Urogenital and caudal dysgenesis in adrenocortical dysplasia (acd) mice is caused by a splicing mutation in a novel telomeric regulator

Exome sequencing identifies variants in infants with sacral agenesis

BACKGROUND

Sacral agenesis (SA) consists of partial or complete absence of the caudal end of the spine and often presents with additional birth defects. Several studies have examined gene variants for syndromic forms of SA, but only one has examined exomes of children with non-syndromic SA.

METHODS

Using buccal cell specimens from families of children with non-syndromic SA, exomes of 28 child-parent trios (eight with and 20 without a maternal diagnosis of pregestational diabetes) and two child-father duos (neither with diagnosis of maternal pregestational diabetes) were exome sequenced.

RESULTS

Three children had heterozygous missense variants in ID1 (Inhibitor of DNA Binding 1), with CADD scores >20 (top 1% of deleterious variants in the genome); two children inherited the variant from their fathers and one from the child's mother. Rare missense variants were also detected in PDZD2 (PDZ Domain Containing 2; N = 1) and SPTBN5 (Spectrin Beta, Non-erythrocytic 5; N = 2), two genes previously suggested to be associated with SA etiology. Examination of variants with autosomal recessive and X-linked recessive inheritance identified five and two missense variants, respectively. Compound heterozygous variants were identified in several genes. In addition, 12 de novo variants were identified, all in different genes in different children.

CONCLUSIONS

To our knowledge, this is the first study reporting a possible association between ID1 and non-syndromic SA. Although maternal pregestational diabetes has been strongly associated with SA, the missense variants in ID1 identified in two of three children were paternally inherited. These findings add to the knowledge of gene variants associated with non-syndromic SA and provide data for future studies.

Differential impact of a dyskeratosis congenita mutation in TPP1 on mouse hematopoiesis and germline

A TPP1 mutation known to cause telomere shortening and bone marrow failure in humans recapitulates telomere loss but results in severe germline defects in mice without impacting murine hematopoiesis.

Telomerase extends chromosome ends in somatic and germline stem cells to ensure continued proliferation. Mutations in genes critical for telomerase function result in telomeropathies such as dyskeratosis congenita, frequently resulting in spontaneous bone marrow failure. A dyskeratosis congenita mutation in TPP1 (K170∆) that specifically compromises telomerase recruitment to telomeres is a valuable tool to evaluate telomerase-dependent telomere length maintenance in mice. We used CRISPR-Cas9 to generate a mouse knocked in for the equivalent of the TPP1 K170∆ mutation (TPP1 K82∆) and investigated both its hematopoietic and germline compartments in unprecedented detail. TPP1 K82∆ caused progressive telomere erosion with increasing generation number but did not induce steady-state hematopoietic defects. Strikingly, K82∆ caused mouse infertility, consistent with gross morphological defects in the testis and sperm, the appearance of dysfunctional seminiferous tubules, and a decrease in germ cells. Intriguingly, both TPP1 K82∆ mice and previously characterized telomerase knockout mice show no spontaneous bone marrow failure but rather succumb to infertility at steady-state. We speculate that telomere length maintenance contributes differently to the evolutionary fitness of humans and mice.

Telomere length set point regulation in human pluripotent stem cells critically depends on the shelterin protein TPP1

Telomere maintenance is essential for the long-term proliferation of human pluripotent stem cells, while their telomere length set point determines the proliferative capacity of their differentiated progeny. The shelterin protein TPP1 is required for telomere stability and elongation, but its role in establishing a telomere length set point remains elusive. Here, we characterize the contribution of the shorter isoform of TPP1 (TPP1S) and the amino acid L104 outside the TEL patch, TPP1’s telomerase interaction domain, to telomere length control. We demonstrate that cells deficient for TPP1S (TPP1S knockout [KO]), as well as the complete TPP1 KO cell lines, undergo telomere shortening. However, TPP1S KO cells are able to stabilize short telomeres, while TPP1 KO cells die. We compare these phenotypes with those of TPP1^L104A/L104A mutant cells, which have short and stable telomeres similar to the TPP1S KO. In contrast to TPP1S KO cells, TPP1^L104A/L104A cells respond to increased telomerase levels and maintain protected telomeres. However, TPP1^L104A/L104A shows altered sensitivity to expression changes of shelterin proteins suggesting the mutation causes a defect in telomere length feedback regulation. Together this highlights TPP1^L104A/L104A as the first shelterin mutant engineered at the endogenous locus of human stem cells with an altered telomere length set point.

Transcriptome analyses of ovarian stroma: tunica albuginea, interstitium and theca interna

The ovary has specialised stromal compartments, including the tunica albuginea, interstitial stroma and theca interna, which develops concurrently with the follicular antrum. To characterise the molecular determinants of these compartments, stroma adjacent to preantral follicles (pre-theca), interstitium and tunica albuginea were laser microdissected (n = 4 per group) and theca interna was dissected from bovine antral follicles (n = 6). RNA microarray analysis showed minimal differences between interstitial stroma and pre-theca, and these were combined for some analyses and referred to as stroma. Genes significantly upregulated in theca interna compared to stroma included INSL3, LHCGR, HSD3B1, CYP17A1, ALDH1A1, OGN, POSTN and ASPN. Quantitative RT-PCR showed significantly greater expression of OGN and LGALS1 in interstitial stroma and theca interna versus tunica and greater expression of ACD in tunica compared to theca interna. PLN was significantly higher in interstitial stroma compared to tunica and theca. Ingenuity pathway, network and upstream regulator analyses were undertaken. Cell survival was also upregulated in theca interna. The tunica albuginea was associated with GPCR and cAMP signalling, suggesting tunica contractility. It was also associated with TGF-β signalling and increased fibrous matrix. Western immunoblotting was positive for OGN, LGALS1, ALDH1A1, ACD and PLN with PLN and OGN highly expressed in tunica and interstitial stroma (each n = 6), but not in theca interna from antral follicles (n = 24). Immunohistochemistry localised LGALS1 and POSTN to extracellular matrix and PLN to smooth muscle cells. These results have identified novel differences between the ovarian stromal compartments.

