Unbalanced translocation in an adult patient with premature ovarian failure and mental retardation detected by spectral karyotyping and array-comparative genomic hybridization

Molecular cytogenetic characterization of a de novo derivative chromosome X with an unbalanced t(X;9) translocation in a fetus and literature review

Partial trisomy 9p is one of the most frequent autosome anomalies in newborn infants featured by craniofacial dysmorphism, intellectual disability and psychomotor growth. Female patients carrying monosomy Xq usually show mild symptoms due to skewed X-chromosome inactivation (XCI). Unbalanced translocation between chromosome X and chromosome 9 is rare in prenatal diagnosis. The skewed inactivation of abnormal X would spread into the extra segment of chromosome 9 presented in the der(X) leading to mild phenotypes. We reported on a fetus with high risk of trisomy 9p(13.32 Mb 9p23-p24.3 duplication)suggested by noninvasive prenatal testing (NIPT), the fetus was normal by ultrasonography. G-banding with trypsin-giemsa (GTG), copy number variations sequencing (CNV-seq) and fluorescence in situ hybridization (FISH) were carried out to delineate the nature of rearrangement. Final karyotype of the fetus was identified as 46,X,der(X)t(X;9)(q27;p23)dn. An unbalanced X-autosome translocation with a deletion of Xqter-q27.2 and a duplication of 9pter-p23 led to mild phenotypes with no obvious alteration by prenatal ultrasonography, or obvious pathological alterations after pregnancy termination.

Genomic markers of ovarian reserve

Ovarian reserve and its utilization, over a reproductive life span, are determined by genetic, epigenetic, and environmental factors. The establishment of the primordial follicle pool and the rate of primordial follicle activation have been under intense study to determine genetic factors that affect reproductive lifespan. Much has been learned from transgenic animal models about the developmental origins of the primordial follicle pool and mechanisms that lead to primordial follicle activation, folliculogenesis, and the maturation of a single oocyte with each menstrual cycle. Recent genome-wide association studies on the age of human menopause have identified approximately 20 loci, and shown the importance of factors involved in double-strand break repair and immunology. Studies to date from animal models and humans show that many genes determine ovarian aging, and that there is no single dominant allele yet responsible for depletion of the ovarian reserve. Personalized genomic approaches will need to take into account the high degree of genetic heterogeneity, family pedigree, and functional data of the genes critical at various stages of ovarian development to predict women's reproductive life span.

Investigating the role of X chromosome breakpoints in premature ovarian failure

The importance of the genetic factor in the aetiology of premature ovarian failure (POF) is emphasized by the high percentage of familial cases and X chromosome abnormalities account for 10% of chromosomal aberrations. In this study, we report the detailed analysis of 4 chromosomal abnormalities involving the X chromosome and associated with POF that were detected during a screening of 269 affected women. Conventional and molecular cytogenetics were valuable tools for locating the breakpoint regions and thus the following karyotypes were defined: 46,X,der(X)t(X;19)(p21.1;q13.42)mat, 46,X,t(X;2)(q21.33;q14.3)dn, 46,X,der(X)t(X;Y)(q26.2;q11.223)mat and 46,X,t(X;13)(q13.3;q31)dn. A bioinformatic analysis of the breakpoint regions identified putative candidate genes for ovarian failure near the breakpoint regions on the X chromosome or on autosomes that were involved in the translocation event. HS6ST1, HS6ST2 and MATER genes were identified and their functions and a literature review revealed an interesting connection to the POF phenotype. Moreover, the 19q13.32 locus is associated with the age of onset of the natural menopause. These results support the position effect of the breakpoint on flanking genes, and cytogenetic techniques, in combination with bioinformatic analysis, may help to improve what is known about this puzzling disorder and its diagnostic potential.

Genetic and molecular analysis of a new unbalanced X;18 rearrangement: localization of the diminished ovarian reserve disease locus in the distal Xq POF1 region

BACKGROUND

Diminished ovarian reserve (DOR) is a heterogeneous disorder causing infertility, characterized by a decreased number of oocytes, the genetic cause of which is still unknown.

METHODS AND RESULTS

We describe a family with a new unbalanced X;18 translocation der(X) associated with either fully attenuated or DOR phenotype in the same family. Cytogenetics and array comparative genomic hybridization (aCGH) studies have revealed the same partial Xq monosomy and partial 18q trisomy in both the 32-year-old female with DOR and the unaffected mother. The genetic analysis has defined a subtelomeric deletion spanning 13.3 Mb from Xq27.3 to -Xqter, which covers the premature ovarian failure locus 1 (POF1); and a duplication spanning 13.4 Mb, from 18q22.1 to 18qter. From a parental-origin study, we have inferred that the rearranged X chromosome is maternally derived. The Xq27 and 18q22 breakpoint regions fall in a region extremely rich in long interspersed nuclear element, a class of retrotransposons able to trigger mispairing and unusual crossovers. X-inactivation studies reveal a skewing of der(X) both in the mother and the proband. Therefore, the phenotypic expression of der(X) is fully attenuated in the fertile mother and partially attenuated in the DOR daughter.

CONCLUSIONS

We report on an unbalanced maternally derived translocation (X;18)(q27;q22) with different intra-familial reproductive performances, ranging from fertility to DOR. Skewed X-inactivation seems to restore the unbalanced genetic make-up, fully silencing the 18q22 trisomy and at least in part the Xq27 monosomy. The chromosomal abnormality observed in this family supports the presence of a DOR susceptibility locus in the distal Xq region and targets the POF1 region for further investigation.