Transcriptional program of bone morphogenetic protein-2-induced epithelial and smooth muscle differentiation of pluripotent human embryonal carcinoma cells

TGFβ Family Signaling Pathways in Pluripotent and Teratocarcinoma Stem Cells' Fate Decisions: Balancing Between Self-Renewal, Differentiation, and Cancer

The transforming growth factor-β (TGFβ) family factors induce pleiotropic effects and are involved in the regulation of most normal and pathological cellular processes. The activity of different branches of the TGFβ family signaling pathways and their interplay with other signaling pathways govern the fine regulation of the self-renewal, differentiation onset and specialization of pluripotent stem cells in various cell derivatives. TGFβ family signaling pathways play a pivotal role in balancing basic cellular processes in pluripotent stem cells and their derivatives, although disturbances in their genome integrity induce the rearrangements of signaling pathways and lead to functional impairments and malignant transformation into cancer stem cells. Therefore, the identification of critical nodes and targets in the regulatory cascades of TGFβ family factors and other signaling pathways, and analysis of the rearrangements of the signal regulatory network during stem cell state transitions and interconversions, are key issues for understanding the fundamental mechanisms of both stem cell biology and cancer initiation and progression, as well as for clinical applications. This review summarizes recent advances in our understanding of TGFβ family functions in naїve and primed pluripotent stem cells and discusses how these pathways are involved in perturbations in the signaling network of malignant teratocarcinoma stem cells with impaired differentiation potential.

Nucleotide excision repair capacity increases during differentiation of human embryonic carcinoma cells into neurons and muscle cells

Embryonic stem cells can self-renew and differentiate, holding great promise for regenerative medicine. They also employ multiple mechanisms to preserve the integrity of their genomes. Nucleotide excision repair, a versatile repair mechanism, removes bulky DNA adducts from the genome. However, the dynamics of the capacity of nucleotide excision repair during stem cell differentiation remain unclear. Here, using immunoslot blot assay, we measured repair rates of UV-induced DNA damage during differentiation of human embryonic carcinoma (NTERA-2) cells into neurons and muscle cells. Our results revealed that the capacity of nucleotide excision repair increases as cell differentiation progresses. We also found that inhibition of the apoptotic signaling pathway has no effect on nucleotide excision repair capacity. Furthermore, RNA-Seq-based transcriptomic analysis indicated that expression levels of four core repair factors, xeroderma pigmentosum (XP) complementation group A (XPA), XPC, XPG, and XPF-ERCC1, are progressively up-regulated during differentiation, but not those of replication protein A (RPA) and transcription factor IIH (TFIIH). Together, our findings reveal that increase of nucleotide excision repair capacity accompanies cell differentiation, supported by the up-regulated transcription of genes encoding DNA repair enzymes during differentiation of two distinct cell lineages.

Expression of a novel splice variant of FRMD7 in developing human fetal brains that is upregulated upon the differentiation of NT2 cells

FRMD7 mutations are associated with X-linked idiopathic congenital nystagmus (ICN); however, the underlying mechanisms whereby mutations of FRMD7 lead to ICN remain unclear. In a previous study, the first FRMD7 splice variant (FRMD7-S) was cloned and identified, and FRMD7-S was hypothesized to play a significant role in neuronal differentiation and development. The present study investigated a novel multiple exon-skipping mRNA splice variant of FRMD7, termed FRMD7_SV2, which was detected in NT2 cells using northern blotting. The mRNA expression levels of FRMD7_SV2 in the developing human fetal brain were examined using reverse transcription polymerase chain reaction (PCR), while the expression levels in NT2 cells treated with retinoid acid (RA) or bone morphogenetic protein-2 were investigated using quantitative PCR. The results revealed that the expression of FRMD7_SV2 was spatially and temporally restricted in human fetal brain development, and was upregulated upon RA-induced neuronal differentiation of the NT2 cells. These results indicated that as a novel splice variant of FRMD7, FRMD7_SV2 may play a role in neuronal development.

miR-18b and miR-518b Target FOXN1 during epithelial lineage differentiation in pluripotent cells