Genome-wide chromatin accessibility and transcriptome profiling show minimal epigenome changes and coordinated transcriptional dysregulation of hedgehog signaling in Danforth's short tail mice

Danforth's short tail (Sd) mice provide an excellent model for investigating the underlying etiology of human caudal birth defects, which affect 1 in 10 000 live births. Sd animals exhibit aberrant axial skeleton, urogenital and gastrointestinal development similar to human caudal malformation syndromes including urorectal septum malformation, caudal regression, vertebral-anal-cardiac-tracheo-esophageal fistula-renal-limb (VACTERL) association and persistent cloaca. Previous studies have shown that the Sd mutation results from an endogenous retroviral (ERV) insertion upstream of the Ptf1a gene resulting in its ectopic expression at E9.5. Though the genetic lesion has been determined, the resulting epigenomic and transcriptomic changes driving the phenotype have not been investigated. Here, we performed ATAC-seq experiments on isolated E9.5 tailbud tissue, which revealed minimal changes in chromatin accessibility in Sd/Sd mutant embryos. Interestingly, chromatin changes were localized to a small interval adjacent to the Sd ERV insertion overlapping a known Ptf1a enhancer region, which is conserved in mice and humans. Furthermore, mRNA-seq experiments revealed increased transcription of Ptf1a target genes and, importantly, downregulation of hedgehog pathway genes. Reduced sonic hedgehog (SHH) signaling was confirmed by in situ hybridization and immunofluorescence suggesting that the Sd phenotype results, in part, from downregulated SHH signaling. Taken together, these data demonstrate substantial transcriptome changes in the Sd mouse, and indicate that the effect of the ERV insertion on Ptf1a expression may be mediated by increased chromatin accessibility at a conserved Ptf1a enhancer. We propose that human caudal dysgenesis disorders may result from dysregulation of hedgehog signaling pathways.

The role of telomere-binding modulators in pluripotent stem cells

Pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs), ESCs derived by somatic cell nuclear transfer (ntESCs), and induced pluripotent stem cells (iPSCs) have unlimited capacity for self-renewal and pluripotency and can give rise to all types of somatic cells. In order to maintain their self-renewal and pluripotency, PSCs need to preserve their telomere length and homeostasis. In recent years, increasing studies have shown that telomere reprogramming is essential for stem cell pluripotency maintenance and its induced pluripotency process. Telomere-associated proteins are not only required for telomere maintenance in both stem cells, their extra-telomeric functions have also been found to be critical as well. Here, we will discuss how telomeres and telomere-associated factors participate and regulate the maintenance of stem cell pluripotency.

Shwachman-Diamond Syndrome Protein SBDS Maintains Human Telomeres by Regulating Telomerase Recruitment

Shwachman-Diamond syndrome (SDS) is a rare pediatric disease characterized by various systemic disorders, including hematopoietic dysfunction. The mutation of Shwachman-Bodian-Diamond syndrome (SBDS) gene has been proposed to be a major causative reason for SDS. Although SBDS patients were reported to have shorter telomere length in granulocytes, the underlying mechanism is still unclear. Here we provide data to elucidate the role of SBDS in telomere protection. We demonstrate that SBDS deficiency leads to telomere shortening. We found that overexpression of disease-associated SBDS mutants or knockdown of SBDS hampered the recruitment of telomerase onto telomeres, while the overall reverse transcriptase activity of telomerase remained unaffected. Moreover, we show that SBDS could specifically bind to TPP1 during the S phase of cell cycle, likely functioning as a stabilizer for TPP1-telomerase interaction. Our findings suggest that SBDS is a telomere-protecting protein that participates in regulating telomerase recruitment.

High-throughput gene expression analysis identifies p53-dependent and -independent pathways contributing to the adrenocortical dysplasia (acd) phenotype

In mammalian cells TPP1, encoded by the Acd gene, is a key component of the shelterin complex, which is required for telomere length maintenance and telomere protection. In mice, a hypomorphic mutation in Acd causes the adrenocortical dysplasia (acd) phenotype, which includes limb and body axis anomalies, and perinatal lethality. p53 deficiency partially rescues limb and body axis anomalies in acd mutant embryos, but not perinatal lethality, implicating p53-independent mechanisms in the acd phenotype. Loss of function of most shelterin proteins results in early embryonic lethality. Thus, study of the hypomorphic acd allele provides a unique opportunity to understand telomere dysfunction at an organismal level. The aim of this study was to identify transcriptome alterations in acd mutant and acd, p53 double mutant embryos to understand the p53-dependent and -independent factors that contribute to the mutant phenotypes in the context of the whole organism. Genes involved in developmental processes, cell cycle, metabolic pathways, tight junctions, axon guidance and signaling pathways were regulated by p53-driven mechanisms in acd mutant embryos, while genes functioning in immune response, and RNA processing were altered independently of p53 in acd, p53 double mutant embryos. To our best of knowledge, this is the first study revealing detailed transcriptomic alterations, reflecting novel p53-dependent and -independent pathways contributing to the acd phenotype. Our data confirm the importance of cell cycle and DNA repair pathways, and suggest novel links between telomere dysfunction and immune system regulation and the splicing machinery. Given the broad applicability of telomere maintenance in growth, development, and genome stability, our data will also provide a rich resource for others studying telomere maintenance and DNA damage responses in mammalian model systems.

Control of Cellular Aging, Tissue Function, and Cancer by p53 Downstream of Telomeres

Telomeres, the nucleoprotein complex at the ends of eukaryotic chromosomes, perform an essential cellular role in part by preventing the chromosomal end from initiating a DNA-damage response. This function of telomeres can be compromised as telomeres erode either as a consequence of cell division in culture or as a normal part of cellular ageing in proliferative tissues. Telomere dysfunction in this context leads to DNA-damage signaling and activation of the tumor-suppressor protein p53, which then can prompt either cellular senescence or apoptosis. By culling cells with dysfunctional telomeres, p53 plays a critical role in protecting tissues against the effects of critically short telomeres. However, as telomere dysfunction worsens, p53 likely exacerbates short telomere-driven tissue failure diseases, including pulmonary fibrosis, aplastic anemia, and liver cirrhosis. In cells lacking p53, unchecked telomere shortening drives chromosomal end-to-end fusions and cycles of chromosome fusion-bridge-breakage. Incipient cancer cells confronting these telomere barriers must disable p53 signaling to avoid senescence and eventually up-regulate telomerase to achieve cellular immortality. The recent findings of highly recurrent activating mutations in the promoter for the telomerase reverse transcriptase (TERT) gene in diverse human cancers, together with the widespread mutations in p53 in cancer, provide support for the idea that circumvention of a telomere-p53 checkpoint is essential for malignant progression in human cancer.

Cell signaling pathways in the adrenal cortex: Links to stem/progenitor biology and neoplasia

The adrenal cortex is a dynamic tissue responsible for the synthesis of steroid hormones, including mineralocorticoids, glucocorticoids, and androgens in humans. Advances have been made in understanding the role of adrenocortical stem/progenitor cell populations in cortex homeostasis and self-renewal. Recently, large molecular profiling studies of adrenocortical carcinoma (ACC) have given insights into proteins and signaling pathways involved in normal tissue homeostasis that become dysregulated in cancer. These data provide an impetus to examine the cellular pathways implicated in adrenocortical disease and study connections, or lack thereof, between adrenal homeostasis and tumorigenesis, with a particular focus on stem and progenitor cell pathways. In this review, we discuss evidence for stem/progenitor cells in the adrenal cortex, proteins and signaling pathways that may regulate these cells, and the role these proteins play in pathologic and neoplastic conditions. In turn, we also examine common perturbations in adrenocortical tumors (ACT) and how these proteins and pathways may be involved in adrenal homeostasis.

