Updates on Rare Genetic Variants, Genetic Testing, and Gene Therapy in Individuals With Obesity.

PURPOSE OF REVIEW: The goal of this paper is to aggregate information on monogenic contributions to obesity in the past five years and to provide guidance for genetic testing in clinical care. RECENT FINDINGS: Advances in sequencing technologies, increasing awareness, access to testing, and new treatments have increased the utilization of genetics in clinical care. There is increasing recognition of the prevalence of rare genetic obesity from variants with mean allele frequency < 5% -new variants in known genes as well as identification of novel genes- causing monogenic obesity. While most of these genes are in the leptin melanocortin pathway, those in adipocytes may also contribute. Common variants may contribute either to higher lifetime tendency for weight gain or provide protection from monogenic obesity. While specific genetic mutations are rare, these segregate in individuals with early-onset severe obesity; thus, collectively genetic etiologies are not as rare. Some genetic conditions are amenable to targeted treatment. Research into the discovery of novel genetic causes as well as targeted treatment is growing over time. The utility of therapeutic strategies based on the genetic risk of obesity is an advancing frontier.

DNA replication in early mammalian embryos is patterned, predisposing lamina-associated regions to fragility.

DNA replication in differentiated cells follows a defined program, but when and how it is established during mammalian development is not known. Here we show using single-cell sequencing, that both bovine and mouse cleavage stage embryos progress through S-phase in a defined pattern. Late replicating regions are associated with the nuclear lamina from the first cell cycle after fertilization, and contain few active origins, and few but long genes. Chromosome breaks, which form spontaneously in bovine embryos at sites concordant with human embryos, preferentially locate to late replicating regions. In mice, late replicating regions show enhanced fragility due to a sparsity of dormant origins that can be activated under conditions of replication stress. This pattern predisposes regions with long neuronal genes to fragility and genetic change prior to segregation of soma and germ line. Our studies show that the formation of early and late replicating regions is among the first layers of epigenetic regulation established on the mammalian genome after fertilization.

Replication stress impairs chromosome segregation and preimplantation development in human embryos.

Human cleavage-stage embryos frequently acquire chromosomal aneuploidies during mitosis due to unknown mechanisms. Here, we show that S phase at the 1-cell stage shows replication fork stalling, low fork speed, and DNA synthesis extending into G2 phase. DNA damage foci consistent with collapsed replication forks, DSBs, and incomplete replication form in G2 in an ATR- and MRE11-dependent manner, followed by spontaneous chromosome breakage and segmental aneuploidies. Entry into mitosis with incomplete replication results in chromosome breakage, whole and segmental chromosome errors, micronucleation, chromosome fragmentation, and poor embryo quality. Sites of spontaneous chromosome breakage are concordant with sites of DNA synthesis in G2 phase, locating to gene-poor regions with long neural genes, which are transcriptionally silent at this stage of development. Thus, DNA replication stress in mammalian preimplantation embryos predisposes gene-poor regions to fragility, and in particular in the human embryo, to the formation of aneuploidies, impairing developmental potential.

Delayed DNA replication in haploid human embryonic stem cells.

Haploid human embryonic stem cells (ESCs) provide a powerful genetic system but diploidize at high rates. We hypothesized that diploidization results from aberrant DNA replication. To test this, we profiled DNA replication timing in isogenic haploid and diploid ESCs. The greatest difference was the earlier replication of the X Chromosome in haploids, consistent with the lack of X-Chromosome inactivation. We also identified 21 autosomal regions that had delayed replication in haploids, extending beyond the normal S phase and into G2/M. Haploid-delays comprised a unique set of quiescent genomic regions that are also underreplicated in polyploid placental cells. The same delays were observed in female ESCs with two active X Chromosomes, suggesting that increased X-Chromosome dosage may cause delayed autosomal replication. We propose that incomplete replication at the onset of mitosis could prevent cell division and result in re-entry into the cell cycle and whole genome duplication.

