteins during early pregnancy has the potential to expose protein species indicative of both the overall health of the fetal-maternal environment and fetal developmental status. In this study, we examined the differential abundance of bovine conceptus fluid proteins (5-50 kDa fraction) from naturally conceived, in vitro fertilisation (IVF) and somatic cell nuclear transfer (SCNT)-derived pregnancies at days 45 and 90 of gestation. In day 45 allantoic fluid (AllF) samples, an atypical cluster of low molecular weight ( approximately 14-16 kDa), low pI (between 3.0 and 4.5 pH units) protein species was increased in three of four IVF samples (30-100-fold increase in protein spot volumes compared to normal). These proteins were identified as paralogs of the bovine cathelicidin antimicrobial protein (CAMP) by MALDI-TOF MS peptide mass fingerprint and MALDI-TOF MS/MS peptide sequence analysis. Peptidoglycan recognition protein and serine (or cysteine) proteinase inhibitor clade B1, were also significantly increased in the corresponding IVF samples. In two of four SCNT AllF samples, a 2-10-fold increase in CAMP protein spot volumes were detected. No aberrant abundance levels of individual protein species were observed in amniotic fluid samples, or in day 90 IVF AllF samples. Identification of unique protein species present in the normal bovine AllF proteome at day 45 is also reported.

Nuclear reprogramming: what has been done and potential avenues for improvements

A major challenge for reproductive biologists is the development of novel strategies to improve cloning efficiency. Even in species for which cloning is relatively successful, like cattle, the efficiency is still unacceptably low. In this review article we critically analyse all approaches that have been suggested by different laboratories in the field so far. As will be discussed below, so far none of these gives rise to a dramatic increase in cloning efficiency. Possibly, a multi-step approach including a pre-treatment of donor cells to modify their chromatin, along with improved culture system for cloned embryos would be the most promising.

Production efficiency and telomere length of the cloned pigs following serial somatic cell nuclear transfer

The aim of the present study was to examine the production efficiency of cloned pigs by serial somatic cell nuclear transfer (SCNT) and to ascertain any changes in the telomere lengths of multiple generations of pigs. Using fetal fibroblasts as the starting nuclear donor cells, porcine salivary gland progenitor cells were collected from the resultant first-generation cloned pigs to successively produce second- and third-generation clones, with no significant differences in production efficiency, which ranged from 1.4% (2/140) to 3.3% (13/391) among the 3 generations. The average telomere lengths (terminal restriction fragment values) for the first, second and third generation clones were 16.3, 18.1 and 20.5 kb, respectively, and were comparable to those in age-matched controls. These findings suggest that third-generation cloned pigs can be produced by serial somatic cell cloning without compromising production efficiency and that the telomere lengths of cloned pigs from the first to third generations are normal.

Production of second-generation cloned cats by somatic cell nuclear transfer

We successfully produced second-generation cloned cats by somatic cell nuclear transfer (SCNT) using skin cells from a cloned cat. Skin cells from an odd-eyed, all-white male cat (G0 donor cat) were used to generate a cloned cat (G1 cloned cat). At 6 months of age, skin cells from the G1 cloned cat were used for SCNT to produce second-generation cloned cats. We compared the in vitro and in vivo development of SCNT embryos that were derived from the G0 donor and G1 cloned donor cat's skin fibroblasts. The nuclei from the G0 donor and G1 cloned donor cat's skin fibroblasts fused with enucleated oocytes with equal rates of fusion (60.7% vs. 58.8%, respectively) and cleavage (66.3% vs. 63.4%). The 2–4-cell SCNT embryos were then transferred into recipients. One of the five recipients of G0 donor derived NT embryos (20%) delivered one live male cloned kitten, whereas 4 of 15 recipients of the G1 cloned donor cat derived NT embryos (26%) delivered a total of seven male second-generation cloned kittens (four live kittens from one surrogate, plus two stillborn kittens, and one live kitten that died 2 d after birth from three other surrogate mothers). The four second-generation cloned kittens from the same surrogate all had a white coat color; three of the four second-generation cloned kittens had two blue eyes, and one of the second-generation cloned kittens had an odd-eye color. Despite low cloning efficiency, cloned cats can be used as donor cats to produce second-generation cloned cats.