The shelterin complex and hematopoiesis

Mammalian chromosomes terminate in stretches of repetitive telomeric DNA that act as buffers to avoid loss of essential genetic information during end-replication. A multiprotein complex known as shelterin prevents recognition of telomeric sequences as sites of DNA damage. Telomere erosion contributes to human diseases ranging from BM failure to premature aging syndromes and cancer. The role of shelterin telomere protection is less understood. Mutations in genes encoding the shelterin proteins TRF1-interacting nuclear factor 2 (TIN2) and adrenocortical dysplasia homolog (ACD) were identified in dyskeratosis congenita, a syndrome characterized by somatic stem cell dysfunction in multiple organs leading to BM failure and other pleiotropic manifestations. Here, we introduce the biochemical features and in vivo effects of individual shelterin proteins, discuss shelterin functions in hematopoiesis, and review emerging knowledge implicating the shelterin complex in hematological disorders.

Mouse models of adrenocortical tumors

The molecular basis of the organogenesis, homeostasis, and tumorigenesis of the adrenal cortex has been the subject of intense study for many decades. Specifically, characterization of tumor predisposition syndromes with adrenocortical manifestations and molecular profiling of sporadic adrenocortical tumors have led to the discovery of key molecular pathways that promote pathological adrenal growth. However, given the observational nature of such studies, several important questions regarding the molecular pathogenesis of adrenocortical tumors have remained. This review will summarize naturally occurring and genetically engineered mouse models that have provided novel tools to explore the molecular and cellular underpinnings of adrenocortical tumors. New paradigms of cancer initiation, maintenance, and progression that have emerged from this work will be discussed.

Mouse Models Recapitulating Human Adrenocortical Tumors: What Is Lacking?

Adrenal cortex tumors are divided into benign forms, such as primary hyperplasias and adrenocortical adenomas (ACAs), and malignant forms or adrenocortical carcinomas (ACCs). Primary hyperplasias are rare causes of adrenocorticotropin hormone-independent hypercortisolism. ACAs are the most common type of adrenal gland tumors and they are rarely "functional," i.e., producing steroids. When functional, adenomas result in endocrine disorders, such as Cushing's syndrome (hypercortisolism) or Conn's syndrome (hyperaldosteronism). By contrast, ACCs are extremely rare but highly aggressive tumors that may also lead to hypersecreting syndromes. Genetic analyses of patients with sporadic or familial forms of adrenocortical tumors (ACTs) led to the identification of potentially causative genes, most of them being involved in protein kinase A (PKA), Wnt/β-catenin, and P53 signaling pathways. Development of mouse models is a crucial step to firmly establish the functional significance of candidate genes, to dissect mechanisms leading to tumors and endocrine disorders, and in fine to provide in vivo tools for therapeutic screens. In this article, we will provide an overview on the existing mouse models (xenografted and genetically engineered) of ACTs by focusing on the role of PKA and Wnt/β-catenin pathways in this context. We will discuss the advantages and limitations of models that have been developed heretofore and we will point out necessary improvements in the development of next generation mouse models of adrenal diseases.

Development of adrenal cortex zonation

The human adult adrenal cortex is composed of the zona glomerulosa (zG), zona fasciculata (zF), and zona reticularis (zR), which are responsible for production of mineralocorticoids, glucocorticoids, and adrenal androgens, respectively. The final completion of cortical zonation in humans does not occur until puberty with the establishment of the zR and its production of adrenal androgens; a process called adrenarche. The maintenance of the adrenal cortex involves the centripetal displacement and differentiation of peripheral Sonic hedgehog-positive progenitors cells into zG cells that later transition to zF cells and subsequently zR cells.

Adrenocortical growth and cancer

The adrenal gland consists of two distinct parts, the cortex and the medulla. Molecular mechanisms controlling differentiation and growth of the adrenal gland have been studied in detail using mouse models. Knowledge also came from investigations of genetic disorders altering adrenal development and/or function. During embryonic development, the adrenal cortex acquires a structural and functional zonation in which the adrenal cortex is divided into three different steroidogenic zones. Significant progress has been made in understanding adrenal zonation. Recent lineage tracing experiments have accumulated evidence for a centripetal differentiation of adrenocortical cells from the subcapsular area to the inner part of the adrenal cortex. Understanding of the mechanism of adrenocortical cancer (ACC) development was stimulated by knowledge of adrenal gland development. ACC is a rare cancer with a very poor overall prognosis. Abnormal activation of the Wnt/β-catenin as well as the IGF2 signaling plays an important role in ACC development. Studies examining rare genetic syndromes responsible for familial ACT have played an important role in identifying genetic alterations in these tumors (like TP53 or CTNNB1 mutations as well as IGF2 overexpression). Recently, genomic analyses of ACT have shown gene expression profiles associated with malignancy as well as chromosomal and methylation alterations in ACT and exome sequencing allowed to describe the mutational landscape of these tumors. This progress leads to a new classification of these tumors, opening new perspectives for the diagnosis and prognostication of ACT. This review summarizes current knowledge of adrenocortical development, growth, and tumorigenesis.

Hoyeraal-Hreidarsson syndrome caused by a germline mutation in the TEL patch of the telomere protein TPP1

Germline mutations in telomere biology genes cause dyskeratosis congenita (DC), an inherited bone marrow failure and cancer predisposition syndrome. Hoyeraal-Hreidarsson syndrome (HH) is a clinically severe variant of DC. Using exome sequencing, Kocak et al. identified mutations in ACD (encoding TPP1), a component of the telomeric shelterin complex, in one family affected by HH. Characterization of the mutations revealed that the single-amino-acid deletion affecting the TEL patch surface of the TPP1 protein significantly compromises both telomerase recruitment and processivity.

Germline mutations in telomere biology genes cause dyskeratosis congenita (DC), an inherited bone marrow failure and cancer predisposition syndrome. DC is a clinically heterogeneous disorder diagnosed by the triad of dysplastic nails, abnormal skin pigmentation, and oral leukoplakia; Hoyeraal-Hreidarsson syndrome (HH), a clinically severe variant of DC, also includes cerebellar hypoplasia, immunodeficiency, and intrauterine growth retardation. Approximately 70% of DC cases are associated with a germline mutation in one of nine genes, the products of which are all involved in telomere biology. Using exome sequencing, we identified mutations in Adrenocortical Dysplasia Homolog (ACD) (encoding TPP1), a component of the telomeric shelterin complex, in one family affected by HH. The proband inherited a deletion from his father and a missense mutation from his mother, resulting in extremely short telomeres and a severe clinical phenotype. Characterization of the mutations revealed that the single-amino-acid deletion affecting the TEL patch surface of the TPP1 protein significantly compromises both telomerase recruitment and processivity, while the missense mutation in the TIN2-binding region of TPP1 is not as clearly deleterious to TPP1 function. Our results emphasize the critical roles of the TEL patch in proper stem cell function and demonstrate that TPP1 is the second shelterin component (in addition to TIN2) to be implicated in DC.