MicroRNAs (miRNAs) regulate myriad biological processes; however, their role in cell fate choice is relatively unexplored. Pluripotent NT2/D1 embryonal carcinoma cells differentiate into an epithelial/smooth muscle phenotype when treated with bone morphogenetic protein-2 (BMP-2). To identify miRNAs involved in epithelial cell development, we performed miRNA profiling of NT2/D1 cells treated with BMP-2 at 6, 12, and 24 h, and on days 6 and 10. Integration of the miRNA profiling data with previously obtained gene expression profiling (GEP) data of NT2/D1 cells treated with BMP-2 at the same time points identified miR-18b and miR-518b as the top two miRNAs with the highest number of up-regulated predicted targets with known functions in epithelial lineage development. Silencing of miR-18b and miR-518b in NT2/D1 cells revealed several up-regulated TFs with functions in epithelial lineage development; among these, target prediction programs identified FOXN1 as the only direct target of both miRNAs. FOXN1 has previously been shown to play an important role in keratinocyte differentiation and epithelial cell proliferation. NT2/D1 and H9 human embryonic stem cells with silenced miR-18b and miR-518b showed up-regulation of FOXN1 and the epithelial markers CDH1, EPCAM, KRT19, and KRT7. A 3'UTR luciferase assay confirmed FOXN1 to be a target of the two miRNAs, and up-regulation of FOXN1 in NT2/D1 cells led to the expression of epithelial markers. Overexpression of the two miRNAs in BMP-2-treated NT2/D1 cells led to down-regulation of FOXN1 and epithelial lineage markers. These results show that miR-18b and miR-518b are upstream controllers of FOXN1-directed epithelial lineage development.

Cisplatin induces resistance by triggering differentiation of testicular embryonal carcinoma cells

Although testicular germ cell tumors are generally quite responsive to treatment with cisplatin, a small fraction of them acquire resistance during therapy. Even when cisplatin treatment is successful the patient is often left with a residual teratoma at the site of the primary tumor suggesting that cisplatin may trigger differentiation in some tumors. Using the human embryonal carcinoma cell line NTera2/D1, we confirmed that exposure to the differentiating agent retinoic acid produced a reduction in pluripotency markers NANOG and POU5F1 (Oct3/4) and an acute concentration-dependent increase in resistance to both cisplatin and paclitaxel that reached as high as 18-fold for cisplatin and 61-fold for paclitaxel within four days. A two day exposure to cisplatin also produced a concentration-dependent decrease in the expression of the NANOG and POU5F1 and increased expression of three markers whose levels increase with differentiation including Nestin, SCG10 and Fibronectin. In parallel, exposure to cisplatin induced up to 6.2-fold resistance to itself and 104-fold resistance to paclitaxel. Paclitaxel did not induce differentiation or resistance to either itself or cisplatin. Neither retinoic acid nor cisplatin induced resistance in cervical or prostate cancer cell lines or other germ cell tumor lines in which they failed to alter the expression of NANOG and POU5F1. Forced expression of NANOG prevented the induction of resistance to cisplatin by retinoic acid. We conclude that cisplatin can acutely induce resistance to itself and paclitaxel by triggering a differentiation response in pluripotent germ cell tumor cells.

Differences in the pattern and regulation of mineral deposition in human cell lines of osteogenic and non-osteogenic origin

Bone marrow-derived mesenchymal stem cells (MSCs) are widely used as a cellular model of bone formation, and can mineralize in vitro in response to osteogenic medium (OM). It is unclear, however, whether this property is specific to cells of mesenchymal origin. We analysed the OM response in 3 non-osteogenic lines, HEK293, HeLa and NTera, compared to MSCs. Whereas HEK293 cells failed to respond to OM conditions, the 2 carcinoma-derived lines NTera and HeLa deposited a calcium phosphate mineral comparable to that present in MSC cultures. However, unlike MSCs, HeLa and NTera cultures did so in the absence of dexamethasone. This discrepancy was confirmed, as bone morphogenetic protein inhibition obliterated the OM response in MSCs but not in HeLa or NTera, indicating that these 2 models can deposit mineral through a mechanism independent of established dexamethasone or bone morphogenetic protein signalling.

Endothelium-derived Netrin-4 supports pancreatic epithelial cell adhesion and differentiation through integrins α2β1 and α3β1

BACKGROUND

Netrins have been extensively studied in the developing central nervous system as pathfinding guidance cues, and more recently in non-neural tissues where they mediate cell adhesion, migration and differentiation. Netrin-4, a distant relative of Netrins 1-3, has been proposed to affect cell fate determination in developing epithelia, though receptors mediating these functions have yet to be identified.

