Reproductive history of a healthy woman with mosaic duplication of chromosome 4p

Somatic/gonadal mosaicism for structural autosomal rearrangements: female predominance among carriers of gonadal mosaicism for unbalanced rearrangements

Background

Mosaicism for chromosomal structural rearrangements (Rea) is rare and the timing and mechanisms of mosaic Rea formation, maintenance, and clinical manifestation are poorly understood. To date, there are no published data on the cytogenetic profile of mosaic Reas. The question as to whether the proportion of abnormal cells in the carrier’s cultured blood is clinically significant remains unanswered. A previous study showed a strong female preponderance among carriers of mosaicism for Rea with pericentromeric breaks, indicating female-specific instability in early embryos. However, there is no corresponding study on male to female sex ratio (SR) among carriers of somatic and/or gonadal mosaicism for non-centromeric Rea. Population rates of mosaic Rea carriers calculated from consecutive series of patients referred for various reasons and from prenatal samples have not been established. Therefore the objectives of the present study were several-fold: (1) a study on profiles of Rea involved, (2) comparative analysis of the proportion of cells with unbalanced Rea in blood cultures from asymptomatic and affected carriers, (3) comparative analysis of SR in carriers of mosaicism for balanced and unbalanced Rea, and (4) determination of the population frequency of mosaicism for autosomal Rea.

Results

One hundred and three cases of mosaicism for autosomal non-centromeric Rea (N/Rea; normal line/structural rearrangement) in which the sex of the carrier had been specified were identified in the literature. Among balanced Rea, there was a prevalence of reciprocal translocations (89 %) over inversions (11 %). Among unbalanced Rea, deletions were the most frequent (40 %), followed by duplications (25 %) and rings (16 %). Derivatives and other chromosome abnormalities were less frequent (9 and 10 %). Eight of eleven (73 %) affected carriers of unbalanced Rea displayed a high proportion (>50 %) of abnormal cells compared to 4/37 (11 %) in asymptomatic carriers, p < 0.0001. Among carriers of mosaicism for balanced Rea there was a slight male predominance, 24 M/22 F, unlike the strong female predominance among carriers of mosaicism for unbalanced Rea, 11 M/46 F, p < 0.0001. Among ten carriers of unbalanced Rea with reproductive failure, only one was a male with infertility, and one was a partner of a woman experiencing recurrent spontaneous abortion. Population rates of mosaics for reciprocal translocaton (N/rcp), inversion (N/inv), and unbalanced Rea (N/unbal Rea) calculated from published data on consecutive series of patients with reproductive failures were 0.02 ‰, 0.005 ‰, and 0.002 ‰, correspondingly. Among 30,376 infertile patients three carriers of mosaicism for balanced Rea were identified (two cases of N/rcp and one case of N/inv), whereas among 26,384 patients with habitual abortion seven carriers were detected (five N/rcp and two N/inv). Among all 56,760 tested patients with reproductive failures only one was found to be a carrier of mosaicism for an unbalanced Rea (N/del, mosaicism for deletion).

Conclusions

A high proportion of Rea cells (>50 %) detected in cultured T-lymphocytes is associated with clinical manifestation of chromosomal imbalance. A strong female prevalence among carriers of mosaicism for unbalanced Rea suggests male-specific selection against abnormal cells rather than impairment of male gametogenesis, as the latter suggests a better prognosis for male fetuses. These findings should be taken into consideration when genetic counseling of patients referred after a diagnosis of mosaicism for an unbalanced rearrangement in a fetus.

Clinical characterization of int22h1/int22h2-mediated Xq28 duplication/deletion: new cases and literature review

Background

Int22h1/int22h2-mediated Xq28 duplication syndrome is caused by ~0.5 Mb chromosomal duplications mediated by nonallelic homologous recombination between intron 22 homologous region 1 (int22h1) and 2 (int22h2), which, in addition to int22h3, are also responsible for inversions disrupting the F8 gene in hemophilia A. This syndrome has recently been described in 9 males with cognitive impairment, behavioral problems, and distinctive facial features; and 6 females with milder phenotypes. The reciprocal deletion was previously reported in a mother and daughter. It was suggested that this deletion may not have phenotypic effects in females because of skewed chromosome X inactivation, but may be embryonic lethal in males.