Inherited bone marrow failure associated with germline mutation of ACD, the gene encoding telomere protein TPP1

Telomerase is a ribonucleoprotein enzyme that is necessary for overcoming telomere shortening in human germ and stem cells. Mutations in telomerase or other telomere-maintenance proteins can lead to diseases characterized by depletion of hematopoietic stem cells and bone marrow failure (BMF). Telomerase localization to telomeres requires an interaction with a region on the surface of the telomere-binding protein TPP1 known as the TEL patch. Here, we identify a family with aplastic anemia and other related hematopoietic disorders in which a 1-amino-acid deletion in the TEL patch of TPP1 (ΔK170) segregates with disease. All family members carrying this mutation, but not those with wild-type TPP1, have short telomeres. When introduced into 293T cells, TPP1 with the ΔK170 mutation is able to localize to telomeres but fails to recruit telomerase to telomeres, supporting a causal relationship between this TPP1 mutation and bone marrow disorders. ACD/TPP1 is thus a newly identified telomere-related gene in which mutations cause aplastic anemia and related BMF disorders.

Multiple facets of TPP1 in telomere maintenance

Telomeres are nucleoprotein complexes that cap the ends of all linear chromosomes and function to prevent aberrant repair and end-to-end chromosome fusions. In somatic cells, telomere shortening is a natural part of the aging process as it occurs with each round of cell division. In germ and stem cells, however, the enzyme telomerase synthesizes telomere DNA to counter-balance telomere shortening and help maintain cellular proliferation. Of the primary telomere end-binding proteins, TPP1 has recently emerged as a primary contributor in protecting telomere DNA and in recruiting telomerase to the telomere ends. In this review, we summarize the current knowledge regarding the role of TPP1 in telomere maintenance.

Telomere biology in stem cells and reprogramming

Telomerase expression in humans is restricted to different populations of stem and progenitor cells, being silenced in most somatic tissues. Efficient telomere homeostasis is essential for embryonic and adult stem cell function and therefore essential for tissue homeostasis throughout organismal life. Accordingly, the mutations in telomerase culminate in reduced stem cell function both in vivo and in vitro and have been associated with tissue dysfunction in human patients. Despite the importance of telomerase for stem cell biology, the mechanisms behind telomerase regulation during development are still poorly understood, mostly due to difficulties in acquiring and maintaining pluripotent stem cell populations in culture. In this chapter, we will analyze recent developments in this field, including the importance of efficient telomere homeostasis in different stem cell types and the role of telomerase in different techniques used for cellular reprogramming.

Hematopoietic stem cells are acutely sensitive to Acd shelterin gene inactivation

The shelterin complex plays dual functions in telomere homeostasis by recruiting telomerase and preventing the activation of a DNA damage response at telomeric ends. Somatic stem cells require telomerase activity, as evidenced by progressive stem cell loss leading to bone marrow failure in hereditary dyskeratosis congenita. Recent work demonstrates that dyskeratosis congenita can also arise from mutations in specific shelterin genes, although little is known about shelterin functions in somatic stem cells. We found that mouse hematopoietic stem cells (HSCs) are acutely sensitive to inactivation of the shelterin gene Acd, encoding TPP1. Homozygosity for a hypomorphic acd allele preserved the emergence and expansion of fetal HSCs but led to profoundly defective function in transplantation assays. Upon complete Acd inactivation, HSCs expressed p53 target genes, underwent cell cycle arrest, and were severely depleted within days, leading to hematopoietic failure. TPP1 loss induced increased telomeric fusion events in bone marrow progenitors. However, unlike in epidermal stem cells, p53 deficiency did not rescue TPP1-deficient HSCs, indicating that shelterin dysfunction has unique effects in different stem cell populations. Because the consequences of telomere shortening are progressive and unsynchronized, acute loss of shelterin function represents an attractive alternative for studying telomere crisis in hematopoietic progenitors.

Conditional mutagenesis of Gata6 in SF1-positive cells causes gonadal-like differentiation in the adrenal cortex of mice

Transcription factor GATA6 is expressed in the fetal and adult adrenal cortex and has been implicated in steroidogenesis. To characterize the role of transcription factor GATA6 in adrenocortical development and function, we generated mice in which Gata6 was conditionally deleted using Cre-LoxP recombination with Sf1-cre. The adrenal glands of adult Gata6 conditional knockout (cKO) mice were small and had a thin cortex. Cytomegalic changes were evident in fetal and adult cKO adrenal glands, and chromaffin cells were ectopically located at the periphery of the glands. Corticosterone secretion in response to exogenous ACTH was blunted in cKO mice. Spindle-shaped cells expressing Gata4, a marker of gonadal stroma, accumulated in the adrenal subcapsule of Gata6 cKO mice. RNA analysis demonstrated the concomitant upregulation of other gonadal-like markers, including Amhr2, in the cKO adrenal glands, suggesting that GATA6 inhibits the spontaneous differentiation of adrenocortical stem/progenitor cells into gonadal-like cells. Lhcgr and Cyp17 were overexpressed in the adrenal glands of gonadectomized cKO vs control mice, implying that GATA6 also limits sex steroidogenic cell differentiation in response to the hormonal changes that accompany gonadectomy. Nulliparous female and orchiectomized male Gata6 cKO mice lacked an adrenal X-zone. Microarray hybridization identified Pik3c2g as a novel X-zone marker that is downregulated in the adrenal glands of these mice. Our findings offer genetic proof that GATA6 regulates the differentiation of steroidogenic progenitors into adrenocortical cells.

Next-generation sequencing identifies the Danforth's short tail mouse mutation as a retrotransposon insertion affecting Ptf1a expression

The semidominant Danforth's short tail (Sd) mutation arose spontaneously in the 1920s. The homozygous Sd phenotype includes severe malformations of the axial skeleton with an absent tail, kidney agenesis, anal atresia, and persistent cloaca. The Sd mutant phenotype mirrors features seen in human caudal malformation syndromes including urorectal septum malformation, caudal regression, VACTERL association, and persistent cloaca. The Sd mutation was previously mapped to a 0.9 cM region on mouse chromosome 2qA3. We performed Sanger sequencing of exons and intron/exon boundaries mapping to the Sd critical region and did not identify any mutations. We then performed DNA enrichment/capture followed by next-generation sequencing (NGS) of the critical genomic region. Standard bioinformatic analysis of paired-end sequence data did not reveal any causative mutations. Interrogation of reads that had been discarded because only a single end mapped correctly to the Sd locus identified an early transposon (ETn) retroviral insertion at the Sd locus, located 12.5 kb upstream of the Ptf1a gene. We show that Ptf1a expression is significantly upregulated in Sd mutant embryos at E9.5. The identification of the Sd mutation will lead to improved understanding of the developmental pathways that are misregulated in human caudal malformation syndromes.