METHODOLOGY/PRINCIPAL FINDINGS

Using human embryonic pancreatic cells as a model of developing epithelium, here we report that Netrin-4 is abundantly expressed in vascular endothelial cells and pancreatic ductal cells, and supports epithelial cell adhesion through integrins α2β1 and α3β1. Interestingly, we find that Netrin-4 recognition by embryonic pancreatic cells through integrins α2β1 and α3β1 promotes insulin and glucagon gene expression. In addition, full genome microarray analysis revealed that fetal pancreatic cell adhesion to Netrin-4 causes a prominent down-regulation of cyclins and up-regulation of negative regulators of the cell cycle. Consistent with these results, a number of other genes whose activities have been linked to developmental decisions and/or cellular differentiation are up-regulated.

CONCLUSIONS/SIGNIFICANCE

Given the recognized function of blood vessels in epithelial tissue morphogenesis, our results provide a mechanism by which endothelial-derived Netrin-4 may function as a pro-differentiation cue for adjacent developing pancreatic cell populations expressing adhesion receptors α2β1 and α3β1 integrins.

Phorbol ester-induced human cytomegalovirus major immediate-early (MIE) enhancer activation through PKC-delta, CREB, and NF-kappaB desilences MIE gene expression in quiescently infected human pluripotent NTera2 cells

The ways in which human cytomegalovirus (HCMV) major immediate-early (MIE) gene expression breaks silence from latency to initiate the viral replicative cycle are poorly understood. A delineation of the signaling cascades that desilence the HCMV MIE genes during viral quiescence in the human pluripotent N-Tera2 (NT2) cell model provides insight into the molecular mechanisms underlying HCMV reactivation. In this model, we show that phorbol 12-myristate 13-acetate (PMA) immediately activates the expression of HCMV MIE RNA and protein and greatly increases the MIE-positive (MIE(+)) NT2 cell population density; levels of Oct4 (pluripotent cell marker) and HCMV genome penetration are unchanged. Decreasing PKC-delta activity (pharmacological, dominant-negative, or RNA interference [RNAi] method) attenuates PMA-activated MIE gene expression. MIE gene activation coincides with PKC-delta Thr505 phosphorylation. Mutations in MIE enhancer binding sites for either CREB (cyclic AMP [cAMP] response element [CRE]) or NF-kappaB (kappaB) partially block PMA-activated MIE gene expression; the ETS binding site is negligibly involved, and kappaB does not confer MIE gene activation by vasoactive intestinal peptide (VIP). The PMA response is also partially attenuated by the RNAi-mediated depletion of the CREB or NF-kappaB subunit RelA or p50; it is not diminished by TORC2 knockdown or accompanied by TORC2 dephosphorylation. Mutations in both CRE and kappaB fully abolish PMA-activated MIE gene expression. Thus, PMA stimulates a PKC-delta-dependent, TORC2-independent signaling cascade that acts through cellular CREB and NF-kappaB, as well as their cognate binding sites in the MIE enhancer, to immediately desilence HCMV MIE genes. This signaling cascade is distinctly different from that elicited by VIP.

Breaking human cytomegalovirus major immediate-early gene silence by vasoactive intestinal peptide stimulation of the protein kinase A-CREB-TORC2 signaling cascade in human pluripotent embryonal NTera2 cells

The triggering mechanisms underlying reactivation of human cytomegalovirus (HCMV) in latently infected persons are unclear. During latency, HCMV major immediate-early (MIE) gene expression breaks silence to initiate viral reactivation. Using quiescently HCMV-infected human pluripotent embryonal NTera2 cells (NT2) to model HCMV reactivation, we show that vasoactive intestinal peptide (VIP), an immunomodulatory neuropeptide, immediately and dose-dependently (1 to 500 nM) activates HCMV MIE gene expression. This response requires the MIE enhancer cyclic AMP response elements (CRE). VIP quickly elevates CREB Ser133 and ATF-1 Ser63 phosphorylation levels, although the CREB Ser133 phosphorylation level is substantial at baseline. VIP does not change the level of HCMV genomes in nuclei, Oct4 (pluripotent cell marker), or hDaxx (cellular repressor of HCMV gene expression). VIP-activated MIE gene expression is mediated by cellular protein kinase A (PKA), CREB, and TORC2. VIP induces PKA-dependent TORC2 Ser171 dephosphorylation and nuclear entry, which likely enables MIE gene activation, as TORC2 S171A (devoid of Ser171 phosphorylation) exhibits enhanced nuclear entry and desilences the MIE genes in the absence of VIP stimulation. In conclusion, VIP stimulation of the PKA-CREB-TORC2 signaling cascade activates HCMV CRE-dependent MIE gene expression in quiescently infected NT2 cells. We speculate that neurohormonal stimulation via this signaling cascade is a possible means for reversing HCMV silence in vivo.

