Genetic counseling in carriers of reciprocal chromosomal translocations involving long arm of chromosome 16

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.

Meiotic and pedigree segregation analyses in carriers of t(4;8)(p16;p23.1) differing in localization of breakpoint positions at 4p subband 4p16.3 and 4p16.1

PURPOSE

The purpose of this study was to compare meiotic segregation in sperm cells from two carriers with t(4;8)(p16;p23.1) reciprocal chromosome translocations (RCTs), differing in localization of the breakpoint positions at the 4p subband-namely, 4p16.3 (carrier 1) and 4p16.1 (carrier 2)-and to compare data of the pedigree analyses performed by direct method.

METHODS

Three-color fluorescent in situ hybridization (FISH) on sperm cells and FISH mapping for the evaluation of the breakpoint positions, data from pedigrees, and direct segregation analysis of the pedigrees were performed.

RESULTS

Similar proportions of normal/balanced and unbalanced sperm cells were found in both carriers. The most common was an alternate type of segregation (about 52 % and about 48 %, respectively). Unbalanced adjacent I and adjacent II karyotypes were found in similar proportions about 15 %. The direct segregation analysis (following Stengel-Rutkowski) of the pedigree of carriers of t(4;8)(p16.1;p23.1) was performed and results were compared with the data of the pedigree segregation analysis obtained earlier through the indirect method. The probability of live-born progeny with unbalanced karyotype for carriers of t(4;8)(p16.1;p23.1) was moderately high at 18.8 %-comparable to the value obtained using the indirect method for the same carriership, which was 12 %. This was, however, markedly lower than the value of 41.2 % obtained through the pedigree segregation indirect analysis estimated for carriers of t(4;8)(p16.3;p23.1), perhaps due to the unique composition of genes present within the 4p16.1-4p 16.3 region.

CONCLUSIONS

Revealed differences in pedigree segregation analysis did not correspond to the very similar profile of meiotic segregation patterns presented by carrier 1 and carrier 2. Most probably, such discordances may be due to differences in embryo survival rates arising from different genetic backgrounds.

Recurrence risks for different pregnancy outcomes and meiotic segregation analysis of spermatozoa in carriers of t(1;11)(p36.22;q12.2)

Cumulative data obtained from two relatively large pedigrees of a unique reciprocal chromosomal translocation (RCT) t(1;11)(p36.22;q12.2) ascertained by three miscarriages (pedigree 1) and the birth of newborn with hydrocephalus and myelomeningocele (pedigree 2) were used to estimate recurrence risks for different pregnancy outcomes. Submicroscopic molecular characterization by fluorescent in situ hybridization (FISH) of RCT break points in representative carriers showed similar rearrangements in both families. Meiotic segregation patterns after sperm analysis by three-color FISH of one male carrier showed all possible outcomes resulting from 2:2 and 3:1 segregations. On the basis of empirical survival data, we suggest that only one form of chromosome imbalance resulting in monosomy 1p36.22→pter with trisomy 11q12.2→qter may be observed in progeny at birth. Segregation analysis of these pedigrees was performed by the indirect method of Stengel-Rutkowski and showed that probability rate for malformed child at birth due to an unbalanced karyotype was 3/48 (6.2±3.5%) after ascertainment correction. The risk for stillbirths/early neonatal deaths was -/48 (<1.1%) and for miscarriages was 17/48 (35.4±6.9%). However, the probability rate for children with a normal phenotype at birth was 28/48 (58.3±7.1%). The results obtained from this study may be used to determine the risks for the various pregnancy outcomes for carriers of t(1;11)(p36.22;q12.2) and can be used for genetic counseling of carriers of this rearrangement.

Genetic counseling in carriers of reciprocal translocations involving two autosomes

One of the main genetic causes involve in the pathogenesis of recurrent abortion is parental chromosomal abnormalities. The central concept in genetic counseling with such families is to estimate the probability of recurrence of unfavorable pregnancy outcomes. The main questions that consultants usually ask are: Why did this happen? What is the risk to be done again?Our cases were two families with repeated miscarriage. The pedigrees were drawn, the chromosomes of couples were studied, and estimation for recurrent risk was done. We tried to answer those two main questions and clear the results for them.Parental chromosome abnormalities were founded after karyotyping with GTG technique at 450 band resolution, revealing 46 chromosomes with balanced translocation of autosomes in one of the partner in both families. Recurrent risk was estimated as "high" for their future pregnancies in each family.Couples in which one partner is the carrier of such balanced translocation have increased risks of infertility, recurrent abortion, and delivery of chromosomally abnormal offspring. Genetic counseling of such couples, therefore, presents a unique challenge and should be considered in dealing with such families.

Reciprocal chromosome translocations involving short arm of chromosome 9 as a risk factor of unfavorable pregnancy outcomes after meiotic malsegregation 2:2

PURPOSE

Genetic counseling of carriers with individual chromosome translocation requires information on how balanced reciprocal chromosome translocations (RCT) will segregate, what possible form of unbalanced embryo/fetus/child can occur, and the survival rates that have been observed in the particular families. We collected new empirical data and evaluated pedigrees of RCT carriers involving 9p in order to improve risk figures.