Birth defects are the leading cause of infant mortality in the United States, accounting for 1 in 5 infant deaths annually. Birth defects that affect development of the caudal portion of the embryo can include malformations of the spine, such as spina bifida, and malformations of the kidneys and lower gastrointestinal tract. Little is known regarding the genetic causes of human caudal birth defects. The Danforth's short tail (Sd) mouse shares many similarities with these caudal birth defects that occur in humans. In this manuscript, we used next-generation sequencing to identify the genetic cause of the Sd mouse phenotype. We found that the Sd mutation is a retrotransposon insertion that inappropriately turns on a nearby gene that is normally important for pancreas development. Future studies of Sd mice will help us understand the pathogenesis of caudal birth defects in humans.

Telomeres-structure, function, and regulation

In mammals, maintenance of the linear chromosome ends (or telomeres) involves faithful replication of genetic materials and protection against DNA damage signals, to ensure genome stability and integrity. These tasks are carried out by the telomerase holoenzyme and a unique nucleoprotein structure in which an array of telomere-associated proteins bind to telomeric DNA to form special protein/DNA complexes. The telomerase complex, which is comprised of telomeric reverse transcriptase (TERT), telomeric RNA component (TERC), and other assistant factors, is responsible for adding telomeric repeats to the ends of chromosomes. Without proper telomere maintenance, telomere length will shorten with successive round of DNA replication due to the so-called end replication problem. Aberrant regulation of telomeric proteins and/or telomerase may lead to abnormalities that can result in diseases such as dyskeratosis congenita (DC) and cancers. Understanding the mechanisms that regulate telomere homeostasis and the factors that contribute to telomere dysfunction should aid us in developing diagnostic and therapeutic tools for these diseases.

Additive effect of TAp63 deficiency on the adrenocortical dysplasia (acd) phenotype

Adrenocortical dysplasia (acd) is a spontaneous autosomal recessive mouse mutation exhibiting caudal truncation, vertebral segmentation defects, hydronephrosis, limb hypoplasia, and perinatal lethality. Acd encodes TPP1, a component of the shelterin complex that maintains telomere integrity, and consequently acd mutant mice have telomere dysfunction and genomic instability. We previously showed that apoptosis is the primary mechanism causing the acd skeletal phenotype, and that p53 deficiency rescues the skeletal defects of the acd phenotype but has no effect on the perinatal lethality. The Trp63 gene encodes multiple isoforms, which play a role in proliferation, apoptosis, and stem/progenitor cell maintenance. Different p63 isoforms exhibit both proapoptotic (TAp63) and antiapoptotic (ΔNp63) functions. We hypothesized that deficiency of proapoptotic TAp63 isoforms might rescue the acd skeletal phenotype, similar to our previous observations with deficiency of p53. Mice heterozygous for a null allele of TAp63 were crossed to heterozygous acd mice to determine the effect of TAp63 deficiency on the acd mutant phenotype. In contrast to our results with the acd × p53 cross, skeletal anomalies were not rescued by deficiency of TAp63. In fact, the limb and vertebral anomalies observed in double-mutant embryos were more severe than those of embryos with the acd mutation alone, demonstrating a dose-dependent effect. These studies suggest that TAp63 isoforms do not facilitate p53-like apoptosis during development in response to acd-mediated telomere dysfunction and are consistent with the proposed roles of TAp63 in maintaining genomic stability.

Evidence of adrenal failure in aging Dax1-deficient mice

Dosage-sensitive sex reversal, adrenal hypoplasia congenita (AHC) critical region on the X chromosome, gene 1 (Dax1) is an orphan nuclear receptor essential for development and function of the mammalian adrenal cortex and gonads. DAX1 was cloned as the gene responsible for X-linked AHC, which is characterized by adrenocortical failure necessitating glucocorticoid replacement. Contrary to these human data, young mice with genetic Dax1 knockout (Dax1(-/Y)) exhibit adrenocortical hyperfunction, consistent with the historic description of Dax1 as a transcriptional repressor that inhibits steroidogenic factor 1-dependent steroidogenesis. This paradox of molecular function and two apparently opposite phenotypes associated with Dax1 deficiency in mice and humans is compounded by the recent observations that under certain circumstances, Dax1 can serve as a transcriptional activator of steroidogenic factor 1. The recently revealed role of Dax1 in embryonic stem cell pluripotency, together with the observation that its expression in the adult adrenal is restricted to the subcapsular cortex, where presumptive undifferentiated progenitor cells reside, has led us to reexamine the phenotype of Dax1(-/Y) mice in order to reconcile the conflicting mouse and human data. In this report, we demonstrate that although young Dax1(-/Y) mice have enhanced steroidogenesis and subcapsular adrenocortical proliferation, as these mice age, they exhibit declining adrenal growth, decreasing adrenal steroidogenic capacity, and a reversal of their initial enhanced hormonal sensitivity. Together with a marked adrenal dysplasia in aging mice, these data reveal that both Dax1(-/Y) mice and patients with X-linked AHC exhibit adrenal failure that is consistent with adrenocortical subcapsular progenitor cell depletion and argue for a significant role of Dax1 in maintenance of these cells.

Adrenocortical stem and progenitor cells: unifying model of two proposed origins

The origins of our understanding of the cellular and molecular mechanisms by which signaling pathways and downstream transcription factors coordinate the specification of adrenocortical cells within the adrenal gland have arisen from studies on the role of Sf1 in steroidogenesis and adrenal development initiated 20 years ago in the laboratory of Dr. Keith Parker. Adrenocortical stem/progenitor cells have been predicted to be undifferentiated and quiescent cells that remain at the periphery of the cortex until needed to replenish the organ, at which time they undergo proliferation and terminal differentiation. Identification of these stem/progenitor cells has only recently been explored. Recent efforts have examined signaling molecules, including Wnt, Shh, and Dax1, which may coordinate intricate lineage and signaling relationships between the adrenal capsule (stem cell niche) and underlying cortex (progenitor cell pool) to maintain organ homeostasis in the adrenal gland.

Telomeric and extra-telomeric roles for telomerase and the telomere-binding proteins

Mammalian telomeres are formed by tandem repeats of the TTAGGG sequence, which are progressively lost with each round of cell division. Telomere protection requires a minimal length of TTAGGG repeats to allow the binding of shelterin, which prevents the activation of a DNA damage response (DDR) at chromosome ends. Telomere elongation is carried out by telomerase. Telomerase can also act as a transcriptional modulator of the Wnt-β-catenin signalling pathway and has RNA-dependent RNA polymerase activity. Dysfunctional telomeres can lead to either cancer or ageing pathologies depending on the integrity of the DDR. This Review discusses the role of telomeric proteins in cancer and ageing through modulating telomere length and protection, as well as regulating gene expression by binding to non-telomeric sites.