Serial cloning of pigs by somatic cell nuclear transfer: restoration of phenotypic normality during serial cloning

Somatic cell nuclear transfer (scNT) is a useful way to create cloned animals. However, scNT clones exhibit high levels of phenotypic instability. This instability may be due to epigenetic reprogramming and/or genomic damage in the donor cells. To test this, we produced transgenic pig fibroblasts harboring the truncated human thrombopoietin (hTPO) gene and used them as donor cells in scNT to produce first-generation (G1) cloned piglets. In this study, 2,818 scNT embryos were transferred to 11 recipients and five G1 piglets were obtained. Among them, a clone had a dimorphic facial appearance with severe hypertelorism and a broad prominent nasal bridge. The other clones looked normal. Second-generation (G2) scNT piglets were then produced using ear cells from a G1 piglet that had an abnormal nose phenotype. We reasoned that, if the phenotypic abnormality of the G1 clone was not present in the G2 and third-generation (G3) clones, or was absent in the G2 clones but reappeared in the G3 clones, the phenotypic instability of the G1 clone could be attributed to faulty epigenetic reprogramming rather than to inherent/accidental genomic damage to the donor cells. Blastocyst rates, cell numbers in blastocyst, pregnancy rates, term placenta weight and ponderal index, and birth weight between G1 and G2 clones did not differ, but were significantly (P < 0.05) lower than control age- and sex-matched piglets. Next, we analyzed global methylation changes during development of the preimplantation embryos reconstructed by donor cells used for the production of G1 and G2 clones and could not find any significant differences in the methylation patterns between G1 and G2 clones. Indeed, we failed to detect the phenotypic abnormality in the G2 and G3 clones. Thus, the phenotypic abnormality of the G1 clone is likely to be due to epigenetic dysregulation. Additional observations then suggested that expression of the hTPO gene in the transgenic clones did not appear to be the cause of the phenotypic abnormality in the G1 clones and that the abnormality was acquired by only a few of the G1 clone's cells during its gestational development.

A cloned toy poodle produced from somatic cells derived from an aged female dog

To date, dogs have been cloned with somatic cell nuclear transfer (SCNT), using donor cells derived from large-breed dogs 2 months to 3 years of age. The objective of the present study was to use SCNT to produce a small-breed dog from ear fibroblasts of an aged poodle, using large-breed oocyte donors and surrogate females, and to determine the origin of its mitochondrial DNA (mtDNA) and the length of its telomeres. Oocytes were derived from large-breed donors, matured in vivo, collected by flushing oviducts, and reconstructed with somatic cells derived from an aged (14-year-old) female toy poodle. Oocytes and donor cells were fused by electric stimuli, activated chemically, and transferred into the oviducts of large-breed recipient females. Overall, 358 activated couplets were surgically transferred into the oviducts of 20 recipient dogs. Two recipients became pregnant; only one maintained pregnancy to term, and a live puppy (weighing 190 g) was delivered by Caesarean section. The cloned poodle was phenotypically and genetically identical to the nuclear donor dog; however, its mtDNA was from the oocyte donor, and its mean telomere length was not significantly different from that of the nuclear donor. In summary, we demonstrated that a small-breed dog could be cloned by transferring activated couplets produced by fusion of somatic cells from a small-breed, aged donor female with enucleated in-vivo-matured oocytes of large-breed females, and transferred into the oviduct of large-breed recipient female dogs.

Telomere length status of somatic cell sheep clones and their offspring

This study was carried out to determine the telomere length status of sheep clones and their offspring, and to examine telomere dynamics and chromosomal abnormalities in culture propagated donor cells. Skin samples were collected from somatic cell nuclear transfer-derived sheep clones, and three of their progeny generated by natural mating. Samples were collected from control animals (n = 35), spanning in age from 1 month to 36 months of age. Genomic DNA was extracted from cell/tissue samples and their telomere lengths were assessed by terminal restriction fragment (TRF) analysis. Results revealed: that (a) sheep clones derived from cultured somatic cells have shortened telomere lengths compared to age-matched controls; (b) the offspring derived from natural mating between clones had normal telomere lengths compared to their age-matched counterparts; and donor cell cultures beyond 20 population doublings had significantly (P < 0.05) shortened telomeres and exhibited a higher numerical and structural chromosomal abnormalities.

