Mitotic stability of small supernumerary marker chromosomes depends on their shape and telomeres - a long term in vitro study

Small supernumerary marker chromosomes derived from human chromosome 11

Introduction: With only 39 reported cases in the literature, carriers of a small supernumerary marker chromosome (sSMC) derived from chromosome 11 represent an extremely rare cytogenomic condition. Methods: Herein, we present a review of reported sSMC(11), add 18 previously unpublished cases, and closely review eight cases classified as 'centromere-near partial trisomy 11' and a further four suited cases from DECIPHER. Results and discussion: Based on these data, we deduced the borders of the pericentric regions associated with clinical symptoms into a range of 2.63 and 0.96 Mb for chromosome 11 short (p) and long (q) arms, respectively. In addition, the minimal pericentric region of chromosome 11 without triplo-sensitive genes was narrowed to positions 47.68 and 60.52 Mb (GRCh37). Furthermore, there are apparent differences in the presentation of signs and symptoms in carriers of larger sSMCs derived from chromosome 11 when the partial trisomy is derived from different chromosome arms. However, the number of informative sSMC(11) cases remains low, with overlapping presentation between p- and q-arm-imbalances. In addition, uniparental disomy (UPD) of 'normal' chromosome 11 needs to be considered in the evaluation of sSMC(11) carriers, as imprinting may be an influencing factor, although no such cases have been reported. Comprehensively, prenatal sSMC(11) cases remain a diagnostic and prognostic challenge.

The past, present, and future for constitutional ring chromosomes: A report of the international consortium for human ring chromosomes

Human ring chromosomes (RCs) are rare diseases with an estimated newborn incidence of 1/50,000 and an annual occurrence of 2,800 patients globally. Over the past 60 years, banding cytogenetics, fluorescence in situ hybridization (FISH), chromosome microarray analysis (CMA), and whole-genome sequencing (WGS) has been used to detect an RC and further characterize its genomic alterations. Ring syndrome featuring sever growth retardation and variable intellectual disability has been considered as general clinical presentations for all RCs due to the cellular losses from the dynamic mosaicism of RC instability through mitosis. Cytogenomic heterogeneity ranging from simple complete RCs to complex rearranged RCs and variable RC intolerance with different relative frequencies have been observed. Clinical heterogeneity, including chromosome-specific deletion and duplication syndromes, gene-related organ and tissue defects, cancer predisposition to different types of tumors, and reproductive failure, has been reported in the literature. However, the patients with RCs reported in the literature accounted for less than 1% of its occurrence. Current diagnostic practice lacks laboratory standards for analyzing cellular behavior and genomic imbalances of RCs to evaluate the compound effects on patients. Under-representation of clinical cases and lack of comprehensive diagnostic analysis make it a challenge for evidence-based interpretation of clinico-cytogenomic correlations and recommendation of follow-up clinical management. Given recent advancements in genomic technologies and organized efforts by international collaborations and patient advocacy organizations, the prospective of standardized cytogenomic diagnosis and evidence-based clinical management for all patients with RCs could be achieved at an unprecedented global scale.

Double aneuploidy mosaicism involving chromosomes 18 and 21 in a neonate

BACKGROUND

Double aneuploidy is common, especially in products of conception, frequently involving a combination of a sex chromosome and an acrocentric chromosome. Double autosomal trisomies are rare with only five cases reported. Double aneuploidy mosaicism involving two different cell lines is rarer with only three cases reported.

CASE PRESENTATION

We report a fourth case of double aneuploidy mosaicism on a baby. Results of a 24-h preliminary chromosome analysis at birth showed a mosaic karyotype, 47,XX,+18[15]/47,XX,+21[8]/48,XX,+21,+mar[7]. Reflex testing to SNP microarray with the same sample collected at birth showed gain of a 77.9 Mb region on chromosome 18 and gain of a 32.5 Mb region on chromosome 21. Microarray did not show any other copy number variants indicating that the marker chromosome may not contain any euchromatic material. A repeat chromosome analysis at 1-year of age showed a mosaic karyotype, 47,XX,+18[76]/47,XX,+21[4] with loss of the marker cell line.

CONCLUSION

Based on our results, we propose that the mosaic double autosomal trisomy in our case was due to two independent non-disjunction events in a normal zygote very early during embryogenesis.

Divergent Levels of Marker Chromosomes in an hiPSC-Based Model of Psychosis

In the process of generating presumably clonal human induced pluripotent stem cells (hiPSCs) from two carriers of a complex structural rearrangement, each having a psychotic disorder, we also serendipitously generated isogenic non-carrier control hiPSCs, finding that the rearrangement occurs as an extrachromosomal marker (mar) element. All confirmed carrier hiPSCs and differentiated neural progenitor cell lines were found to be mosaic. We caution that mar elements may be difficult to functionally evaluate in hiPSC cultures using currently available methods, as it is difficult to distinguish cells with and without mar elements in live mosaic cultures.

Multiple Small Supernumerary Marker Chromosomes Resulting from Maternal Meiosis I or II Errors

We present 2 cases with multiple de novo supernumerary marker chromosomes (sSMCs), each derived from a different chromosome. In a prenatal case, we found mosaicism for an sSMC(4), sSMC(6), sSMC(9), sSMC(14) and sSMC(22), while a postnatal case had an sSMC(4), sSMC(8) and an sSMC(11). SNP-marker segregation indicated that the sSMC(4) resulted from a maternal meiosis II error in the prenatal case. Segregation of short tandem repeat markers on the sSMC(8) was consistent with a maternal meiosis I error in the postnatal case. In the latter, a boy with developmental/psychomotor delay, autism, hyperactivity, speech delay, and hypotonia, the sSMC(8) was present at the highest frequency in blood. By comparison to other patients with a corresponding duplication, a minimal region of overlap for the phenotype was identified, with CHRNB3 and CHRNA6 as dosage-sensitive candidate genes. These genes encode subunits of nicotinic acetylcholine receptors (nAChRs). We propose that overproduction of these subunits leads to perturbed component stoichiometries with dominant negative effects on the function of nAChRs, as was shown by others in vitro. With the limitation that in each case only one sSMC could be studied, our findings demonstrate that different meiotic errors lead to multiple sSMCs. We relate our findings to age-related aneuploidy in female meiosis and propose that predivision sister-chromatid separation during meiosis I or II, or both, may generate multiple sSMCs.