Zebrafish as a model system to study the physiological function of telomeric protein TPP1

Telomeres are specialized chromatin structures at the end of chromosomes. Telomere dysfunction can lead to chromosomal abnormalities, DNA damage responses, and even cancer. In mammalian cells, a six-protein complex (telosome/shelterin) is assembled on the telomeres through the interactions between various domain structures of the six telomere proteins (POT1, TPP1, TIN2, TRF1, TRF2 and RAP1), and functions in telomere maintenance and protection. Within the telosome, TPP1 interacts directly with POT1 and TIN2 and help to mediate telosome assembly. Mechanisms of telomere regulation have been extensively studied in a variety of model organisms. For example, the physiological roles of telomere-targeted proteins have been assessed in mice through homozygous inactivation. In these cases, early embryonic lethality has prevented further studies of these proteins in embryogenesis and development. As a model system, zebrafish offers unique advantages such as genetic similarities with human, rapid developmental cycles, and ease of manipulation of its embryos. In this report, we detailed the identification of zebrafish homologues of TPP1, POT1, and TIN2, and showed that the domain structures and interactions of these telosome components appeared intact in zebrafish. Importantly, knocking down TPP1 led to multiple abnormalities in zebrafish embryogenesis, including neural death, heart malformation, and caudal defect. And these embryos displayed extensive apoptosis. These results underline the importance of TPP1 in zebrafish embryogenesis, and highlight the feasibility and advantages of investigating the signaling pathways and physiological function of telomere proteins in zebrafish.

Introduction to telomeres and telomerase

Telomeres are ends of chromosomes that play an important part in the biology of eukaryotic cells. Through the coordinated action of the telomerase and networks of other proteins and factors, the length and integrity of telomeres are maintained to prevent telomere dysfunction that has been linked to senescence, aging, diseases, and cancer. The tools and assays being used to study telomeres are being broadened, which has allowed us to derive a more detailed, high-resolution picture of the various players and pathways at work at the telomeres.

Adrenal cortex ontogenesis

During the early phases of development, adrenal glands share a common origin with kidneys and gonads. The action of diverse transcription factors, signalling pathways and endocrine signals is required for the individualization of the adrenal primordium and its subsequent differentiation into an adult adrenal gland, with massive remodelling taking place around the time of birth in humans. Here I summarize the most important steps by which the adrenal cortex is shaped and present an overview of the current understanding of the genes and molecular pathways implicated in adrenal development and involved in the pathogenesis of its congenital diseases. Evidence is accumulating that some pivotal factors acting during adrenocortical development also play an important role to regulate the growth of adrenocortical tumors, representing promising therapeutical targets for a biology-oriented therapy.

Role of shelterin in cancer and aging

Mammalian telomeres are formed by tandem repeats of the TTAGGG sequence bound by a specialized six-protein complex known as shelterin, which has fundamental roles in the regulation of telomere length and telomere capping. In the past, the study of mice genetically modified for telomerase components has been instrumental to demonstrate the role of telomere length in cancer and aging. Recent studies using genetically modified mice for shelterin proteins have highlighted an equally important role of telomere-bound proteins in cancer and aging, even in the presence of proficient telomerase activity and normal telomere length. In this review, we will focus on recent findings, suggesting a role of shelterin components in cancer and aging.

TPP1 is required for TERT recruitment, telomere elongation during nuclear reprogramming, and normal skin development in mice

The TPP1/ACD protein (hereafter TPP1) is a component of the shelterin complex at mammalian telomeres. Here we find that Tpp1-deficient mouse embryonic fibroblasts (MEFs) show increased chromosomal instability including sister chromatid fusions and chromosomes with multitelomeric signals related to telomere fragility. Tpp1 deletion decreases both TERT (the telomerase catalytic subunit) binding to telomeres in MEFs and telomerase function at chromosome ends in vivo. Abrogation of Tpp1 abolished net telomere elongation in the context of nuclear reprogramming of MEFs into induced pluripotent stem cells, whereas Tpp1 deletion in stratified epithelia of Tpp1(Delta/Delta)K5-Cre mice resulted in perinatal death, severe skin hyperpigmentation, and impaired hair follicle morphogenesis. p53 deficiency rescues skin hyperpigmentation and hair growth in these mice, indicating that p53 restricts proliferation of Tpp1-deficient cells. These results suggest a telomere-capping model where TPP1 protects telomere integrity and regulates telomerase recruitment to telomeres, thereby preventing early occurrence of degenerative pathologies.

Defending the end zone: studying the players involved in protecting chromosome ends

The linear nature of eukaryotic chromosomes leaves natural DNA ends susceptible to triggering DNA damage responses. Telomeres are specialized nucleoprotein structures that comprise the "end zone" of chromosomes. Besides having specialized sequences and structures, there are six resident proteins at telomeres that play prominent roles in protecting chromosome ends. In this review, we discuss this team of proteins, termed shelterin, and how it is involved in regulating DNA damage signaling, repair and replication at telomeres.

Acquired monosomy 7 myelodysplastic syndrome in a child with clinical features suggestive of dyskeratosis congenita and IMAGe association

We describe a case of acquired monosomy 7 myelodysplastic syndrome (MDS) in a boy with congenital adrenocortical insufficiency, genital anomalies, growth delay, skin hyperpigmentation, and chronic lung disease. Some of his clinical manifestations were suggestive of dyskeratosis congenita (DC), while other features resembled IMAGe association. DC has been linked to mutations in telomere maintenance genes. The genetic basis of IMAGe association is unknown, although mice harboring a mutation in a telomere maintenance gene, Tpp1, have adrenal hypoplasia congenita. We considered the possibility that this patient has a defect in telomere function resulting in features of both DC and IMAGe association.

Telomeres and telomerase in cancer

Myriad genetic and epigenetic alterations are required to drive normal cells toward malignant transformation. These somatic events commandeer many signaling pathways that cooperate to endow aspiring cancer cells with a full range of biological capabilities needed to grow, disseminate and ultimately kill its host. Cancer genomes are highly rearranged and are characterized by complex translocations and regional copy number alterations that target loci harboring cancer-relevant genes. Efforts to uncover the underlying mechanisms driving genome instability in cancer have revealed a prominent role for telomeres. Telomeres are nucleoprotein structures that protect the ends of eukaryotic chromosomes and are particularly vulnerable due to progressive shortening during each round of DNA replication and, thus, a lifetime of tissue renewal places the organism at risk for increasing chromosomal instability. Indeed, telomere erosion has been documented in aging tissues and hyperproliferative disease states-conditions strongly associated with increased cancer risk. Telomere dysfunction can produce the opposing pathophysiological states of degenerative aging or cancer with the specific outcome dictated by the integrity of DNA damage checkpoint responses. In most advanced cancers, telomerase is reactivated and serves to maintain telomere length and emerging data have also documented the capacity of telomerase to directly regulate cancer-promoting pathways. This review covers the role of telomeres and telomerase in the biology of normal tissue stem/progenitor cells and in the development of cancer.