Developmental, behavioral, and physiological phenotype of cloned mice

Cloning from adult somatic cells has been successful in at least ten species. Although generating viable cloned mammals from adult cells is technically feasible, prenatal and perinatal mortality is high and live cloned offspring have had health problems. This chapter summarizes the health consequences of cloning in mice and discusses possible mechanisms through which these conditions may arise. These studies have further significance as other assisted reproductive techniques (ART) also involve some of the same procedures used in cloning, and there are some reports that offspring generated by ART display aberrant phenotypes as well. At the moment, the long-term consequences of mammalian cloning remain poorly characterized. Data available thus far suggest that we should use this technology with great caution until numerous questions are addressed and answered.

Reproductive fertility of cloned male cats derived from adult somatic cell nuclear transfer

This study was designed to investigate the reproductive fertility by the natural breeding of cloned male cats with domestic female cats, and to measure endocrine hormone concentration related to male reproduction such as testosterone, leutinizing hormone (LH), and follicle stimulating hormone (FSH). Cloned A, B, C, and D cats produced three, two, four, and five kittens after natural mating with four domestic female cats, respectively, despite later puberty of the cloned D cat than those of the other cloned male cats. Three of the 14 kittens expressed an odd eye color, which was produced by 1 and 2 from cloned A and B cats. The eye color of the other F1 kittens varied from nine brown to two blue. Body weight at birth ranged from 72.9 to 134.0 g. Although clone D had a poorer libido and entered puberty later than those of the other cloned male cats, he produced gonadal hormones within the average range. Four of the cloned male cats had normal fertility. The concentration of gonadal hormones in cloned male cats was similar to two control and donor cats. The concentration of testosterone was not significantly different among clones A, B, C, D, and control cats (5.99 +/- 5.68; 3.46 +/- 2.81; 6.41 +/- 2.17; 3.75 +/- 0.34; 4.0 +/- 3.63 ng/mL, p < 0.05). The concentrations of LH and FSH were not significantly different among the cloned cats (p < 0.05). Seven male and seven female (in total 14) kittens were produced by the natural breeding with four domestic female cats. These results indicated that cloned male cats have normal reproductive fertility and lie within the normal range of gonadal hormone production. All F1 kittens were produced by natural breeding and delivery, and are still alive and have normal growth health (27 months age).

Premature Chromosome Condensation Is Not Essential for Nuclear Reprogramming in Bovine Somatic Cell Nuclear Transfer1

Premature chromosome condensation (PCC) was believed to promote nuclear reprogramming and to facilitate cloning by somatic cell nuclear transfer (NT) in mammalian species. However, it is still uncertain whether PCC is necessary for the successful reprogramming of an introduced donor nucleus in cattle. In the present study, fused NT embryos were subjected to immediate activation (IA, simultaneous fusion and activation), delayed activation (DA, activation applied 4 h postfusion), and IA with aged oocytes (IAA, activation at the same oocyte age as group DA). The morphologic changes, such as nuclear swelling, the occurrence of PCC, and microtubule/aster formation, were analyzed in detail by laser-scanning confocal microscopy. When embryos were subjected to IA in both IA and IAA groups, the introduced nucleus gradually became swollen, and a pronuclear-like structure formed within the oocyte, but PCC was not observed. In contrast, delaying embryo activation resulted in 46.5%-91.2% of NT embryos exhibiting PCC. This PCC was observed beginning at 4 h postcell fusion and was shown as one, two, or multiple chromosomal complexes. Subsequently, a diversity of pronuclear-like structures existed in NT embryos, characterized as single, double, and multiple nuclei. In the oocytes exhibiting PCC, the assembled spindle structure was observed to be an interactive mass, closely associated with condensed chromosomes, but no aster had formed. Regardless of whether they were subjected to IA, IAA, or DA treatments, if the oocytes contained pronuclear-like structures, either one or two asters were observed in proximity to the nuclei. A significantly higher rate of development to blastocysts was achieved in embryos that were immediately activated (IA, 59.1%; IAA, 40.7%) than in those for which activation was delayed (14.2%). The development rate was higher in group IA than in group IAA, but it was not significant (P = 0.089). Following embryo transfer, there was no statistically significant difference in the pregnancy rates (Day 70) between two of the groups (group IA, 11.7%, n = 94 vs. group DA, 12.3%, n = 130; P > 0.05) or live term development (group IA, 4.3% vs. group DA, 4.6%; P > 0.05). Our study has demonstrated that the IA of bovine NT embryos results in embryos with increased competence for preimplantational development. Moreover, PCC was shown to be unnecessary for the reprogramming of a transplanted somatic genome in a cattle oocyte.