Reduced replication fork speed promotes pancreatic endocrine differentiation and controls graft size.

Limitations in cell proliferation are important for normal function of differentiated tissues and essential for the safety of cell replacement products made from pluripotent stem cells, which have unlimited proliferative potential. To evaluate whether these limitations can be established pharmacologically, we exposed pancreatic progenitors differentiating from human pluripotent stem cells to small molecules that interfere with cell cycle progression either by inducing G1 arrest or by impairing S phase entry or S phase completion and determined growth potential, differentiation, and function of insulin-producing endocrine cells. We found that the combination of G1 arrest with a compromised ability to complete DNA replication promoted the differentiation of pancreatic progenitor cells toward insulin-producing cells and could substitute for endocrine differentiation factors. Reduced replication fork speed during differentiation improved the stability of insulin expression, and the resulting cells protected mice from diabetes without the formation of cystic growths. The proliferative potential of grafts was proportional to the reduction of replication fork speed during pancreatic differentiation. Therefore, a compromised ability to enter and complete S phase is a functionally important property of pancreatic endocrine differentiation, can be achieved by reducing replication fork speed, and is an important determinant of cell-intrinsic limitations of growth.

Allele-Specific Chromosome Removal after Cas9 Cleavage in Human Embryos.

Correction of disease-causing mutations in human embryos holds the potential to reduce the burden of inherited genetic disorders and improve fertility treatments for couples with disease-causing mutations in lieu of embryo selection. Here, we evaluate repair outcomes of a Cas9-induced double-strand break (DSB) introduced on the paternal chromosome at the EYS locus, which carries a frameshift mutation causing blindness. We show that the most common repair outcome is microhomology-mediated end joining, which occurs during the first cell cycle in the zygote, leading to embryos with non-mosaic restoration of the reading frame. Notably, about half of the breaks remain unrepaired, resulting in an undetectable paternal allele and, after mitosis, loss of one or both chromosomal arms. Correspondingly, Cas9 off-target cleavage results in chromosomal losses and hemizygous indels because of cleavage of both alleles. These results demonstrate the ability to manipulate chromosome content and reveal significant challenges for mutation correction in human embryos.

Pluripotent stem cells with low differentiation potential contain incompletely reprogrammed DNA replication.

Reprogrammed pluripotent stem cells (PSCs) are valuable for research and potentially for cell replacement therapy. However, only a fraction of reprogrammed PSCs are developmentally competent. Genomic stability and accurate DNA synthesis are fundamental for cell development and critical for safety. We analyzed whether defects in DNA replication contribute to genomic instability and the diverse differentiation potentials of reprogrammed PSCs. Using a unique single-molecule approach, we visualized DNA replication in isogenic PSCs generated by different reprogramming approaches, either somatic cell nuclear transfer (NT-hESCs) or with defined factors (iPSCs). In PSCs with lower differentiation potential, DNA replication was incompletely reprogrammed, and genomic instability increased during replicative stress. Reprogramming of DNA replication did not correlate with DNA methylation. Instead, fewer replication origins and a higher frequency of DNA breaks in PSCs with incompletely reprogrammed DNA replication were found. Given the impact of error-free DNA synthesis on the genomic integrity and differentiation proficiency of PSCs, analyzing DNA replication may be a useful quality control tool.

Efficient SNP editing in haploid human pluripotent stem cells.

