Lia Knegt

The atypical 16p11.2 deletion: A not so atypical microdeletion syndrome?

One of the recently recognized microdeletion syndromes is the 16p11.2 deletion syndrome (593u2009kb; ∼29.5u2009Mb to ∼30.1u2009Mb), associated with developmental delay, autism spectrum disorder, epilepsy, and obesity. Less frequently reported is a smaller 220u2009kb deletion, adjacent and distal to this 16p11.2 deletion, which has been referred to as the atypical 16p11.2 deletion (220u2009kb; ∼28.74u2009Mb to ∼28.95u2009Mb). We describe three patients with this deletion and update the manifestations in two sibs who have been described as possibly new entity in this Journal in 1997 [Bakker and Hennekam (1997); Am J Med Genet 70:312–314] and were recently found to have the “atypical 16p11.2 deletion” as well. Patients show a developmental delay, behavioral problems, and unusual facial morphology (prominent forehead, downslanted, and narrow palpebral fissures), and some are obese. We suggest that this “atypical” deletion may turn out to become a microdeletion syndrome that will be recognizable in the future, or at least to show a phenotype that is recognizable in retrospect. As it may no longer be so “atypical,” we suggest renaming the entity “distal 16p11.2 deletion,” to distinguish it from the common proximal 16p11.2 deletion.

Congenital thrombocytopenia in a neonate with an interstitial microdeletion of 3q26.2q26.31.

Interstitial deletions encompassing the 3q26.2 region are rare. Only one case‐report was published this far describing a patient with an interstitial deletion of 3q26.2 (involving the MDS1‐EVI1 complex (MECOM)) and congenital thrombocytopenia. In this report we describe a case of a neonate with congenital thrombocytopenia and a constitutional 4.52u2009Mb deletion of 3q26.2q26.31 including TERC and the first 2 exons of MECOM, involving MDS1 but not EVI1. The deletion was demonstrated by array‐CGH on lymphocytes. Our report confirms that congenital thrombocytopenia can be due to a constitutional deletion of 3q26.2 involving MECOM. We suggest that in case of unexplained neonatal thrombocytopenia, with even just slight facial dysmorphism, DNA microarray on peripheral blood should be considered early in the diagnostic work‐up.

Severe fluoropyrimidine toxicity due to novel and rare DPYD missense mutations, deletion and genomic amplification affecting DPD activity and mRNA splicing

Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in the catabolism of 5-fluorouracil (5FU). Genetic variations in DPD have emerged as predictive risk factors for severe fluoropyrimidine toxicity. Here, we report novel and rare genetic variants underlying DPD deficiency in 9 cancer patients presenting with severe fluoropyrimidine-associated toxicity. All patients possessed a strongly reduced DPD activity, ranging from 9 to 53% of controls. Analysis of the DPD gene (DPYD) showed the presence of 21 variable sites including 4 novel and 4 very rare aberrations: 3 missense mutations, 2 splice-site mutations, 1 intronic mutation, a deletion of 21 nucleotides and a genomic amplification of exons 9-12. Two novel/rare variants (c.2843T>C, c.321+1G>A) were present in multiple, unrelated patients. Functional analysis of recombinantly-expressed DPD mutants carrying the p.I948T and p.G284V mutation showed residual DPD activities of 30% and 0.5%, respectively. Analysis of a DPD homology model indicated that the p.I948T and p.G284V mutations may affect electron transfer and the binding of FAD, respectively. cDNA analysis showed that the c.321+1G>A mutation in DPYD leads to skipping of exon 4 immediately upstream of the mutated splice-donor site in the process of DPD pre-mRNA splicing. A lethal toxicity in two DPD patients suggests that fluoropyrimidines combined with other therapies such as radiotherapy might be particularly toxic for DPD deficient patients. Our study advocates a more comprehensive genotyping approach combined with phenotyping strategies for upfront screening for DPD deficiency to ensure the safe administration of fluoropyrimidines.

Complete and partial XYLT1 deletion in a patient with neonatal short limb skeletal dysplasia.

We report on a boy with a neonatal short limb skeletal dysplasia with serious medical complications, associated with one intragenic and one complete deletion of XYLT1. XYLT1 mutations have recently been reported as causative in recessive Desbuquois skeletal dysplasia (DBSD), but the skeletal features in our patient do not fit this diagnosis. It is possible that the phenotype of XYLT1 mutations extends to more aspecific types of short limb skeletal dysplasias and not to DBSD alone.

An inactivating mutation in the histone deacetylase SIRT6 causes human perinatal lethality.