Caudal regression in adrenocortical dysplasia (acd) mice is caused by telomere dysfunction with subsequent p53-dependent apoptosis

Adrenocortical dysplasia (acd) is a spontaneous autosomal recessive mouse mutation that exhibits a pleiotropic phenotype with perinatal lethality. Mutant acd embryos have caudal truncation, vertebral segmentation defects, hydronephrosis, and limb hypoplasia, resembling humans with Caudal Regression syndrome. Acd encodes Tpp1, a component of the shelterin complex that maintains telomere integrity, and consequently acd mutant mice have telomere dysfunction and genomic instability. While the association between genomic instability and cancer is well documented, the association between genomic instability and birth defects is unexplored. To determine the relationship between telomere dysfunction and embryonic malformations, we investigated mechanisms leading to the caudal dysgenesis phenotype of acd mutant embryos. We report that the caudal truncation is caused primarily by apoptosis, not altered cell proliferation. We show that the apoptosis and consequent skeletal malformations in acd mutants are dependent upon the p53 pathway by genetic rescue of the limb hypoplasia and vertebral anomalies with p53 null mice. Furthermore, rescue of the acd phenotype by p53 deficiency is a dosage-sensitive process, as acd/acd, p53(-/-) double mutants exhibit preaxial polydactyly. These findings demonstrate that caudal dysgenesis in acd embryos is secondary to p53-dependent apoptosis. Importantly, this study reinforces a significant link between genomic instability and birth defects.

Genetic p53 deficiency partially rescues the adrenocortical dysplasia phenotype at the expense of increased tumorigenesis

Telomere dysfunction and shortening induce chromosomal instability and tumorigenesis. In this study, we analyze the adrenocortical dysplasia (acd) mouse, harboring a mutation in Tpp1/Acd. Additional loss of p53 dramatically rescues the acd phenotype in an organ-specific manner, including skin hyperpigmentation and adrenal morphology, but not germ cell atrophy. Survival to weaning age is significantly increased in Acd(acd/acd) p53(-/-) mice. On the contrary, p53(-/-) and p53(+/-) mice with the Acd(acd/acd) genotype show a decreased tumor-free survival, compared with Acd(+/+) mice. Tumors from Acd(acd/acd) p53(+/-) mice show a striking switch from the classic spectrum of p53(-/-) mice toward carcinomas. The acd mouse model provides further support for an in vivo role of telomere deprotection in tumorigenesis.

Review paper: origin and molecular pathology of adrenocortical neoplasms

Neoplastic adrenocortical lesions are common in humans and several species of domestic animals. Although there are unanswered questions about the origin and evolution of adrenocortical neoplasms, analysis of human tumor specimens and animal models indicates that adrenocortical tumorigenesis involves both genetic and epigenetic alterations. Chromosomal changes accumulate during tumor progression, and aberrant telomere function is one of the key mechanisms underlying chromosome instability during this process. Epigenetic changes serve to expand the size of the uncommitted adrenal progenitor population, modulate their phenotypic plasticity (i.e., responsiveness to extracellular signals), and increase the likelihood of subsequent genetic alterations. Analyses of heritable and spontaneous types of human adrenocortical tumors documented alterations in either cell surface receptors or their downstream effectors that impact neoplastic transformation. Many of the mutations associated with benign human adrenocortical tumors result in dysregulated cyclic adenosine monophosphate signaling, whereas key factors and/or signaling pathways associated with adrenocortical carcinomas include dysregulated expression of the IGF2 gene cluster, activation of the Wnt/beta-catenin pathway, and inactivation of the p53 tumor suppressor. A better understanding of the factors and signaling pathways involved in adrenal tumorigenesis is necessary to develop targeted pharmacologic and genetic therapies.

In search of adrenocortical stem and progenitor cells

Scientists have long hypothesized the existence of tissue-specific (somatic) stem cells and have searched for their location in different organs. The theory that adrenocortical organ homeostasis is maintained by undifferentiated stem or progenitor cells can be traced back nearly a century. Similar to other organ systems, it is widely believed that these rare cells of the adrenal cortex remain relatively undifferentiated and quiescent until needed to replenish the organ, at which time they undergo proliferation and terminal differentiation. Historical studies examining cell cycle activation by label retention assays and regenerative potential by organ transplantation experiments suggested that the adrenocortical progenitors reside in the outer periphery of the adrenal gland. Over the past decade, the Hammer laboratory, building on this hypothesis and these observations, has endeavored to understand the mechanisms of adrenocortical development and organ maintenance. In this review, we summarize the current knowledge of adrenal organogenesis. We present evidence for the existence and location of adrenocortical stem/progenitor cells and their potential contribution to adrenocortical carcinomas. Data described herein come primarily from studies conducted in the Hammer laboratory with incorporation of important related studies from other investigators. Together, the work provides a framework for the emerging somatic stem cell field as it relates to the adrenal gland.

Kcnq1ot1/Lit1 noncoding RNA mediates transcriptional silencing by targeting to the perinucleolar region

The Kcnq1ot1 antisense noncoding RNA has been implicated in long-range bidirectional silencing, but the underlying mechanisms remain enigmatic. Here we characterize a domain at the 5' end of the Kcnq1ot1 RNA that carries out transcriptional silencing of linked genes using an episomal vector system. The bidirectional silencing property of Kcnq1ot1 maps to a highly conserved repeat motif within the silencing domain, which directs transcriptional silencing by interaction with chromatin, resulting in histone H3 lysine 9 trimethylation. Intriguingly, the silencing domain is also required to target the episomal vector to the perinucleolar compartment during mid-S phase. Collectively, our data unfold a novel mechanism by which an antisense RNA mediates transcriptional gene silencing of chromosomal domains by targeting them to distinct nuclear compartments known to be rich in heterochromatic machinery.

Adrenal gland tumorigenesis after gonadectomy in mice is a complex genetic trait driven by epistatic loci

Postgonadectomy adrenocortical tumorigenesis is a strain-specific phenomenon in inbred mice, assumed to be caused by elevated LH secretion and subsequent ectopic LH receptor (LHR) overexpression in adrenal gland. However, the molecular mechanisms of this cascade of events remain unknown. In this study, we took advantage of the mouse strain dependency of the phenotype to unravel its genetic basis. Our results present the first genome-wide screening related to this pathology in two independent F2 and backcross populations generated between the neoplastic DBA/2J and the nonsusceptible C57BL/6J strains. Surprisingly, the postgonadectomy elevation of serum LH was followed by similar up-regulation of adrenal LHR expression in both parental strains and their crosses, irrespective of their tumor status, indicating that it is not the immediate cause of the tumorigenesis. Linkage analysis revealed one major significant locus for the tumorigenesis on chromosome 8, modulated by epistasis with another quantitative trait locus on chromosome 18. Weight gain, a secondary phenotype after gonadectomy, showed a significant but separate quantitative trait locus on chromosome 7. Altogether, postgonadectomy adrenocortical tumorigenesis in DBA/2J mice is a dominant trait that is not a direct consequence of adrenal LHR expression but is driven by a complex genetic architecture. Analysis of candidate genes in the tumorigenesis linkage region showed that Sfrp1 (secreted frizzled-related protein 1), a tumor suppressor gene, is differentially expressed in the neoplastic areas. These findings may have relevance to the human pathogenesis of macronodular adrenal hyperplasia and adrenocortical tumors in postmenopausal women and why some of them develop obesity.