PURPOSE: To correct a potentially damaging mutation in haploid human embryonic stem cells. METHODS: Exome sequencing was performed on DNA extracted from parthenogenetically derived embryonic stem cell line (pES12). An SLC10A2 gene mutation, which affects bile acid transport, was chosen as mutation of interest in this proof of concept study to attempt correction in human pluripotent haploid cells. Confirmation of the mutation was verified, and guide RNA and a correction template was designed in preparation of performing CRISPR. Haploid cells underwent serial fluorescence activated cell sorting (FACS) with Hoechst 33342 to create an increasingly haploid (1n) enriched culture. Nucleofection was performed on p. 37 and then cells were sorted for 1n DNA content with +GFP to identify the haploid cells that expressed Cas9 tagged with GFP. RESULTS: 104,686 haploid GFP + cells were collected. Cells were cultured, individual colonies picked, and 48 clones were sent for Sanger sequencing. CRIPSR efficiency was 77.1%, with 7/48 (14.6%) clones resulting in a corrected SLC10A2 mutation. Confirmation of persistence of haploid cells was achieved with repeated FACS sorting and centromere quantification. Given the large number of passages and exposure to CRISPR, we also performed analysis of karyotypes and of off-target effects. Cells evaluated were karyotypically normal and there was no evident off target effects. CONCLUSIONS: CRISPR/Cas9 can be effectively utilized to edit mutations in haploid human embryonic stem cells. Establishment and maintenance of a haploid cell culture provides a novel way to utilize CRISPR/Cas9 in gene editing, particularly in the study of recessive alleles.

Distinct Imprinting Signatures and Biased Differentiation of Human Androgenetic and Parthenogenetic Embryonic Stem Cells.

Genomic imprinting is an epigenetic mechanism that results in parent-of-origin monoallelic expression of specific genes, which precludes uniparental development and underlies various diseases. Here, we explored molecular and developmental aspects of imprinting in humans by generating exclusively paternal human androgenetic embryonic stem cells (aESCs) and comparing them with exclusively maternal parthenogenetic ESCs (pESCs) and bi-parental ESCs, establishing a pluripotent cell system of distinct parental backgrounds. Analyzing the transcriptomes and methylomes of human aESCs, pESCs, and bi-parental ESCs enabled the characterization of regulatory relations at known imprinted regions and uncovered imprinted gene candidates within and outside known imprinted regions. Investigating the consequences of uniparental differentiation, we showed the known paternal-genome preference for placental contribution, revealed a similar bias toward liver differentiation, and implicated the involvement of the imprinted gene IGF2 in this process. Our results demonstrate the utility of parent-specific human ESCs for dissecting the role of imprinting in human development and disease.

ß-Apo-10'-carotenoids Modulate Placental Microsomal Triglyceride Transfer Protein Expression and Function to Optimize Transport of Intact ß-Carotene to the Embryo.

ß-Carotene is an important source of vitamin A for the mammalian embryo, which depends on its adequate supply to achieve proper organogenesis. In mammalian tissues, ß-carotene 15,15'-oxygenase (BCO1) converts ß-carotene to retinaldehyde, which is then oxidized to retinoic acid, the biologically active form of vitamin A that acts as a transcription factor ligand to regulate gene expression. ß-Carotene can also be cleaved by ß-carotene 9',10'-oxygenase (BCO2) to form ß-apo-10'-carotenal, a precursor of retinoic acid and a transcriptional regulator per se The mammalian embryo obtains ß-carotene from the maternal circulation. However, the molecular mechanisms that enable its transfer across the maternal-fetal barrier are not understood. Given that ß-carotene is transported in the adult bloodstream by lipoproteins and that the placenta acquires, assembles, and secretes lipoproteins, we hypothesized that the aforementioned process requires placental lipoprotein biosynthesis. Here we show that ß-carotene availability regulates transcription and activity of placental microsomal triglyceride transfer protein as well as expression of placental apolipoprotein B, two key players in lipoprotein biosynthesis. We also show that ß-apo-10'-carotenal mediates the transcriptional regulation of microsomal triglyceride transfer protein via hepatic nuclear factor 4α and chicken ovalbumin upstream promoter transcription factor I/II. Our data provide the first in vivo evidence of the transcriptional regulatory activity of ß-apocarotenoids and identify microsomal triglyceride transfer protein and its transcription factors as the targets of their action. This study demonstrates that ß-carotene induces a feed-forward mechanism in the placenta to enhance the assimilation of ß-carotene for proper embryogenesis.

