Partial tetrasomy 18 mosaicism: Pre and postnatal diagnosis of a unique pattern

ObjectiveTo present prenatal diagnosis and cytogenetic characterization of a unique pattern of partial tetrasomy 18 mosaicism.Case reportA 34-year-old woman underwent amniocentesis at 25 weeks of gestation due to anomalies detected in obstetric ultrasound. It revealed a de novo supernumerary partial isochromosome 18 in 11 of 37 metaphases of cultured amniocytes. The karyotype was 47,XX,+idic(18) (q12.3)[11]/46,XX[26]. Elective cesarean section was performed at 33 weeks of gestational age due to anhydramnios. A female symmetric small for gestational age baby with dysmorphic features and an Apgar score of 9/10/10 was born. She had a good clinical outcome during hospitalization. Postnatal peripheral blood karyotype was normal. Interphase fluorescence in situ hybridization in a sample of the oral mucosa confirmed the prenatal diagnosis. At three months of corrected age she had a normal psychomotor development.ConclusionTo the best of our knowledge, this is the first reported case of mosaic partial tetrasomy 18 including segments of the long arm. This newborn's relatively mild phenotype highlights the challenges of prenatal genetic counselling in mosaic cases with fetal anomalies.     

The embryo battle against adverse genomes: Are de novo terminal deletions the rescue of unfavorable zygotic imbalances?

De novo distal deletions are structural variants considered to be already present in the zygote. However, investigations especially in the prenatal setting have documented that they are often in mosaic with cell lines in which the same deleted chromosome shows different types of aberrations such as: 1) neutral copy variants with loss of heterozygosity that replace the deleted region with equivalent portions of the homologous chromosome and create distal uniparental disomy (UPD); 2) derivative chromosomes where the deleted one ends with the distal region of another chromosome or has the shape of a ring; 3) U-type mirror dicentric or inv-dup del rearrangements. Unstable dicentrics had already been entailed as causative of terminal deletions even when no trace of the reciprocal inv-dup del had been detected. To clarify the mechanism of origin of distal deletions, we examined PubMed using as keywords: complex/mosaic chromosomal deletions, distal UPD, U-type dicentrics, inv-dup del chromosomes, excluding the recurrent inv-dup del(8p)s which are known to originate by NAHR at the maternal meiosis. The literature has shown that U-type dicentrics leading to nearly complete trisomy and therefore incompatible with zygotic survival underlie many types of de novo unbalanced rearrangements, including terminal deletions. In the early embryo, the position of the postzygotic breaks of the dicentric, the different ways of acquiring telomeres by the broken portions and the selection of the most favorable cell lines in the different tissues determine the prevalence of one or the other rearrangement. Multiple lines with simple terminal deletions, inv-dup dels, unbalanced translocations and segmental UPDs can coexist in various mosaic combinations although it is rare to identify them all in the blood.Regarding the origin of the dicentric, among the 30 cases of non-recurrent inv-dup del with sufficient genotyping information, paternal origin was markedly prevalent with consistently identical polymorphisms within the duplication region, regardless of parental origin. The non-random parental origin made any postzygotic origin unlikely and suggested the occurrence of these dicentrics mainly in spermatogenesis.This study strengthens the evidence that non-recurrent de novo structural rearrangements are often secondary to the rescue of a zygotic genome incompatible with embryo survival.     

A multistep approach to the genotype-phenotype analysis of Polish patients with tuberous sclerosis complex

The aim of this study was to evaluate a cost-effective diagnostic strategy for identification of casual variants for tuberous sclerosis complex (TSC) in the Polish population and to correlate the genetic results with selected phenotypic features.Fifty-five patients, aged 3–44 years, with a clinical diagnosis of TSC were enrolled into the study. All patients received a three-step analysis: next generation sequencing screening (NGS), multiplex ligation-dependent probe amplification (MLPA) and deep sequencing.This multistep approach obtained positive results in 51/55 (93%) patients: of the 51 positives TSC1 variants were observed in 16 (31%) and TSC2 variants in 35 (69%); these included 13 novel variants and two patients with mosaicism. Four patients (7%) had no mutation identified (NMI). Among the TSC1 gene variants, there were five nonsense, four frameshift, three large deletions, two missense and two splicing variants. For the TSC2 gene, 11 were missense, eight splicing, six frameshift, four large deletions, two in-frame deletions and four nonsense variants. The patients with TSC2 changes had their clinical diagnosis of TSC at a younger age than those with TSC1 changes (one year vs three years, p = 0.041). The TSC2 group demonstrated a higher number of major symptoms per patient (p = 0.04). Subependymal giant cell astrocytoma with concomitance of other brain lesions was more common in patients with missense mutations in either gene (23% vs 0%, p = 0.02).Such a multistep molecular diagnostic strategy could increase the possibility of detecting causal variants for TSC and may allow detection of mosaicism at low levels. Missense pathogenic variants in TSC1 or TSC2 gene might be associated with a higher risk of brain lesions.     

Genetics of common malformations

Advanced technology has recently allowed us to study rare Mendelian disorders in an unprecedented manner. The same technology should allow us also to study more common malformations. Many of these are not caused by a variant in a single Mendelian gene but by interplay between series of genetic variants and exogenous influences. Likely the site from which the DNA is derived is of great importance in studying malformations as mosaicism may be much more common than earlier anticipated. Factors other than simple variants in our genomic DNA should be considered in the studies as well. Not only is recognition of someone's liability to disease important, but also determining exogenous factors involved in malformations should receive more attention as it may allow us decrease the burden of malformations in humans.     