Transgenic animal production and animal biotechnology

Considerable progress has been made in methods for production of transgenic livestock; beginning with pronuclear microinjection over 20 years ago. New methods, including the use of viral vectors, sperm-mediated gene transfer and somatic cell cloning, have overcome many of the limitations of pronuclear microinjection. It is now possible to not only readily make simple insertional genetic modifications, but also to accomplish, more complex, homozygous gene targeting and artificial chromosome transfer in livestock.

Production of Cloned Mice and ES Cells from Adult Somatic Cells by Nuclear Transfer: How to Improve Cloning Efficiency?

Although it has now been 10 years since the first cloned mammals were generated from somatic cells using nuclear transfer (NT), most cloned embryos usually undergo developmental arrest prior to or soon after implantation, and the success rate for producing live offspring by cloning remains below 5%. The low success rate is believed to be associated with epigenetic errors, including abnormal DNA hypermethylation, but the mechanism of "reprogramming" is unclear. We have been able to develop a stable NT method in the mouse in which donor nuclei are directly injected into the oocyte using a piezo-actuated micromanipulator. Especially in the mouse, only a few laboratories can make clones from adult somatic cells, and cloned mice are never successfully produced from most mouse strains. However, this technique promises to be an important tool for future research in basic biology. For example, NT can be used to generate embryonic stem (NT-ES) cell lines from a patient's own somatic cells. We have shown that NT-ES cells are equivalent to ES cells derived from fertilized embryos and that they can be generated relatively easily from a variety of mouse genotypes and cell types of both sexes, even though it may be more difficult to generate clones directly. In general, NT-ES cell techniques are expected to be applied to regenerative medicine; however, this technique can also be applied to the preservation of genetic resources of mouse strain instead of embryos, oocytes and spermatozoa. This review describes how to improve cloning efficiency and NT-ES cell establishment and further applications.

Telomere length analysis in goat clones and their offspring

Incomplete epigenetic reprogramming of the donor genome is believed to be the cause behind the high rate of developmental mortality and post-natal anomalies observed in animal clones. It appears that overt phenotypic abnormalities are not transmitted to their progeny suggesting that epigenetic errors are corrected in the germline of clones. Here, we show variation in telomere lengths among Nigerian dwarf goat clones derived from different somatic cell types and that the offspring of two male clones have significantly shorter telomere lengths than age-matched noncloned animals. Telomere lengths were significantly shorter in skin biopsies of goat clones derived from adult granulosa cells compared to those measured for controls. Telomere lengths were highly variable in male goat clones reconstructed from fetal fibroblasts but their mean terminal repeat fragment (TRF) length was within normal range of normal goats. However, in the progeny of two male clones, mean TRF lengths were considerably shorter than age-matched controls for both skin and leukocyte samples. Evidence for possible inheritance of shortened telomeres was obtained by measuring telomere lengths in testicular biopsies obtained from the clones, which when compared with those from noncloned animals of a similar age were significantly shorter. The offspring exhibited telomere lengths intermediate to the TRF values obtained for their cloned fathers' and age-matched control testes. These results demonstrate that telomere length reprogramming in clones is dependent on the type of donor cell used and that the progeny of clones may inherit telomere length alterations acquired through the cloning procedure.

Growth and fertility of bulls cloned from the somatic cells of an aged and infertile bull

In the present study, somatic cell cloning technology was used to produce eight newborn calves from an aged, infertile bull. Average birth weight of these calves was significantly higher than that of calves produced using AI. Four of the cloned calves died during the peripartum period; the remaining four (Clones A-D) survived and were used in this study. Two of the surviving calves (Clones C and D) were castrated; growth rates of the intact and castrated clones were similar to those of intact and castrated bulls, respectively, that had been derived by AI. Both uncastrated bulls (Clones A and B) began to produce normal semen at approximately 12 months of age. Semen produced by these clones, and their nuclear donor, was subsequently used for IVF; the proportion of IVM-IVF oocytes developing to the blastocyst stage was 23.4% (50/214), 28.4% (52/183) and 30.9% (63/204), respectively. Conception rates for AI were 54.5% (12/22) and 62.7% (64/102) for semen derived from Clone A and from the nuclear donor, respectively. The length of pregnancy and birth weight of the calves derived from semen collected from clones were similar to those of calves obtained by conventional AI using semen from their nuclear donor. Therefore, sires cloned from the somatic cells of an aged and infertile bull had normal fertility.