Methods

Array comparative genomic hybridization analyses were performed using oligonucleotide-based chromosomal microarray. Chromosome X inactivation studies were performed at the AR (androgen receptor) and FMR1 (fragile X mental retardation 1) loci.

Results

We present here 5 males and 6 females with int22h1/int22h2-mediated Xq28 duplication syndrome. The males manifested cognitive impairment, behavioral problems, and distinctive facial features. Two of the six females manifested mild cognitive impairment. This duplication was maternally inherited, and skewed chromosome X inactivation was observed in the majority of females carrying the duplication. We also report the reciprocal deletion in a mother and daughter with overweight, but normal cognition. In addition, we present the first case of a prenatally diagnosed de novo int22h1/int22h2-mediated deletion in a healthy female infant. We reviewed individuals previously reported with similar or overlapping rearrangements and evaluated the potential roles of genes in the rearrangement region.

Conclusions

The similarity of clinical features among individuals with the int22h1/int22h2-mediated Xq28 duplication supports the notion that this duplication causes a recognizable syndrome that affects males with females exhibiting milder phenotypes. It is suggested that the observed cognitive impairment in this syndrome results from increased dosage of RAB39B gene located within the duplicated region. Increased dosage of CLIC2 may also contribute to the phenotype. The reciprocal deletion results in skewed chromosome X inactivation and no clinical phenotype in females. Review of overlapping deletions suggests that hemizygous loss of VBP1 may be the cause for the proposed male lethality associated with this deletion.

Structural genetic variation in the context of somatic mosaicism

Somatic mosaicism is the result of postzygotic de novo mutation occurring in a portion of the cells making up an organism. Structural genetic variation is a very heterogeneous group of changes, in terms of numerous types of aberrations that are included in this category, involvement of many mechanisms behind the generation of structural variants, and because structural variation can encompass genomic regions highly variable in size. Structural variation rapidly evolved as the dominating type of changes behind human genetic diversity, and the importance of this variation in biology and medicine is continuously increasing. In this review, we combine the evidence of structural variation in the context of somatic cells. We discuss the normal and disease-related somatic structural variation. We review the recent advances in the field of monozygotic twins and other models that have been studied for somatic mutations, including other vertebrates. We also discuss chromosomal mosaicism in a few prime examples of disease genes that contributed to understanding of the importance of somatic heterogeneity. We further highlight challenges and opportunities related to this field, including methodological and practical aspects of detection of somatic mosaicism. The literature devoted to interindividual variation versus papers reporting on somatic variation suggests that the latter is understudied and underestimated. It is important to increase our awareness about somatic mosaicism, in particular, related to structural variation. We believe that further research of somatic mosaicism will prove beneficial for better understanding of common sporadic disorders.

2q31.2q32.3 deletion syndrome: report of an adult patient

A 36-year-old patient with a disorder characterized by severe mental retardation, behavioral problems, dysmorphic face, "muscular build," and hand/foot anomalies, is reported. Following a diagnosis of de novo pericentric inversion of chromosome 8 based on standard cytogenetic analysis, a subsequent 75 kb array-CGH investigation disclosed a deletion spanning for about 13.7 Mb in the 2q31.2q32.3 region. Whole painting of chromosome 8 established the intrachromosomal nature of the rearrangement and FISH analysis with locus-specific probes confirmed the deletion on the long arm of chromosome 2. The deleted region, clinical outcome, and medical history in this patient are mainly superimposable to those reported in a published 8-year-old boy, suggesting that this genomic segment is prone to rearrangements and its hemizygosity gives rise to a clinically recognizable syndrome. The role of some genes mapping in the deleted region and related with distinct disorders is discussed. Interestingly, deletion of MSTN gene, a negative regulator of muscle growth, was associated in our patient with a "muscular build," a feature which could be regarded as a handle for clinical recognition of this syndrome.

