Mosaicism for structural non-centromeric autosomal rearrangements in disease-defined carriers: sex differences in the rearrangements profile and maternal age distributions

Characterization of a rare mosaic unbalanced translocation of t(3;12) in a patient with neurodevelopmental disorders

Background

Unbalanced translocations may be de novo or inherited from one parent carrying the balanced form and are usually present in all cells. Mosaic unbalanced translocations are extremely rare with a highly variable phenotype depending on the tissue distribution and level of mosaicism. Mosaicism for structural chromosomal abnormalities is clinically challenging for diagnosis and counseling due to the limitation of technical platforms and complex mechanisms, respectively. Here we report a case with a tremendously rare maternally-derived mosaic unbalanced translocation of t(3;12), and we illustrate the unreported complicated mechanism using single nucleotide polymorphism (SNP) array, fluorescence in situ hybridization (FISH), and chromosome analyses.

Case presentation

An 18-year-old female with a history of microcephaly, pervasive developmental disorder, intellectual disability, sensory integration disorder, gastroparesis, and hypotonia presented to our genetics clinic. She had negative karyotype by parental report but no other genetic testing performed previously. SNP microarray analysis revealed a complex genotype including 8.4 Mb terminal mosaic duplication on chromosome 3 (3p26.3->3p26.1) with the distal 5.7 Mb involving two parental haplotypes and the proximal 2.7 Mb involving three parental haplotypes, and a 6.1 Mb terminal mosaic deletion on chromosome 12 (12p13.33->12p13.31) with no evidence for a second haplotype. Adjacent to the mosaic deletion is an interstitial mosaic copy-neutral region of homozygosity (1.9 Mb, 12p13.31). The mother of this individual was confirmed by chromosome analysis and FISH that she carries a balanced translocation, t(3;12)(p26.1;p13.31).

Conclusion

Taken together, the proband, when at the stage of a zygote, likely carried the derivative chromosome 12 from this translocation, and a postzygotic mitotic recombination event occurred between the normal paternal chromosome 12 and maternal derivative chromosome 12 to “correct” the partial 3p trisomy and partial deletion of 12p. To the best of our knowledge, it is the first time to report the mechanism utilizing a combined cytogenetic and cytogenomic approach, and we believe it expands our knowledge of mosaic structural chromosomal disorders and provides new insight into clinical management and genetic counseling.

Factors affecting clinical manifestation of chromosomal imbalance in carriers of segmental autosomal mosaicism: differential impact of gender

Mosaicism for unbalanced chromosomal rearrangements segmental mosaicism (SM) is rare, both in patients referred for cytogenetic testing and in prenatal diagnoses. In contrast, in preimplantation embryos SM is a frequent finding and, therefore, is even more challenging. However, there is no consistency among results of published studies on the clinical outcomes of embryos with SM, primarily due to the small number of reported cases. Moreover, there is the problem of predicting the potential for the optimal development of a mosaic embryo to a healthy individual. Therefore, we suggested comparing factors predisposing to favorable and poor prognoses, identified in postnatal and prenatal cohorts of SM carriers, with those obtained from studies on preimplantation embryos. We analyzed 580 published cases of SM including (i) postnatally diagnosed affected carriers, (ii) clinically asymptomatic carriers, (iii) prenatally diagnosed carriers, and (iv) miscarriages. We observed a concordance with preimplantation diagnoses regarding the clinical significance of the extent of mosaicism as well as a predominance of deletions over other types of rearrangements. However, there is no concordance regarding excessive involvement of chromosomes 1, 5, and 9 in unbalanced rearrangements and a preferential involvement of larger chromosomes compared to short ones. Paternal age was not found to be associated with SM in postnatally disease-defined individuals. We have identified maternal age and preferential involvement of chromosome 18 in rearrangements associated with clinical manifestations. Male predominance was found among normal pregnancy outcomes and among disease-defined carriers of rearrangements resulting in a gain of genomic material. Female predominance was found among abnormal pregnancy outcomes, among disease-defined carriers of loss and gain/loss rearrangements, and among transmitting carriers of gonadal SM, both affected and asymptomatic. According to data obtained from “post-embryo” studies, clinical manifestations of chromosomal imbalance are associated with a high proportion of abnormal cells, female gender, the type of rearrangement and involved chromosome(s), and maternal age. We believe these data are instructive in the challenging medical genetic counseling of parents faced with no option other than transfer of an embryo with segmental mosaicism.

Mosaicism for structural non-centromeric autosomal rearrangement in prenatal diagnoses: evidence for sex-specific selection against chromosomal abnormalities

Background

Mosaicism for chromosome rearrangements is common in preimplantation diagnoses, yet is rare in prenatal diagnoses as well as in other groups of patients referred to cytogenetic testing. Consequently, there is a lack of detailed studies on this kind of mosaicism in all groups of patients. Previous reports have identified a deficit of males among asymptomatic carriers of N/unbalanced Rea. Three mechanisms were proposed for explaining this phenomenon, including a high instability in the early female embryonic development, a male-specific selection against abnormal cells in the early embryo development, or a high intrauterine lethality of male carriers. To address these possibilities, we have performed a meta-analysis of male-to-female ratio (sex ratio, SR) in prenatally diagnosed and in spontaneously aborted carriers of mosaic Rea.

Results

One hundred and twenty one prenatally detected cases of normal cell line/autosome rearrangement mosaicism (N/Rea) with known carriers’ sex were identified from the literature. Carriers of N/unbalanced Rea presented with 38 abnormal and 28 normal/apparently normal outcomes while carriers of N/balanced Rea presented with 24 normal and 3 abnormal outcomes. 58% of carriers of N/unbalanced Rea with an abnormal outcome displayed a high proportion (> 50%) of amniocytes with the abnormality compared to 25% of carriers with normal/apparently normal outcome. More female carriers of N/unbalanced Rea were identified with an abnormal outcome (15 M/23F) in contrast to a notable male predominance (18 M/10F) among those with normal outcome. Additionally, among spontaneously aborted carriers of N/unbalanced Rea, there was a strong female predominance (7 M/23F).

Conclusion

Previous reports have identified a deficit of male among asymptomatic carriers of N/unbalanced Rea. The current data suggests a male-specific selection against chromosomal abnormalities.

Electronic supplementary material

The online version of this article (10.1186/s13039-017-0346-0) contains supplementary material, which is available to authorized users.