Skewed X-Chromosome Inactivation in Scleroderma

Scleroderma is a female-prevalent autoimmune disease of unclear etiology. Two fundamental gender differences, skewed X-chromosome inactivation (XCI) and pregnancy-related microchimerism, have been implicated in scleroderma. We investigated the XCI patterns of female scleroderma patients and the parental origin of the inactive X chromosome in those patients having skewed XCI patterns (>80%). In addition, we investigated whether a correlation exists between XCI patterns and microchimerism in a well-characterized cohort. About 195 female scleroderma patients and 160 female controls were analyzed for the androgen receptor locus to assess XCI patterns in the DNA extracted from peripheral blood cells. Skewed XCI was observed in 67 (44.9%) of 149 informative patients and in 10 of 124 healthy controls (8.0%) [odds ratio (OR) = 9.3 (95% confidence interval (CI) 4.3-20.6, P < 0.0001)]. Extremely skewed XCI (>90%) was present in 44 of 149 patients (29.5%) but only in 3 of 124 controls (2.4%; OR = 16.9; 95% CI 4.8-70.4, P < 0.0001). Parental origin of the inactive X chromosome was investigated for ten patients for whom maternal DNA was informative, and the inactive X chromosome was of maternal origin in eight patients and of paternal origin in two patients. Skewed XCI mosaicism could be considered as an important risk factor in scleroderma.     

Mosaic mutations of the<Emphasis Type="Italic"> FLN1</Emphasis> gene cause a mild phenotype in patients with periventricular heterotopia

X-linked periventricular nodular heterotopia (PNH) (OMIM 300049) is a neuronal migration disorder, associated with mutations of the FLN1 gene (Xq28), accompanied by severe epilepsy and normal to mildly impaired cognitive function in affected women. The recurrence risk has been estimated to be 50% in daughters of affected women, with early post-natal lethality in boys. Mutation analysis [denaturing high-performance liquid chromatography (DHPLC) and sequencing], performed in a woman and a man with PNH, was suggestive of somatic mosaicism. Both patients were investigated using single nucleotide primer extension (SNuPE) and DHPLC. To better characterize mosaicism in the affected man, SNuPE-DHPLC analysis was also performed on a pool of hair roots and single hair roots. The affected woman had features of PNH on magnetic resonance imaging. She had well-controlled epilepsy and normal cognitive function. She was mosaic for a nucleotide insertion (c.568_569ingG). SNuPE-DHPLC findings showed 17% of mutant allele. The affected man had classical PNH and was mosaic for an A>G substitution (intron 11 acceptor splice site). SNuPE-DHPLC on both leukocyte and hair root DNA revealed 42% and 69% of mutant allele. Single hair root analysis confirmed that this patient did not harbor the mutation in all ectodermal derivative cells. His daughter had not inherited the mutation. Phenotypic heterogeneity associated with X-linked PNH may depend on the type of mutation, its location on the protein, as well as on somatic mosaicism. Mosaicism can influence the recurrence risk rates in affected women. Mosaic mutations in men may not be transmitted to their daughters, masking the X-linked nature of the disorder.     

Radiological features in Trisomy 8

Radiological study may prompt the correct diagnosis of Trisomy 8 mosaicism when the clinical features are mild and could be overlooked. Skeletal features characteristic of Trisomy 8 are found in the skull, elbows, spine, pelvis, hands and feet.     

Chromosomal abnormalities in embryos derived from testicular sperm extraction

OBJECTIVE: To compare the rate of chromosome abnormalities in embryos obtained from karyotypically normal patients with nonobstructive azoospermia undergoing testicular sperm extraction (TESE) to those from patients undergoing intracytoplasmic sperm injection (ICSI) with ejaculated sperm. DESIGN: Retrospective analysis. SETTING: IVF centers. PATIENT(S): Male partners had either nonobstructive zoospermia or oligospermia. INTERVENTION(S) : Preimplantation genetic diagnosis. Chromosome enumeration was performed by fluorescence in situ hybridization (FISH). Embryos classified as abnormal were reanalyzed to study mosaicism. MAIN OUTCOME MEASURE(S): Chromosome abnormalities in embryos. RESULT(S): Embryos from ICSI cycles with ejaculated sperm (group 1) were 41.8% normal, 26.2% aneuploid, and 26.5% mosaic. In contrast, the embryos from ICSI cycles with TESE for nonobstructive azoospermia (group 2) were 22% normal, 17% aneuploid, and 53% mosaic. The difference in mosaicism rate between the two groups of embryos was highly significant. CONCLUSION(S): The present study results indicate a high incidence of mosaicism in embryos derived from TESE in men with a severe deficit in spermatogenesis. Sperm derived from TESE for nonobstructive azoospermia may have a higher rate of compromised or immature centrosome structures leading to mosaicism in the embryo.     

45,X/46,XY mosaicism

The clinical findings in ten patients with 45,X/46,XY mosaicism are described. Three girls presented with short stature, delayed sexual development or Turner-like stigmata without signs of virilization. Bilaterally gonadoblastomas were found in two girls, and the gonads in one of these girls also contained mucinous cystadenomas. The remaining seven patients were raised as boys. Three had scrotal hypospadias and mixed gonadal dysgenesis. Three presented as male pseudohermaphrodites with scrotal or penoscrotal hypospadias and bilateral testes. One male was diagnosed in adulthood because of gynecomastia, but had normal male external genitals. The clinical findings illustrate the wide spectrum of phenotypic manifestations of 45,X/46,XY mosaicism, ranging from females with Turner-like phenotypes, phenotypic males and females with mixed gonadal dysgenesis, male pseudohermaphroditism to almost phenotypic normal males.