Prenatal diagnosis of ROR-2 related Robinow syndrome presenting with fetal ultrasound findings of mesomelia, vertebral, digital and genital abnormalities.

Autosomal recessive ROR2-Robinow syndrome is caused by pathogenic variants in the ROR2 gene. Fetal ultrasound done on our patient at 24 + 3/7 weeks gestation showed macrocephaly, brachycephaly, flat face, prominent forehead, mild frontal bossing, lower thoracic hemivertebrae, digital abnormalities and micropenis. Fetal trio whole exome sequencing done on amniocytes showed two pathogenic compound heterozygous variants in the ROR2 gene, c.1324 C > T; p.(Arg442*) maternally inherited and c.1366dup; p.(Leu456Profs*3) apparently de novo. c.1324 C > T; p.(Arg442*) is a nonsense variant resulting in protein truncation reported to be associated with RRS3. c.1366dup; p.(Leu456Profs*3) is a frameshift variant predicted to result in protein truncation reported to segregate with the disease in multiple affected individuals from a single large family with distal symphalangism of the fourth finger. Fetal autopsy following pregnancy termination showed a large head with low-set ears, facial abnormalities, mesomelic bone shortening, hemivertebra, fused S3 and S4 vertebral bodies, several fused rib heads and short penis with buried shaft.

Building Healthy Babies: A Mixed-Methods Needs Assessment for a Pre-Conception Program in Ontario.

OBJECTIVES: The objective of this study was to gather Ontario clinicians' and public members' views on the design of a pre-conception patient education program. METHODS: In this mixed-methods study, online surveys comprised of rank order, multiple choice, and short answer questions were completed by clinicians and public members. Semi-structured focus groups consisting of 2-6 participants each were then held via videoconference. Demographic variables and survey responses were analyzed quantitatively using descriptive and summary statistics. Descriptive thematic qualitative analysis using the constant comparative method of grounded theory was completed on each transcript to generate themes. RESULTS: A total of 168 public members and 43 clinicians in Ontario completed surveys, while 11 clinicians and 11 public members participated in the focus groups. A pre-conception program in Ontario was felt to be important. An individual appointment with a primary care provider was the favoured program format per survey responses, whereas a virtual format with an interactive component was preferred among focus group participants. Important topics to include were pre-conception health (infertility, genetic screening, folic acid), prenatal and postpartum counselling (diet, activity, substance use, prenatal care, postpartum course), and medical optimization in pregnancy (high-risk medical conditions, medications, mental health). Both groups emphasized the need to consider accommodations for marginalized populations and various cultures and languages. CONCLUSION: A standardized pre-conception patient education program is felt to be of high value by Ontario clinicians and public members. A pre-conception program may help improve obstetrical outcomes and decrease rates of major congenital anomalies in Ontario.

Anterior extension of the choroid plexus into the frontal horns of the fetal lateral cerebral ventricles: Prenatal findings and postnatal outcome.

OBJECTIVE: To evaluate the short- and long-term outcome of fetuses with evidence of extension of the choroid plexus into the frontal horns. METHODS: This is a retrospective cohort study of fetuses diagnosed with isolated choroid plexi extending into the frontal horns. Fetuses with major central nervous system anomalies were excluded. Ultrasound and fetal/postnatal magnetic resonance imaging (MRI) were evaluated. Postnatal outcomes, including developmental assessment, were obtained. RESULTS: Twenty nine fetuses were diagnosed with choroid plexus extension (22 unilateral and 7 bilateral). Gestational age at diagnosis was 19.3 weeks. Three cases (10.3%) presented with nonspecific extra-CNS findings. At presentation, 8/29 (28%) cases had single/multiple choroid plexus cysts (CPC). Twenty-six (89.6%) cases underwent antenatal MRI. On MRI, four cases had punctate susceptibility weighted imaging (SWI) foci suggesting trace hemosiderin and two cases had ventriculomegaly. Antenatal follow-up demonstrated resolution of the choroid plexus extension in 90% (18/20). Gestational age at delivery was 39.6 weeks. All had normal neurologic examinations within 24 h of life. Postnatal MRI studies were notable for deep venous differences in seven cases. Long-term clinical outcome was assessed in 14 cases with a median follow-up of 1.75 years, with normal neurodevelopment reported in 13/14 (92.8%). CONCLUSIONS: Most fetuses with an anterior extension of the choroid plexus as the sole sonographic finding had favorable outcomes.