Expression of imprinted genes is aberrant in deceased newborn cloned calves and relatively normal in surviving adult clones

Cattle are the species used most frequently for the development of assisted reproductive technologies, such as nuclear transfer. Cattle cloning can be performed by a large number of laboratories around the world, and the efficiency of nuclear transfer in cattle is the highest among all species in which successful cloning has been achieved. However, an understanding of the expression of imprinted genes in this important species is lacking. In the present study, real time reverse transcription polymerase chain reaction (RT-PCR) was utilized to quantify the expression of the bovine Igf2, Igf2r, and H19 genes in eight major organs (brain, bladder, heart, kidney, liver, lung, spleen, and thymus) of somatic cell cloned calves that died shortly after birth, in three tissues (skin, muscle, and liver) of healthy clones that survived to adulthood, and in corresponding tissues of control animals from natural reproduction. We found that, deceased bovine cloned calves exhibited abnormal expression of all three genes studied in various organs. Large variations in the expression levels of imprinted genes were also seen among these clones, which were produced from the same genetic donor. In surviving adult clones, however, the expression of these imprinted genes was largely normal, except for the expression of the Igf2 gene in muscle, which was highly variable. Our data showed disruptions of expression of imprinted genes in bovine clones, which is possibly due to incomplete reprogramming of donor cell nuclei during nuclear transfer, and these abnormalities may be associated with the high neonatal mortality in cloned animals; clones that survived to adulthood, however, are not only physically healthy but also relatively normal at the molecular level of those three imprinted genes.

Meat and milk compositions of bovine clones

The technology is now available for commercial cloning of farm animals for food production, but is the food safe for consumers? Here, we provide data on >100 parameters that compare the composition of meat and milk from beef and dairy cattle derived from cloning to those of genetic- and breed-matched control animals from conventional reproduction. The cloned animals and the comparators were managed under the same conditions and received the same diet. The composition of the meat and milk from the clones were largely not statistically different from those of matched comparators, and all parameters examined were within the normal industry standards or previously reported values. The data generated from our match-controlled experiments provide science-based information desired by regulatory agencies to address public concerns about the safety of meat and milk from somatic animal clones.

Cloning and assisted reproductive techniques: Influence on early development and adult phenotype

Over the past 40 years, our increased understanding and development of cell and molecular biology has allowed even greater advances in reproductive biology. This is most evident by the development of various aspects of assisted reproductive techniques (ART), generation of transgenic animals, and most recently generation of mammals through somatic cell cloning. To date, cloning from adult somatic cells has been successful in at least 10 mammalian species. Although generating viable cloned mammals from adult cells is technically feasible and the list of successes will only continue to grow with time, prenatal and perinatal mortality is high and live cloned offspring have not been without health problems. The success of many of the proposed applications of the cloning technique obviously depends upon the health and survival of founder animals generated by nuclear transfer. This article summarizes the health consequences of cloning in mice, and discusses possible mechanisms through which these conditions may arise. In addition, we discuss the effects of ART in animal models and in humans. ART also involves some of the same procedures used in cloning, and there are reports that offspring generated by ART sometimes display aberrant phenotypes as well. It is important to point out that although these techniques do sometimes produce abnormalities, the majority of offspring are born apparently normal and survive to adulthood. Additionally, we must emphasize that the effects of ART and cloning observed in animal models do not necessarily indicate that they will occur in humans. In this article, we review studies examining the phenotype of animals generated by cloning and various ART, and discuss clinical implications of these findings.

Achieving immortality in the C. elegans germline

Germline immortality is a topic that has intrigued theoretical biologists interested in aging for over a century. The germ cell lineage can be passed from one generation to the next, indefinitely. In contrast, somatic cells are typically only needed for a single generation and are then discarded. Germ cells may, therefore, harbor rejuvenation mechanisms that enable them to proliferate for eons. Such processes are thought to be either absent from or down-regulated in somatic cells, although cell non-autonomous forms of rejuvenation are formally possible. A thorough description of mechanisms that foster eternal youth in germ cells is lacking. The mysteries of germline immortality are being addressed in the nematode Caenorhabditis elegans by studying mutants that reproduce normally for several generations but eventually become sterile. The mortal germline mutants probably become sterile as a consequence of accumulating various forms of heritable cellular damage. Such mutants are abundant, indicating that several different biochemical pathways are required to rejuvenate the germline. Thus, forward genetics should help to define mechanisms that enable the germline to achieve immortality.