Pathway activation in large B-cell non-Hodgkin lymphoma cell lines by doxorubicin reveals prognostic markers of in vivo response

The principal curative agent in the front-line treatment of patients with diffuse large B-cell lymphoma (DLBCL) is the anthracycline, doxorubicin. To define pathways that may have a functional role in the response of DLBCL in vivo to doxorubicin-based therapies, seven DLBCL cell lines were treated with doxorubicin and the cellular response evaluated. Expression profiling of responses revealed changes in levels of genes consistent with discrete pathway activation that were confirmed functionally. The two most sensitive cell lines (Ly3 and Ly10) displayed activation of the TP53 pathway but not in the remaining five (Ly1, Ly2, Ly4, Ly7 and Ly8), where TP53 mutations were identified. In this latter group, a G2/M delay was invoked. NF-kappaB pathway activation was evident in Ly1 which with Ly4 displayed the most chemoresistant response. Treatment of Ly1 after doxorubicin with the proteasomic inhibitor, bortezomib, additively increased the cytotoxic effect of doxorubicin. Chemoresistance of Ly4 was associated with loss of chromosome 2 (0-9 Mbp) that in vivo was highly correlated with adverse outcome. Thus, the response of DLBCL in vivo and in vitro is defined by several distinct molecular and genetic pathways which is, perhaps, not surprising given the heterogeneous clinical, morphologic and genetic nature of DLBCL.

Human pluripotent embryonal carcinoma NTERA2 cl.D1 cells maintain their typical morphology in an angiomyogenic medium

Background

Pluripotent embryonal carcinomas are good potential models, to study, "in vitro," the mechanisms that control differentiation during embryogenesis. The NTERA2cl.D1 (NT2/D1) cell line is a well known system of ectodermal differentiation. Retinoic acid (RA) induces a dorsal pattern of differentiation (essentially neurons) and bone morphogenetic protein (BMP) or hexamethylenebisacetamide (HMBA) induces a more ventral (epidermal) pattern of differentiation. However, whether these human cells could give rise to mesoderm derivatives as their counterpart in mouse remained elusive. We analyzed the morphological characteristics and transcriptional activation of genes pertinent in cardiac muscle and endothelium differentiation, during the growth of NT2/D1 cells in an inductive angiomyogenic medium with or without Bone Morphogenetic Protein 2 (BMP2).

Results

Our experiments showed that NT2/D1 maintains their typical actin organization in angiomyogenic medium. Although the beta myosin heavy chain gene was never detected, all the other 15 genes analyzed maintained their expression throughout the time course of the experiment. Among them were early and late cardiac, endothelial, neuronal and teratocarcinoma genes.

Conclusion

Our results suggest that despite the NT2/D1 cells natural tendency to differentiate into neuroectodermal lineages, they can activate genes of mesodermal lineages. Therefore, we believe that these pluripotent cells might still be a good model to study biological development of mesodermal derivatives, provided the right culture conditions are met.

Constitutive gene expression predisposes morphogen-mediated cell fate responses of NT2/D1 and 27X-1 human embryonal carcinoma cells