Challenges experienced by genetic counselors while they provided counseling about mosaic embryos.

Objective: To survey genetic counselors (GCs) who have counseled about mosaic embryos regarding the challenges they faced in counseling this patient population and assess their need for more resources to support their practice. Design: Self-administered online survey. Setting: Academic university. Study Population: Seventy-eight GCs primarily from the United States and Canada. Interventions: Genetic counselors completed a quantitative survey with an embedded qualitative component. Quantitative data were analyzed by descriptive statistics. An inductive thematic analysis was performed on open-text responses. Main Outcome Measures: Genetic counselors were asked what clinical activities relating to mosaic embryos they performed. They were then asked to rate how challenging each activity was to perform using a 5-point scale; a rating of 4 or 5 was defined as highly challenging. Open-text questions enabled GCs to describe factors that they felt contributed to these challenges. Results: The challenges reported by GCs included the uncertainty of outcomes in offspring after mosaic embryo transfer, limited guidelines available to assist clinicians with counseling about mosaic embryos, and ranking mosaic embryos by suitability for transfer. The contributing factors suggested by participants included limited outcome data, limited GC involvement in pretest counseling for preimplantation genetic testing for aneuploidy (PGT-A), and perceived inconsistency in counseling practices across clinics. Genetic counselors differed in their genetic testing recommendations for pregnancies conceived after mosaic embryo transfer. Amniocentesis and postnatal assessment were recommended by 85% and 49% of GCs, respectively, and 15% recommended chorionic villus sampling and noninvasive prenatal testing. Almost all (92%) reported a need for more resources, such as standardized guidelines, more outcome data, and continuing education on PGT-A and mosaicism. Conclusions: This study describes challenges experienced by GCs while they counseled about mosaic embryos. Our findings demonstrate a need for more outcome data on mosaic embryo pregnancies and for evidence-based clinical guidelines. The differing recommendations for prenatal genetic testing among GCs in the study warrant further research into contributing factors. We strongly recommend that pretest counseling, including a discussion regarding mosaicism, is provided to all couples considering PGT-A to reduce counseling challenges and to promote patients' informed decision-making.

Diagnostic yield of genome sequencing for prenatal diagnosis of fetal structural anomalies.

OBJECTIVE: Genome sequencing (GS >30x) is beginning to be adopted as a comprehensive genome-wide test for the diagnosis of rare disease in the post-natal setting. Recent studies demonstrated the utility of exome sequencing (ES) in prenatal diagnosis, we investigate the potential benefits for GS to act as a comprehensive prenatal test for diagnosis of fetal abnormalities. METHODS: We performed GS on a prospective cohort of 37 singleton fetuses with ultrasound-identified structural abnormalities undergoing invasive prenatal testing. GS was performed in parallel with standard diagnostic testing, and the prioritized variants were classified according to ACMG guidelines and reviewed by a panel of board-certified laboratory and clinical geneticists. RESULTS: Diagnostic sequence variants were identified in 5 fetuses (14%), with pathogenic variants found in NIPBL, FOXF1, RERE, AMMECR1, and FLT4. A further 7 fetuses (19%) had variants of uncertain significance (VUS) that may explain the phenotypes. Importantly, GS also identified all pathogenic variants reported by clinical microarray (2 CNVs, 5%). CONCLUSION: Prenatal GS offered diagnoses (sequence variants and CNVs) in 19% of fetuses with structural anomalies. GS has the potential of replacing multiple consecutive tests, including microarray, gene panels, and WES, to provide the most comprehensive analysis in a timely manner necessary for prenatal diagnosis.

Mapping the cellular origin and early evolution of leukemia in Down syndrome.

Children with Down syndrome have a 150-fold increased risk of developing myeloid leukemia, but the mechanism of predisposition is unclear. Because Down syndrome leukemogenesis initiates during fetal development, we characterized the cellular and developmental context of preleukemic initiation and leukemic progression using gene editing in human disomic and trisomic fetal hematopoietic cells and xenotransplantation. GATA binding protein 1 (GATA1) mutations caused transient preleukemia when introduced into trisomy 21 long-term hematopoietic stem cells, where a subset of chromosome 21 microRNAs affected predisposition to preleukemia. By contrast, progression to leukemia was independent of trisomy 21 and originated in various stem and progenitor cells through additional mutations in cohesin genes. CD117+/KIT proto-oncogene (KIT) cells mediated the propagation of preleukemia and leukemia, and KIT inhibition targeted preleukemic stem cells.