It has been well established that histone and DNA modifications are critical to maintaining the equilibrium between pluripotency and differentiation during early embryogenesis. Mutations in key regulators of DNA methylation have shown that the balance between gene regulation and function is critical during neural development in early years of life. However, there have been no identified cases linking epigenetic regulators to aberrant human development and fetal demise. Here, we demonstrate that a homozygous inactivating mutation in the histone deacetylase SIRT6 results in severe congenital anomalies and perinatal lethality in four affected fetuses. In vitro, the amino acid change at Asp63 to a histidine results in virtually complete loss of H3K9 deacetylase and demyristoylase functions. Functionally, SIRT6 D63H mouse embryonic stem cells (mESCs) fail to repress pluripotent gene expression, direct targets of SIRT6, and exhibit an even more severe phenotype than Sirt6-deficient ESCs when differentiated into embryoid bodies (EBs). When terminally differentiated toward cardiomyocyte lineage, D63H mutant mESCs maintain expression of pluripotent genes and fail to form functional cardiomyocyte foci. Last, human induced pluripotent stem cells (iPSCs) derived from D63H homozygous fetuses fail to differentiate into EBs, functional cardiomyocytes, and neural progenitor cells due to a failure to repress pluripotent genes. Altogether, our study described a germline mutation in SIRT6 as a cause for fetal demise, defining SIRT6 as a key factor in human development and identifying the first mutation in a chromatin factor behind a human syndrome of perinatal lethality.

Genetic Analyses in Small-for-Gestational-Age Newborns

ContextnSmall for gestational age (SGA) can be the result of fetal growth restriction, which is associated with perinatal morbidity and mortality. Mechanisms that control prenatal growth are poorly understood.nnnObjectivenThe aim of the current study was to gain more insight into prenatal growth failure and determine an effective diagnostic approach in SGA newborns. We hypothesized that one or more copy number variations (CNVs) and disturbed methylation and sequence variants may be present in genes associated with fetal growth.nnnDesignnA prospective cohort study of subjects with a low birth weight for gestational age.nnnSettingnThe study was conducted at an academic pediatric research institute.nnnPatientsnA total of 21 SGA newborns with a mean birth weight below the first centile and a control cohort of 24 appropriate-for-gestational-age newborns were studied.nnnInterventionsnArray comparative genomic hybridization, genome-wide methylation studies, and exome sequencing were performed.nnnMain Outcome MeasuresnThe numbers of CNVs, methylation disturbances, and sequence variants.nnnResultsnThe genetic analyses demonstrated three CNVs, one systematically disturbed methylation pattern, and one sequence variant explaining SGA. Additional methylation disturbances and sequence variants were present in 20 patients. In 19 patients, multiple abnormalities were found.nnnConclusionnOur results confirm the influence of a large number of mechanisms explaining dysregulation of fetal growth. We concluded that CNVs, methylation disturbances, and sequence variants all contribute to prenatal growth failure. These genetic workups can be an effective diagnostic approach in SGA newborns.

Widespread domain-like perturbations of DNA methylation in whole blood of Down syndrome neonates

Introduction Down syndrome (DS) is the most frequent genetic cause of intellectual disability. Despite the fact that more than 50 years have passed since the discovery of its genetic aberrations, the exact pathogenesis of the DS phenotype has remained largely unexplained. It was recently hypothesized that the DS pathogenesis involves complex (epi)genetic, molecular and cellular determinants. To date, many reports have addressed epigenetic aberrations associated with DS at different developmental stages/ages and tissue types, but to our best knowledge not in DS newborns. This study aimed to investigate genome-wide methylation patterns in DS newborns compared to non-trisomic newborns. Method We analyzed blood samples obtained from ten newborns with DS and five age-matched non-trisomic newborns. Epigenetic profiles were obtained from extracted DNA using the Illumina Infinium 450K array. Since aberrant blood cell distribution is known to be present in DS, we applied two distinct models: with and without correction for estimated blood cell distribution. Results Differentially methylated position (DMP) analysis of the uncorrected model detected 19525 significant hits (51,2% hypomethylated). In the corrected model, we found 121953 significant DMPs (49,8% hypomethylated). Independent of the used model we observed a chromosome 21 dosage effect. Moreover, we detected 46 and 145 differentially methylated regions in the uncorrected and corrected model respectively, both showing hypomethylation overrepresentation. Replication analyses of DMPs and DMRs found by Bacalini et al. (2015) showed a large overlap. Conclusion In this study, we found methylation profile differences between DS newborns and controls reflecting a systematically affected epigenetic profile. The observed chromosome 21 dosage effect suggests the involvement of affected essential regulatory factors/regions or altered expression of chromatin modeling enzymes located on chromosome 21. Additional research is necessary to substantiate these hypotheses.

Trisomy 4 mosaicism : Delineation of the phenotype

Trisomy 4 mosaicism in liveborns is very rare. We describe a 17‐month‐old girl with trisomy 4 mosaicism. Clinical findings in this patient are compared to previously reported patients. Based on the few descriptions available in the literature the common phenotype of trisomy 4 mosaicism seems to consist of IUGR, low birth weight/length/OFC, congenital heart defects, characteristic thumb anomalies (aplasia/hypoplasia), skin abnormalities (hypo‐/hyperpigmentation), several dysmorphic features, and likely some degree of intellectual disability. When trisomy 4 mosaicism is suspected clinicians should be aware that a normal karyotype in lymphocytes does not exclude mosaicism for trisomy 4. This report contributes to a further delineation of the phenotype associated with trisomy 4 mosaicism.