Familial deletion of (8)(q24.13q24.22) associated with a normal phenotype

Maternal germline factors associated with aneuploid pregnancy loss: a systematic review

BACKGROUND

Miscarriage describes the spontaneous loss of pregnancy before the threshold of viability; the vast majority occur before 12 weeks of gestation. Miscarriage affects one in four couples and is the most common complication of pregnancy. Chromosomal abnormalities of the embryo are identified in ∼50% of first trimester miscarriages; aneuploidy accounts for 86% of these cases. The majority of trisomic miscarriages are of maternal origin with errors occurring during meiotic division of the oocytes. Chromosome segregation errors in oocytes may be sporadic events secondary to advancing maternal age; however, there is increasing evidence to suggest possible maternal germline contributions to this.

OBJECTIVE AND RATIONALE

The objective of this review was to appraise critically the existing evidence relating to maternal germline factors associated with pregnancy loss secondary to embryo aneuploidy, identify limitations in the current evidence base and establish areas requiring further research.

SEARCH METHODS

The initial literature search was performed in September 2019 and updated in January 2021 using the electronic databases OVID MEDLINE, EMBASE and the Cochrane Library. No time or language restrictions were applied to the searches and only primary research was included. Participants were women who had suffered pregnancy loss secondary to numerical chromosomal abnormalities of the embryo. Study identification and subsequent data extraction were performed by two authors independently. The Newcastle-Ottawa Scale was used to judge the quality of the included studies. The results were synthesized narratively.

OUTCOMES

The literature search identified 2198 titles once duplicates were removed, of which 21 were eligible for inclusion in this systematic review. They reported on maternal germline factors having variable degrees of association with pregnancy loss of aneuploid origin. The Online Mendelian Inheritance in Man (OMIM) gene ontology database was used as a reference to establish the functional role currently attributed to the genes reported. The majority of the cases reported and included were secondary to the inheritance of maternal structural factors such as Robertsonian translocations, deletions and insertions. Germline factors with a plausible role in aneuploid pregnancy loss of maternal origin included skewed X-inactivation and CGG repeats in the fragile X mental retardation (FMR1) gene. Studies that reported the association of single gene mutations with aneuploid pregnancy loss were conflicting. Single gene mutations with an uncertain or no role in aneuploid pregnancy loss included mutations in synaptonemal complex protein 3 (SYCP3), mitotic polo-like kinase 4 (PLK4) and meiotic stromal antigen 3 (STAG3) spindle integrity variants and 5,10-methylenetetrahydrofolate reductase (MTHFR).

WIDER IMPLICATIONS

Identifying maternal genetic factors associated with an increased risk of aneuploidy will expand our understanding of cell division, non-disjunction and miscarriage secondary to embryo aneuploidy. The candidate germline factors identified may be incorporated in a screening panel for women suffering miscarriage of aneuploidy aetiology to facilitate counselling for subsequent pregnancies.

An 11.4-Mb Interstitial Deletion in a Fetus with No Apparent Phenotypic Alterations

A prenatal case of a de novo interstitial deletion distal to 8q24 was reported. Ultrasound examination and postmortem evaluation demonstrated no apparent phenotypic alterations. Array CGH showed an 11.4-Mb loss in chromosome 8 ranging from 8q24.13 to 8q24.23. This case partially overlaps the 2 cases previously described in the literature.

Homozygous deletions of a copy number change detected by array CGH: a new cause for mental retardation?

We describe two unrelated patients with mental retardation and normal karyotypes found to have relatively large homozygous deletions (>150 kb) of different regions detected by array comparative genomic hybridization (aCGH). Patient 1 showed a 157-214 kb deletion at 8q24.2, containing BAC clone RP11-17M8. This patient was born to phenotypically normal parents and has microcephaly, distinctive craniofacial features, brachymetacarpia, brachymetatarsia and severe mental retardation. This BAC clone is listed as a copy number variant on the Database of Genomic Variants (http://projects.tcag.ca/variation/). Heterozygosity for the deletion was found in the mother (father is deceased) and uniparental disomy of chromosome 8 was excluded. Patient 2 showed a 812-902 kb deletion at 12q21.1, containing BAC clone RP11-89P15. This region was not listed in any public database as a known variant. This patient has mild craniofacial dysmorphic features, bifid uvula, peripheral pulmonic stenosis and developmental delay. Heterozygosity for this deletion was confirmed in the phenotypically normal parents and two normal siblings, but surprisingly, homozygosity for the deletion in an apparently normal younger sibling brings into question whether this large homozygous copy number change (CNC) is causal. Homozygous deletions of CNCs have not previously been reported in association with a phenotype or mental retardation. These cases represent homozygosity for presumably benign CNCs, and while causality for the phenotypes cannot be confirmed, similar deletions are bound to be identified more frequently as aCGH is used with increasing regularity. Such homozygous deletions should be viewed as potentially clinically relevant.