Heterozygous NOTCH1 deletion associated with variable congenital heart defects.

Pathogenic heterozygous variants in the NOTCH1 gene are known to be associated with both left and right-sided congenital cardiac anomalies with strikingly incomplete penetrance and variable phenotypic expressivity. De novo NOTCH1 whole gene deletion has been reported rarely in the literature and its association with cardiac defects is less well established. Here, we report four cases of NOTCH1 gene deletion from two families associated with a spectrum of congenital heart defects from bicuspid aortic valve to complex cardiac anomalies. This is the first description of a familial NOTCH1 deletion, showing apparently high penetrance, which may be unique to this mechanism of disease. Immunohistochemical staining of cardiac tissue demonstrated reduced levels of NOTCH1 expression in both the left and right ventricular outflow tracts. These cases suggest that haploinsufficiency caused by NOTCH1 gene deletion is associated with both mild and severe cardiac defects, similar to those caused by pathogenic variants in the gene, but with apparently higher, if not complete, penetrance.

Homozygous GLUL deletion is embryonically viable and leads to glutamine synthetase deficiency.

Glutamine synthetase (GS) is the enzyme responsible for the biosynthesis of glutamine, providing the only source of endogenous glutamine necessary for several critical metabolic and developmental pathways. GS deficiency, caused by pathogenic variants in the glutamate-ammonia ligase (GLUL) gene, is a rare autosomal recessive inborn error of metabolism characterized by systemic glutamine deficiency, persistent moderate hyperammonemia, and clinically devastating seizures and multi-organ failure shortly after birth. The four cases reported thus far were caused by homozygous GLUL missense variants. We report a case of GS deficiency caused by homozygous GLUL gene deletion, diagnosed prenatally and likely representing the most severe end of the spectrum. We expand the known phenotype of this rare condition with novel dysmorphic, radiographic and neuropathologic features identified on post-mortem examination. The biallelic deletion identified in this case also included the RNASEL gene and was associated with immune dysfunction in the fetus. This case demonstrates that total absence of the GLUL gene in humans is viable beyond the embryonic period, despite the early embryonic lethality found in GLUL animal models.

The variability of SMARCA4-related Coffin-Siris syndrome: Do nonsense candidate variants add to milder phenotypes?

SMARCA4 encodes a central ATPase subunit in the BRG1-/BRM-associated factors (BAF) or polybromo-associated BAF (PBAF) complex in humans, which is responsible in part for chromatin remodeling and transcriptional regulation. Variants in this and other genes encoding BAF/PBAF complexes have been implicated in Coffin-Siris Syndrome, a multiple congenital anomaly syndrome classically characterized by learning and developmental differences, coarse facial features, hypertrichosis, and underdevelopment of the fifth digits/nails of the hands and feet. Individuals with SMARCA4 variants have been previously reported and appear to display a variable phenotype. We describe here a cohort of 15 unrelated individuals with SMARCA4 variants from the Coffin-Siris syndrome/BAF pathway disorders registry who further display variability in severity and degrees of learning impairment and health issues. Within this cohort, we also report two individuals with novel nonsense variants who appear to have a phenotype of milder learning/behavioral differences and no organ-system involvement.

Loss-of-Function Variants in PPP1R12A: From Isolated Sex Reversal to Holoprosencephaly Spectrum and Urogenital Malformations.

In two independent ongoing next-generation sequencing projects for individuals with holoprosencephaly and individuals with disorders of sex development, and through international research collaboration, we identified twelve individuals with de novo loss-of-function (LoF) variants in protein phosphatase 1, regulatory subunit 12a (PPP1R12A), an important developmental gene involved in cell migration, adhesion, and morphogenesis. This gene has not been previously reported in association with human disease, and it has intolerance to LoF as illustrated by a very low observed-to-expected ratio of LoF variants in gnomAD. Of the twelve individuals, midline brain malformations were found in five, urogenital anomalies in nine, and a combination of both phenotypes in two. Other congenital anomalies identified included omphalocele, jejunal, and ileal atresia with aberrant mesenteric blood supply, and syndactyly. Six individuals had stop gain variants, five had a deletion or duplication resulting in a frameshift, and one had a canonical splice acceptor site loss. Murine and human in situ hybridization and immunostaining revealed PPP1R12A expression in the prosencephalic neural folds and protein localization in the lower urinary tract at critical periods for forebrain division and urogenital development. Based on these clinical and molecular findings, we propose the association of PPP1R12A pathogenic variants with a congenital malformations syndrome affecting the embryogenesis of the brain and genitourinary systems and including disorders of sex development.