Human embryonal carcinoma (EC) cell lines exhibit considerable heterogeneity in their levels of pluripotency. Thus, NT2/D1 cells differentiate into neural lineages upon exposure to all-trans retinoic acid (ATRA) and non-neural epithelial lineages upon exposure to bone morphogenetic protein-2 (BMP-2). In contrast, 27X-1 cells differentiate into extra-embryonic endodermal (ExE) cells upon treatment with either morphogen. To understand the molecular basis for the differential responses of the two cell lines, we performed gene expression profiling at the undifferentiated EC cell line state to identify constitutive differences in gene expression. NT2/D1 cells preferentially expressed transcripts associated with neurectodermal development, whereas 27X-1 cells expressed high levels of transcripts associated with mesendodermal characteristics. We then determined temporal expression profiles of 27X-1 cells during ExE differentiation upon treatment with ATRA and BMP-2 and compared the data with changes in gene expression observed during BMP-2- and ATRA-induced differentiation of NT2/D1 cells. ATRA and BMP-2 induced distinct sets of transcription factors and phenotypic markers in the two EC cell lines, underlying distinct lineage choices. Although 27X-1 differentiation yielded comprehensive gene expression profiles of parietal endodermal lineages, we were able to use the combined analysis of 27X-1 data with data derived from yolk sac tumors for the identification of transcripts associated with visceral endoderm formation. Our results demonstrate constitutive differences in the levels of pluripotency between NT2/D1 and 27X-1 cells that correlate with lineage potential. This study also demonstrates that EC cells can serve as robust models to investigate early lineage choices during both embryonic and extra-embryonic human development.

Epicardial cells of human adults can undergo an epithelial-to-mesenchymal transition and obtain characteristics of smooth muscle cells in vitro

Myocardial and coronary development are both critically dependent on epicardial cells. During cardiomorphogenesis, a subset of epicardial cells undergoes an epithelial-to-mesenchymal transition (EMT) and invades the myocardium to differentiate into various cell types, including coronary smooth muscle cells and perivascular and cardiac interstitial fibroblasts. Our current knowledge of epicardial EMT and the ensuing epicardium-derived cells (EPDCs) comes primarily from studies of chick and mouse embryonic development. Due to the absence of an in vitro culture system, very little is known about human EPDCs. Here, we report for the first time the establishment of cultures of primary epicardial cells from human adults and describe their immunophenotype, transcriptome, transducibility, and differentiation potential in vitro. Changes in morphology and beta-catenin staining pattern indicated that human epicardial cells spontaneously undergo EMT early during ex vivo culture. The surface antigen profile of the cells after EMT closely resembles that of subepithelial fibroblasts; however, only EPDCs express the cardiac marker genes GATA4 and cardiac troponin T. After infection with an adenovirus vector encoding the transcription factor myocardin or after treatment with transforming growth factor-beta1 or bone morphogenetic protein-2, EPDCs obtain characteristics of smooth muscle cells. Moreover, EPDCs can undergo osteogenesis but fail to form adipocytes or endothelial cells in vitro. Cultured epicardial cells from human adults recapitulate at least part of the differentiation potential of their embryonic counterparts and represent an excellent model system to explore the biological properties and therapeutic potential of these cells.

Testicular Germ Cell Tumors: A Paradigm for the Successful Treatment of Solid Tumor Stem Cells

Treatment of testicular germ cell tumors (TGCTs) has been a success primarily due to the exquisite responsiveness of this solid tumor to cisplatin-based therapy. Despite the promise of cure for the majority of TGCT patients, the effectiveness of therapy for some patients is limited by toxicity and the problem of resistance. There is compelling rationale to further understand the biology of TGCTs in order to better treat other solid tumors and to address the shortcomings of present TGCT therapies. TGCTs contain undifferentiated pluripotent stem cells, known as embryonal carcinoma, that share many properties with human embryonic stem cells. The importance of cancer stem cells in the initiation, progression and treatment of solid tumors is beginning to emerge. We discuss TGCTs in the context of solid tumor curability and targeted cancer stem cell therapy.

Transcriptional Program Associated with IFN-αResponse of Renal Cell Carcinoma

Metastatic renal cell carcinoma (RCC) is refractory to therapy; however, 10%-20% of patients respond favorably with interferon-alpha (IFN-alpha) treatment. To understand the molecular basis of response to IFN-alpha therapy, we performed global gene expression analysis of sensitive and resistant RCC cell lines in the absence and in the presence of IFN-alpha, using high-density oligonucleotide arrays to detect differentially expressed genes. In the absence of IFN-alpha, no significant differences in gene expression were observed between six sensitive and six resistant cell lines. Gene expression analysis following a time course of IFN-alpha2b treatment in one sensitive (SK-RC-17) and one resistant (SK-RC-12) cell line revealed that 484 and 354 transcripts, respectively, were modulated. A considerable number of these transcripts were similarly modulated between the two cell types that included several known targets of IFN signaling associated with antiviral and immunomodulatory activity. A further analysis of gene expression pattern in response to IFN revealed that several transcripts associated with proapoptotic function were upregulated in the sensitive cells. In the resistant cells, transcripts associated with cell survival and proliferation were induced, and key apoptotic molecules were suppressed. This study suggests that the IFN-alpha response of individual RCC tumors is determined by the expression pattern of genes in the apoptosis vs. survival and proliferation pathways rather than by alterations in expression of one or more individual genes.