Alcohol exposure in utero perturbs retinoid homeostasis in adult rats.

BACKGROUND: Maternal alcohol exposure and adult alcohol intake have been shown to perturb the metabolism of various micro- and macro-nutrients, including vitamin A and its derivatives (retinoids). Therefore, it has been hypothesized that the well-known detrimental consequences of alcohol consumption may be due to deregulations of the metabolism of such nutrients rather than to a direct effect of alcohol. Alcohol exposure in utero also has long-term harmful consequences on the health of the offspring with mechanisms that have not been fully clarified. Disruption of tissue retinoid homeostasis has been linked not only to abnormal embryonic development, but also to various adult pathological conditions, including cancer, metabolic disorders and abnormal lung function. We hypothesized that prenatal alcohol exposure may permanently perturb tissue retinoid metabolism, predisposing the offspring to adult chronic diseases. METHODS: Serum and tissues (liver, lung and prostate from males; liver and lung from females) were collected from 60-75 day-old sprague dawley rats born from dams that were: (I) fed a liquid diet containing 6.7% alcohol between gestational day 7 and 21; or (II) pair-fed with isocaloric liquid diet during the same gestational window; or (III) fed ad libitum with regular rat chow diet throughout pregnancy. Serum and tissue retinoid levels were analyzed by reverse-phase high-performance liquid chromatography (HPLC). Serum retinol-binding protein (RBP) levels were measured by western blot analysis, and liver, lung and prostate mRNA levels of lecithin-retinol acyltransferase (LRAT) were measured by qPCR. RESULTS: Retinyl ester levels were significantly reduced in the lung of both males and females, as well as in the liver and ventral prostate of males born from alcohol-fed dams. Tissue LRAT mRNA levels remained unchanged upon maternal alcohol treatment. CONCLUSIONS: Prenatal alcohol exposure in rats affects retinoid metabolism in adult life, in a tissue- and sex-dependent manner. We propose that the alcohol-induced perturbations of vitamin A metabolism may predispose to detrimental consequnces on adult health.

Tissue- and sex-specific effects of ß-carotene 15,15' oxygenase (BCO1) on retinoid and lipid metabolism in adult and developing mice.

In mammals, ß-carotene-15,15'-oxygenase (BCO1) is the main enzyme that cleaves ß-carotene, the most abundant vitamin A precursor, to generate retinoids (vitamin A derivatives), both in adult and developing tissues. We previously reported that, in addition to this function, BCO1 can also influence the synthesis of retinyl esters, the storage form of retinoids, in the mouse embryo at mid-gestation. Indeed, lack of embryonic BCO1 impaired both lecithin-dependent and acyl CoA-dependent retinol esterification, mediated by lecithin:retinol acyltransferase (LRAT) and acyl CoA:retinol acyltransferase (ARAT), respectively. Furthermore, embryonic BCO1 also influenced the ester pools of cholesterol and diacylglycerol. In this report, we gained novel insights into this alternative function of BCO1 by investigating whether BCO1 influenced embryonic retinoid and lipid metabolism in a tissue-dependent manner. To this end, livers and brains from wild-type and BCO1-/- embryos at mid-gestation were analyzed for retinoid and lipid content, as well as gene expression levels. We also asked whether or not the role of BCO1 as a regulator of lecithin- and acyl CoA-dependent retinol esterification was exclusively restricted to the developing tissues. Thus, a survey of retinol and retinyl ester levels in adult tissues of wild-type, BCO1-/-, LRAT-/- and LRAT-/-BCO1-/- mice was performed. We showed that the absence of BCO1 affects embryonic retinoid and lipid homeostasis in a tissue-specific manner and that retinyl ester formation is also influenced by BCO1 in a few adult tissues (pancreas, lung, heart and adipose) in a sex-dependent manner.