A large data resource of genomic copy number variation across neurodevelopmental disorders.

Copy number variations (CNVs) are implicated across many neurodevelopmental disorders (NDDs) and contribute to their shared genetic etiology. Multiple studies have attempted to identify shared etiology among NDDs, but this is the first genome-wide CNV analysis across autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), schizophrenia (SCZ), and obsessive-compulsive disorder (OCD) at once. Using microarray (Affymetrix CytoScan HD), we genotyped 2,691 subjects diagnosed with an NDD (204 SCZ, 1,838 ASD, 427 ADHD and 222 OCD) and 1,769 family members, mainly parents. We identified rare CNVs, defined as those found in <0.1% of 10,851 population control samples. We found clinically relevant CNVs (broadly defined) in 284 (10.5%) of total subjects, including 22 (10.8%) among subjects with SCZ, 209 (11.4%) with ASD, 40 (9.4%) with ADHD, and 13 (5.6%) with OCD. Among all NDD subjects, we identified 17 (0.63%) with aneuploidies and 115 (4.3%) with known genomic disorder variants. We searched further for genes impacted by different CNVs in multiple disorders. Examples of NDD-associated genes linked across more than one disorder (listed in order of occurrence frequency) are NRXN1, SEH1L, LDLRAD4, GNAL, GNG13, MKRN1, DCTN2, KNDC1, PCMTD2, KIF5A, SYNM, and long non-coding RNAs: AK127244 and PTCHD1-AS. We demonstrated that CNVs impacting the same genes could potentially contribute to the etiology of multiple NDDs. The CNVs identified will serve as a useful resource for both research and diagnostic laboratories for prioritization of variants.

Impact of introduction of noninvasive prenatal testing on uptake of genetic testing in fetuses with central nervous system anomalies.

OBJECTIVE: To evaluate the impact of introduction of noninvasive prenatal testing (NIPT) on the uptake of invasive testing in pregnancies complicated by fetal central nervous system (CNS) anomalies. METHODS: Retrospective review of all singleton pregnancies complicated by fetal CNS anomalies seen at a single tertiary center between 2010 and 2017. Cases who had undergone invasive testing or NIPT prior to the diagnosis of the CNS anomaly were excluded. Cases were segregated according to whether they were seen prior to introduction of NIPT (group A, 2010-2013) or thereafter (group B, 2014-2017). We examined the rate of invasive and noninvasive genetic testing in each group. RESULTS: We retrieved 500 cases: 308 (62%) were isolated CNS anomalies, and 192 (38%) had additional structural anomalies. In the total cohort, 165 women (33%) underwent expectant management with no further prenatal genetic testing, 166 (33%) had invasive testing, 52 (10%) had NIPT, and 117 pregnancies (23%) were terminated without further prenatal investigations. The introduction of NIPT significantly decreased the number of pregnancies having no testing (44% group A vs 22% in group B, p < .0001), particularly in the group presenting with isolated ventriculomegaly, but did not affect the uptake of invasive testing (34% vs 32%, respectively; p = .61). NIPT would have missed 4% of pathogenic copy number variants (CNVs) in the group of cases with isolated brain anomalies and 11% of CNVs in cases with complex anomalies. CONCLUSIONS: Uptake of invasive prenatal testing in fetuses with brain anomalies was not affected by NIPT. However, the incidence of no genetic testing was significantly reduced. NIPT was a suboptimal testing strategy in this population as it missed a significant number of subchromosomal genetic anomalies.

TRPV6 Variants Interfere with Maternal-Fetal Calcium Transport through the Placenta and Cause Transient Neonatal Hyperparathyroidism.