Two mosaic terminal inverted duplications arising post-zygotically: Evidence for possible formation of neo-telomeres

Objective

To elucidate the structure of terminal inverted duplications and to investigate potential mechanisms of formation in two cases where there was mosaicism with cells of apparently normal karyotype.

Results

A karyotype [46,XY,inv dup(4)(p16.3p15.1)/46,XY] performed on blood lymphocytes from a patient referred for developmental delay (case 1) demonstrated a normal karyotype in 60% of cells with a terminal inverted duplication 4p in the remainder. In case 2, referred for multiple fetal anomalies on an ultrasound scan, 33% of amniocyte colonies were karyotypically normal, with a terminal inv dup 10p in the remainder [46,XX,inv dup(10)(p15.3p11)/46,XX]. Duplicated FISH signals for GATA3 and NEBL loci (in case 2), and for the Wolf-Hirschhorn locus (case 1) confirmed the inverted structure of both duplications. In the GTL banded normal cells from both cases, there was a cryptic deletion detected by FISH of one copy of the subtelomere 4p (case 1, probe GS-36P21), and subtelomere 10p (case 2, probe GS-306F7). At pter on both inv dup chromosomes there was no FISH signal present for the specific subtelomere probe. However, a positive pantelomeric probe signal was detected at 4 pter and 10 pter in both the cryptically-deleted chromosomes and the inv dup chromosomes in the respective cell lines of both cases.

Conclusion

An inv dup structure was evident for both cases on GTL bands, and confirmed by the various FISH studies. The presence of telomere (TTAGGG repeat) sequences at pter on the inv dup chromosomes (where more proximal chromosome specific subtelomeric probes were negative) was indicated by the pantelomeric probe signals in both cases. We conclude the most likely mechanism of origin in both cases was by sub-telomeric breakage in the zygote at pter, and delayed repair/rearrangement until after one or more subsequent mitotic divisions. In these divisions, at least one breakage-fusion-bridge cycle occurred, to produce inverted duplications. It is proposed then that two differently "repaired" daughter cells proliferated in parallel. In one daughter cell line (with an overtly normal karyotype) there was deletion of the subtelomere and presumed repair through capping by a neo-telomere (i.e. "healing", as initially proposed by McClintock). This occurred in both cases presented. In the other daughter cell of each case, it is proposed that chromosome stabilization was achieved (after replication) by sister chromatid reunion to form a dicentric, which broke at a subsequent anaphase, to form an inverted duplication (with loss of the reciprocal product, and the other daughter cell line). One inv dup was repaired without an interstitial specific subtelomere (case 1) and one was repaired with a duplicated specific interstitial subtelomere (case 2). After repair TTAGGG repeats were detected by FISH at each respective new pter.

Duplication 18q21.31-q22.2

We report on three siblings with mild mental retardation and minor dysmorphic features carrying an interstitial duplication of the long arm of chromosome 18 inherited from a healthy mosaic carrier mother. The duplicated region spanned between 18q21.31 and 18q22.2 for about 12 Mb. The distal duplications of 18q are rare and only a small number of subjects, manifesting quite different clinical outcomes, have been described. However, in most of these cases, molecular characterization was not available. We have reviewed nine patients, including three familial cases, displaying overlapping duplicated regions, and compared them with the present individuals in an attempt to delineate karyotype-phenotype correlation.

The first 4p euchromatic variant in a healthy carrier having an unusual reproductive history

We report on the molecular cytogenetics studies in a healthy couple who had had three pregnancies which ended in a termination of pregnancy (TOP). In two of them, prenatal sonogram showed fetal dwarfism and in the third one, a chromosome alteration was found in the amniocentesis. A previous pregnancy ended in a healthy girl. A high-resolution G-band karyotype (550-850 bands), together with Fluorescence in situ Hybridization (FISH) techniques, detected in the father a 4p interstitial euchromatic duplication. This chromosome duplication appears to be a previously undescribed euchromatic variant (EV). We discuss the possibility that the 4p paternal EV could be involved in the clinical and genetic findings of the three TOPs.