Chromosome abnormalities without phenotypic consequences

Some changes in chromosome morphology, detected during cytogenetic analysis, are not associated with clinical defects. Therefore a proper discrimination of harmless variants from true abnormalities, especially during prenatal diagnosis, is crucial to allow precise counseling. In this review we described chromosome variants and examples of chromosome anomalies that are considered to be unrelated to phenotypic consequences. The correlation between the presence of marker chromosomes and a risk of clinical signs is also discussed. Structural rearrangements of heterochromatic material, satellite polymorphism, or fragile sites, are well-known examples of common chromosome variation. However, the absence of clinical effects has also been reported in some cases of chromosome abnormalities concerning euchromatin. Such euchromatic anomalies were divided into 2 categories: unbalanced chromosome abnormalities (UBCAs), such as deletions or duplications, and euchromatic variants (EVs). Recently so-called molecular karyotyping, especially whole-genome screening by the use of high-resolution array-CGH technique, contributed to revealing a high number of previously unknown small genomic variations, which seem to be asymptomatic, as they are present in phenotypically normal individuals.

Interstitial deletion of 6q without phenotypic effect

Cytogenetically detectable euchromatic deletions without phenotypic consequences are rarely encountered. We report on a 34-year-old woman with normal intelligence referred for karyotyping because of recurrent abortions. With the exception of a bicuspid aortic valve without hemodynamic consequences, which is a common minor anomaly in the general population, no dysmorphic features were found on physical examination. Conventional chromosome analysis (GTG-banding) revealed an interstitial deletion in the long arm of chromosome 6. With array comparative genomic hybridization (array-CGH) the size of the deletion was estimated to be between 9.9 and 11.6 Mb and the refined karyotype was 46,XX,del(6)(q22.31q23.1).

Directly transmitted unbalanced chromosome abnormalities and euchromatic variants

In total, 200 families were reviewed with directly transmitted, cytogenetically visible unbalanced chromosome abnormalities (UBCAs) or euchromatic variants (EVs). Both the 130 UBCA and 70 EV families were divided into three groups depending on the presence or absence of an abnormal phenotype in parents and offspring. No detectable phenotypic effect was evident in 23/130 (18%) UBCA families ascertained mostly through prenatal diagnosis (group 1). In 30/130 (23%) families, the affected proband had the same UBCA as other phenotypically normal family members (group 2). In the remaining 77/130 (59%) families, UBCAs had consistently mild consequences (group 3). In the 70 families with established EVs of 8p23.1, 9p12, 9q12, 15q11.2, and 16p11.2, no phenotypic effect was apparent in 38/70 (54%). The same EV was found in affected probands and phenotypically normal family members in 30/70 families (43%) (group 2), and an EV co-segregated with mild phenotypic anomalies in only 2/70 (3%) families (group 3). Recent evidence indicates that EVs involve copy number variation of common paralogous gene and pseudogene sequences that are polymorphic in the normal population and only become visible at the cytogenetic level when copy number is high. The average size of the deletions and duplications in all three groups of UBCAs was close to 10 Mb, and these UBCAs and EVs form the "Chromosome Anomaly Collection" at http://www.ngrl.org.uk/Wessex/collection. The continuum of severity associated with UBCAs and the variability of the genome at the sub-cytogenetic level make further close collaboration between medical and laboratory staff essential to distinguish clinically silent variation from pathogenic rearrangement.

Segmental haplosufficiency: transmitted deletions of 2p12 include a pancreatic regeneration gene cluster and have no apparent phenotypic consequences

Segmental aneuploidy usually has phenotypic consequences but unbalanced rearrangements without phenotypic consequences have also been reported. In particular, harmless deletions of G-dark bands 5p14 and 16q21 have each been found in more than one independent family. Here, we report two families that were ascertained at prenatal diagnosis and had similar overlapping deletions that removed most of the gene poor G-dark band 2p12. PCR mapping showed that the deletions had a minimum size of 6.1 and 6.9 Mb with at least 13 hemizygous loci including a cluster of six pancreatic islet-regenerating genes. These deletions had no apparent phenotypic consequences in eight family members. In contrast, a third family was ascertained through a child with Wilm's tumour; both the child and his mother had more proximal deletions, developmental delay and some dysmorphic features. The deletion had a minimum size of 5.7 Mb and extended into the gene-rich area of 2p11.2. These results are consistent with the idea that there may be segments of the genome that are consistently haplosufficient. The introduction of higher resolution methods of dosage analysis into diagnostic laboratories is already revealing more transmitted abnormalities of uncertain significance. As a result, published cases of transmitted imbalances have been collected as a guide to the possible significance of such findings in the future (see the 'Chromosome Anomaly Collection' at www.som.soton.ac.uk/research/geneticsdiv).