Transient neonatal hyperparathyroidism (TNHP) is etiologically a heterogeneous condition. One of the etiologies is an insufficient maternal-fetal calcium transport through the placenta. We report six subjects with homozygous and/or compound-heterozygous mutations in the gene encoding the transient receptor potential cation channel, subfamily V, member 6 (TRPV6), an epithelial Ca2+-selective channel associated with this condition. Exome sequencing on two neonates with skeletal findings consistent with neonatal hyperparathyroidism identified homozygous frameshift mutations before the first transmembrane domain in a subject born to first-cousins parents of Pakistani descent as well as compound-heterozygous mutations (a combination of a frameshift mutation and an intronic mutation that alters mRNA splicing) in an individual born to a non-consanguineous couple of African descent. Subsequently, targeted mutation analysis of TRPV6 performed on four other individuals (born to non-consanguineous Japanese parents) with similar X-rays findings identified compound-heterozygous mutations. The skeletal findings improved or resolved in most subjects during the first few months of life. We identified three missense variants (at the outer edges of the second and third transmembrane domains) that alter the localization of the TRPV6: one recurrent variant at the S2-S3 loop and two recurrent variants (in the fourth ankyrin repeat domain) that impair TRPV6 stability. Compound heterozygous loss-of-function mutations for the pathogenic frameshift allele and the allele with an intronic c.607+5G>A mutation resulted in the most severe phenotype. These results suggest that TNHP is an autosomal-recessive disease caused by TRPV6 mutations that affect maternal-fetal calcium transport.

RAC1 Missense Mutations in Developmental Disorders with Diverse Phenotypes.

RAC1 is a widely studied Rho GTPase, a class of molecules that modulate numerous cellular functions essential for normal development. RAC1 is highly conserved across species and is under strict mutational constraint. We report seven individuals with distinct de novo missense RAC1 mutations and varying degrees of developmental delay, brain malformations, and additional phenotypes. Four individuals, each harboring one of c.53G>A (p.Cys18Tyr), c.116A>G (p.Asn39Ser), c.218C>T (p.Pro73Leu), and c.470G>A (p.Cys157Tyr) variants, were microcephalic, with head circumferences between -2.5 to -5 SD. In contrast, two individuals with c.151G>A (p.Val51Met) and c.151G>C (p.Val51Leu) alleles were macrocephalic with head circumferences of +4.16 and +4.5 SD. One individual harboring a c.190T>G (p.Tyr64Asp) allele had head circumference in the normal range. Collectively, we observed an extraordinary spread of ∼10 SD of head circumferences orchestrated by distinct mutations in the same gene. In silico modeling, mouse fibroblasts spreading assays, and in vivo overexpression assays using zebrafish as a surrogate model demonstrated that the p.Cys18Tyr and p.Asn39Ser RAC1 variants function as dominant-negative alleles and result in microcephaly, reduced neuronal proliferation, and cerebellar abnormalities in vivo. Conversely, the p.Tyr64Asp substitution is constitutively active. The remaining mutations are probably weakly dominant negative or their effects are context dependent. These findings highlight the importance of RAC1 in neuronal development. Along with TRIO and HACE1, a sub-category of rare developmental disorders is emerging with RAC1 as the central player. We show that ultra-rare disorders caused by private, non-recurrent missense mutations that result in varying phenotypes are challenging to dissect, but can be delineated through focused international collaboration.

Genome-wide placental DNA methylation analysis of severely growth-discordant monochorionic twins reveals novel epigenetic targets for intrauterine growth restriction.

BACKGROUND: Intrauterine growth restriction (IUGR), which refers to reduced fetal growth in the context of placental insufficiency, is etiologically heterogeneous. IUGR is associated not only with perinatal morbidity and mortality but also with adult-onset disorders, such as cardiovascular disease and diabetes, posing a major health burden. Placental epigenetic dysregulation has been proposed as one mechanism that causes IUGR; however, the spectrum of epigenetic pathophysiological mechanisms leading to IUGR remains to be elucidated. Monozygotic monochorionic twins are particularly affected by IUGR, in the setting of severe discordant growth. Because monozygotic twins have the same genotype at conception and a shared maternal environment, they provide an ideal model system for studying epigenetic dysregulation of the placenta. RESULTS: We compared genome-wide placental DNA methylation patterns of severely growth-discordant twins to identify novel candidate genes for IUGR. Snap-frozen placental samples for eight severely growth-discordant monozygotic monochorionic twin pairs were obtained at delivery from each twin. A high-resolution DNA methylation array platform was used to identify methylation differences between IUGR and normal twins. Our analysis revealed differentially methylated regions in the promoters of eight genes: DECR1, ZNF300, DNAJA4, CCL28, LEPR, HSPA1A/L, GSTO1, and GNE. The largest methylation differences between the two groups were in the promoters of DECR1 and ZNF300. The significance of these group differences was independently validated by bisulfite pyrosequencing, implicating aberrations in fatty acid beta oxidation and transcriptional regulation, respectively. Further analysis of the array data identified methylation changes most prominently affecting the Wnt and cadherin pathways in the IUGR cohort. CONCLUSIONS: Our results suggest that IUGR in monozygotic twins is associated with impairments in lipid metabolism and transcriptional regulation as well as cadherin and Wnt signaling. We show that monozygotic monochorionic twins discordant for growth provide a useful model to study one type of the epigenetic placental dysregulation that drives IUGR.