BMP signaling controls PASMC KV channel expression in vitro and in vivo

Bone morphogenetic proteins (BMPs) have been implicated in the pathogenesis of familial pulmonary arterial hypertension. The type 2 receptor (BMPR2) is required for recognition of all BMPs. Transgenic mice with a smooth muscle cell-targeted mutation in this receptor (SM22-tet-BMPR2(delx4+)) developed increased pulmonary artery pressure, associated with a modest increase in arterial muscularization, after 8 wk of transgene activation (West J, Fagan K, Steudel W, Fouty B, Lane K, Harral J, Hoedt-Miller M, Tada Y, Ozimek J, Tuder R, and Rodman DM. Circ Res 94: 1109-1114, 2004). In the present study, we show that these transgenic mice developed increased right ventricular pressures after only 1 wk of transgene activation, without significant remodeling of the vasculature. We then tested the hypothesis that the increased pulmonary artery pressure due to loss of BMPR2 signaling was mediated by reduced K(V) channel expression. There was decreased expression of K(V)1.1, K(V)1.5, and K(V)4.3 mRNA isolated from whole lung. Western blot confirmed decreased K(V)1.5 protein in these lungs. Human pulmonary artery smooth muscle cells (PASMC) treated with recombinant BMP2 had increased K(V)1.5 protein and macroscopic K(V) current density, which was blocked by anti-K(V)1.5 antibody. In vivo, nifedipine, a selective L-type Ca(2+) channel blocker, reduced RV systolic pressure in these dominant-negative BMPR2 mice to levels seen in control animals. This suggests that activation of L-type Ca(2+) channels caused by reduced K(V)1.5 mediates increased pulmonary artery pressure in these animals. These studies suggest that BMP regulates K(V) channel expression and that loss of this signaling pathway in PASMC through a mutation in BMPR2 is sufficient to cause pulmonary artery vasoconstriction.

Milk growth factor (MGF)-induced differentiation of NT2/D1 cells

The differentiation activity of milk growth factor (MGF, 200 ng/ml), which also has proliferative activity, was investigated in NT2/D1 cells relative to that of retinoic acid (RA, 10(-7) M). MGF suppressed the proliferation of NT2/D1 cells to the same extent as RA after cultivation for 2x4 days. MGF enhanced Fas expression in NT2/D1 cells and prevented the decrease of Fas expression when RA was also added. MGF induced the synthesis of alpha-smooth muscle actin (alpha-SM-actin) in NT2/D1 cells without fibrils, but RA did not have such a potent activity. MGF extended glial fibrillary acidic protein (GFAP) that existed in a local area of NT2/D1 cell cytoplasm. On the other hand, RA enhanced GFAP expression and dispersed it throughout the cells. MGF slightly induced neurofilament-medium size (NF-M) synthesis in NT2/D1 cells that RA induced in the cells. MGF was less effective than RA in stimulating the synthesis of epinephrine in the cells, and the additive effect of MGF and RA enhanced epinephrine synthesis. While dopamine synthesis was less effectively stimulated by MGF than by RA, an additive effect of MGF and RA for dopamine synthesis was not observed in the cells. It was thus found that MGF differentiated NT2/D1 cells through alpha-SM-actin-synthesis.

Systematic segregation to mutant mitochondrial DNA and accompanying loss of mitochondrial DNA in human NT2 teratocarcinoma Cybrids

In this study a well-characterized pathological mutation at nucleotide position 3243 of human mitochondrial DNA was introduced into human rho(0) teratocarcinoma (NT2) cells. In cloned and mixed populations of NT2 cells heteroplasmic for the mutation, mitotic segregation toward increasing levels of mutant mitochondrial DNA always occurred. Rapid segregation was frequently followed by complete loss of mitochondrial DNA. These findings support the idea that pathological mitochondrial DNA mutations are particularly deleterious in specific cell types, which can explain some of the tissue-specific aspects of mitochondrial DNA diseases. Moreover, these findings suggest that mitochondrial DNA depletion may be an important and widespread feature of mitochondrial DNA disease.