MATERIAL AND METHODS

Empirical data on 241 pregnancies of 70 carriers were collected from 32 pedigrees of carriers of RCT at risk for a single 9p segment imbalance (RCT9p) from the literature and unpublished data. The probability rates of particular types of pathology have been calculated according to the method of Stengel-Rutkowski and Stene. Cytogenetic interpretation was based on GTG, RBG and FISH techniques.

RESULTS

The probability rate for unbalanced offspring at birth for the whole group of pedigrees was calculated as 17.8+/-3% (33/185) (high risk). Considering the size of the imbalanced segment of 9p, the probability rates for RCT carriers with a breakpoint position at 9p22 at 9p13 and at 9p11.2 were estimated separately, and were found as 21.2+/-4.4% (18/85), 25+/-8.8% (6/24) and 11.8+/-3.7% (9/76), respectively. For unbalanced fetuses at 2nd prenatal diagnosis, we found the risk value as 57.9+/-11.3 % (11/19). The risk value for unkaryotyped stillbirths/early deaths of newborns and miscarriages were 5.4+/-1.7% (10/185) (medium risk) and 13+/-2.8% (rate 24/185) (high risk) respectively.

CONCLUSIONS

Our results showed that the recurrence probability rates are different for particular categories of unfavorable pregnancy outcomes. How much they are dependent on the size of 9p chromosome segments taking part in the imbalance needs further studies based on a larger number of observations.

Further delineation of the phenotype maps for partial trisomy 16q24 and Jacobsen syndrome by a subtle familial translocation t(11;16)(q24.2;q24.1)

We report on two cases of distal monosomy 11q and partial trisomy 16q due to a familial subtle translocation detected by FISH subtelomere screening. Exact breakpoint analyses by FISH with panels of BAC probes demonstrated a 9.3-9.5 megabase partial monosomy of 11q24.2-qter and a 4.9-5.4 megabase partial trisomy of 16q24.1-qter. The index patient displayed craniofacial dysmorphisms, mild mental retardation and postnatal growth retardation, muscular hypotonia, mild periventricular leukodystrophy, patent ductus arteriosus, thrombocytopenia, recurrent infections, inguinal hernia, cryptorchidism, pes equinovarus, and hearing deficiencies. In his mother's cousin who bears the identical unbalanced translocation, mild mental retardation, patent ductus arteriosus, hypogammaglobulinemia, recurrent infections, unilateral kidney hypoplasia, pes equinovarus, and hearing deficiencies were reported. Since only four descriptions of cryptic or subtle partial trisomies 16q have been published to date, our patients contribute greatly to the delineation of the phenotype of this genomic imbalance. In contrast to this, terminal deletions of the long arm of chromosome 11 cause a haploinsufficiency disorder (Jacobsen syndrome) in which karyotype-phenotype correlations are already being established. Here, our findings contribute to the refinement of a phenotype map for several Jacobsen syndrome features including abnormal brain imaging, renal malformations, thrombocytopenia/pancytopenia, inguinal hernia, testicular ectopy, pes equinovarus, and hearing deficiency.

Risk evaluation of carriers with chromosome reciprocal translocation t(7;13)(q34;q13) and concomitant meiotic segregation analyzed by FISH on ejaculated spermatozoa

We performed the segregation analysis of a relatively large pedigree of t(7;13)(q34;q13) carriers together with the sperm karyotype analysis of the one carrier using a tri-color fluorescence in situ hybridization (FISH) method. The risk assessments for unfavorable pregnancy outcomes in a series of 36 pregnancies in eight reciprocal chromosome translocation (RCT) couples of carriers were estimated directly from a pedigree after ascertainment correction. The individual probability rate for unbalanced child was predicted according to Stengel-Rutkowski and co-workers. The unbalanced karyotypes in the form of monosomy 7q34-->qter and trisomy 13q13-->qter were detected among stillborn/early death newborns with holoprosencephaly (HPE), cyclopia and other malformations. Based on clinical description of unkaryotyped stillbirth progeny, it can be assumed that the phenotype distinctions were connected with the unbalanced karyotype from 2:2 segregation (monosomy 7q with trisomy 13q) and 3:1 segregation as interchange trisomy 13 (Patau syndrome). Probability rates for miscarriages, stillbirth/early death were 12.9 +/- 6% (4/31) and 29 +/- 8.2% (9/31), respectively. The results of the meiotic segregation pattern indicated the rate of unbalanced spermatozoa for about 60%, with the unusual high rate (29.4%) of 3:1 segregant (i.e., 13.4% of the tertiary segregation and 16% of the interchange segregation). Adjacent-1 segregation followed with 23.5% and adjacent-2 followed with 7.2% of analyzed spermatozoa. The high rate of unbalanced gametes in comparison to the number of stillborn/early death and miscarriages detected in pedigree suggests a strong selection against unbalanced chromosomal constitutions during fetal development. It corresponds to a very small probability rate (about 0.3%) of viable unbalanced progeny from 3:1 meiotic segregation predicted for maternal carriers. This knowledge can be used in genetic counseling of families with similar RCT ascertained in a different way.