Whole Genome Sequencing Expands Diagnostic Utility and Improves Clinical Management in Pediatric Medicine.

The standard of care for first-tier clinical investigation of the etiology of congenital malformations and neurodevelopmental disorders is chromosome microarray analysis (CMA) for copy number variations (CNVs), often followed by gene(s)-specific sequencing searching for smaller insertion-deletions (indels) and single nucleotide variant (SNV) mutations. Whole genome sequencing (WGS) has the potential to capture all classes of genetic variation in one experiment; however, the diagnostic yield for mutation detection of WGS compared to CMA, and other tests, needs to be established. In a prospective study we utilized WGS and comprehensive medical annotation to assess 100 patients referred to a paediatric genetics service and compared the diagnostic yield versus standard genetic testing. WGS identified genetic variants meeting clinical diagnostic criteria in 34% of cases, representing a 4-fold increase in diagnostic rate over CMA (8%) (p-value = 1.42e-05) alone and >2-fold increase in CMA plus targeted gene sequencing (13%) (p-value = 0.0009). WGS identified all rare clinically significant CNVs that were detected by CMA. In 26 patients, WGS revealed indel and missense mutations presenting in a dominant (63%) or a recessive (37%) manner. We found four subjects with mutations in at least two genes associated with distinct genetic disorders, including two cases harboring a pathogenic CNV and SNV. When considering medically actionable secondary findings in addition to primary WGS findings, 38% of patients would benefit from genetic counseling. Clinical implementation of WGS as a primary test will provide a higher diagnostic yield than conventional genetic testing and potentially reduce the time required to reach a genetic diagnosis.

Compound heterozygous mutations in the noncoding RNU4ATAC cause Roifman Syndrome by disrupting minor intron splicing.

Roifman Syndrome is a rare congenital disorder characterized by growth retardation, cognitive delay, spondyloepiphyseal dysplasia and antibody deficiency. Here we utilize whole-genome sequencing of Roifman Syndrome patients to reveal compound heterozygous rare variants that disrupt highly conserved positions of the RNU4ATAC small nuclear RNA gene, a minor spliceosome component that is essential for minor intron splicing. Targeted sequencing confirms allele segregation in six cases from four unrelated families. RNU4ATAC rare variants have been recently reported to cause microcephalic osteodysplastic primordial dwarfism, type I (MOPD1), whose phenotype is distinct from Roifman Syndrome. Strikingly, all six of the Roifman Syndrome cases have one variant that overlaps MOPD1-implicated structural elements, while the other variant overlaps a highly conserved structural element not previously implicated in disease. RNA-seq analysis confirms extensive and specific defects of minor intron splicing. Available allele frequency data suggest that recessive genetic disorders caused by RNU4ATAC rare variants may be more prevalent than previously reported.

Basic concepts of epigenetics.

Several types of epigenetic marks facilitate the complex patterning required for normal human development. These epigenetic marks include DNA methylation at CpG dinucleotides, covalent modifications of histone proteins, and noncoding RNAs (ncRNAs). They function in a highly orchestrated manner, regulating mitotically heritable differences in gene expression potential without altering the primary DNA sequence. In germ cells and the developing embryo, genome-wide epigenetic reprogramming drives the erasure and reestablishment of correct epigenetic patterns at critical developmental time periods and in specific cell types. Two specific types of epigenetic regulation established in early development include X-chromosome inactivation and genomic imprinting; they regulate gene expression in a dosage-dependent and parent-of-origin-specific manner, respectively. Both genetic and environmental factors impact epigenetic marks, generating phenotypic variation that ranges from normal variation to human disease. Aberrant epigenetic patterning can lead to a variety of human disorders, including subfertility and imprinting disorders.

Measuring a toddler's mouthful: toxicologic considerations.

The volume of a toddler's mouthful needs to be defined to estimate poisoning risk. We calculated a toddler's mouthful in 66 healthy children (1.5-4.5 years old) by measuring apple juice remaining in a cup after sipping. Mean volume was 9.3 mL (range, 3.5-29; 95% confidence interval, 8-11), twofold larger than commonly quoted values.