Tpp1/Acd maintains genomic stability through a complex role in telomere protection

Telomeres serve to protect the ends of chromosomes, and failure to maintain telomeres can lead to dramatic genomic instability. Human TPP1 was identified as a protein which interacts with components of a telomere cap complex, but does not directly bind to telomeric DNA. While biochemical interactions indicate a function in telomere biology, much remains to be learned regarding the roles of TPP1 in vivo. We previously reported the positional cloning of the gene responsible for the adrenocortical dysplasia (acd) mouse phenotype, which revealed a mutation in the mouse homologue encoding TPP1. We find that cells from homozygous acd mice harbor chromosomes fused at telomere sequences, demonstrating a role in telomere protection in vivo. Surprisingly, our studies also reveal fusions and radial structures lacking internal telomere sequences, which are not anticipated from a simple deficiency in telomere protection. Employing spectral karyotyping and telomere FISH in a combined approach, we have uncovered a striking pattern; fusions with telomeric sequences involve nonhomologous chromosomes while those lacking telomeric sequences involve homologues. Together, these studies show that Tpp1/Acd plays a vital role in telomere protection, but likely has additional functions yet to be defined.

The transcriptional status but not the imprinting control region determines allele-specific histone modifications at the imprinted H19 locus

Genomic imprinting governs allele-specific gene expression in an epigenetically heritable manner. The characterization of histone modifications at imprinted gene loci is incomplete, and whether specific histone marks determine transcription or are dependent on it is not understood. Using chromatin immunoprecipitations, we examined in multiple cell types and in an allele-specific manner the active and repressive histone marks of several imprinted loci, including H19, KvDMR1, Snrpn promoter/exon 1, and IG-DMR imprinting control regions. Expressed alleles are enriched for specific actively modified histones, including H3 di- and trimethylated at Lys4 and acetylated histones H3 and H4, while their silent counterparts are associated with repressive marks such as H3 trimethylated at Lys9 alone or in combination with H3 trimethylated at Lys27 and H4/H2A symmetrically dimethylated at Arg3. At H19, allele-specific histone modifications occur throughout the entire locus, including nontranscribed regions such as the differentially methylated domain (DMD) as well as sequences in the H19 gene body that are not differentially methylated. Significantly, the presence of active marks at H19 depends on transcriptional activity and occurs even in the absence of the DMD. These findings suggest that histone modifications are dependent on the transcriptional status of imprinted alleles and illuminate epigenetic mechanisms of genomic imprinting.

Novel polymorphisms and lack of mutations in the ACD gene in patients with ACTH resistance syndromes

OBJECTIVE

ACTH resistance is a feature of several human syndromes with known genetic causes, including familial glucocorticoid deficiency (types 1 and 2) and triple A syndrome. However, many patients with ACTH resistance lack an identifiable genetic aetiology. The human homolog of the Acd gene, mutated in a mouse model of adrenal insufficiency, was sequenced in 25 patients with a clinical diagnosis of familial glucocorticoid deficiency or triple A syndrome.

DESIGN

A 3.4 kilobase genomic fragment containing the entire ACD gene was analysed for mutations in all 25 patients.

SETTING

Samples were obtained by three investigators from different institutions.

PATIENTS

The primary cohort consisted of 25 unrelated patients, primarily of European or Middle Eastern descent, with a clinical diagnosis of either familial glucocorticoid deficiency (FGD) or triple A syndrome. Patients lacked mutations in other genes known to cause ACTH resistance, including AAAS for patients diagnosed with triple A syndrome and MC2R and MRAP for patients diagnosed with familial glucocorticoid deficiency. Thirty-five additional patients with adrenal disease phenotypes were added to form an expanded cohort of 60 patients.

MEASUREMENTS

Identification of DNA sequence changes in the ACD gene in the primary cohort and analysis of putative ACD haplotypes in the expanded cohort.

RESULTS

No disease-causing mutations were found, but several novel single nucleotide polymorphisms (SNPs) and two putative haplotypes were identified. The overall frequency of SNPs in ACD is low compared to other gene families.

CONCLUSIONS

No mutations were identified in ACD in this collection of patients with ACTH resistance phenotypes. However, the newly identified SNPs in ACD should be more closely examined for possible links to disease.

Telomere protection by mammalian Pot1 requires interaction with Tpp1

The shelterin complex at mammalian telomeres contains the single-stranded DNA-binding protein Pot1, which regulates telomere length and protects chromosome ends. Pot1 binds Tpp1, the shelterin component that connects Pot1 to the duplex telomeric DNA-binding proteins Trf1 and Trf2. Control of telomere length requires that Pot1 binds Tpp1 as well as the single-stranded telomeric DNA, but it is not known whether the protective function of Pot1 depends on Tpp1. Alternatively, Pot1 might function similarly to the Pot1-like proteins of budding and fission yeast, which have no known Tpp1-like connection to the duplex telomeric DNA. Using mutant mouse cells with diminished Tpp1 levels, RNA interference directed to mouse Tpp1 and Pot1, and complementation of mouse Pot1 knockout cells with human and mouse Pot1 variants, we show here that Tpp1 is required for the protective function of mammalian Pot1 proteins.

Arabidopsis POT1 associates with the telomerase RNP and is required for telomere maintenance

POT1 is a single-copy gene in yeast and humans that encodes a single-strand telomere binding protein required for chromosome end protection and telomere length regulation. In contrast, Arabidopsis harbors multiple, divergent POT-like genes that bear signature N-terminal OB-fold motifs, but otherwise share limited sequence similarity. Here, we report that plants null for AtPOT1 show no telomere deprotection phenotype, but rather exhibit progressive loss of telomeric DNA. Genetic analysis indicates that AtPOT1 acts in the same pathway as telomerase. In vitro levels of telomerase activity in pot1 mutants are significantly reduced and are more variable than wild-type. Consistent with this observation, AtPOT1 physically associates with active telomerase particles. Although low levels of AtPOT1 can be detected at telomeres in unsynchronized cells and in cells arrested in G2, AtPOT1 binding is significantly enhanced during S-phase, when telomerase is thought to act at telomeres. Our findings indicate that AtPOT1 is a novel accessory factor for telomerase required for positive telomere length regulation, and they underscore the coordinate and extraordinarily rapid evolution of telomere proteins and the telomerase enzyme.

IMAGe association and congenital adrenal hypoplasia: no disease-causing mutations found in the ACD gene

The spontaneous mouse mutant adrenocortical dysplasia (acd) is characterized by defects in the adrenals, kidneys, and gonads of adult mutant mice and by caudal dysgenesis and vertebral segmentation defects in acd embryos. This association of defects mirrors those identified in patients with known or suspected abnormalities in adrenocortical development, including adrenal hypoplasia congenita and IMAGe association. The identification of the Acd gene in mice has prompted the study of its human homolog ACD, which has recently been shown to be a regulator of telomere length. Sequencing of ACD in 15 patients revealed no coding mutations, but three novel SNPs were identified.