Inherited Unbalanced Reciprocal Translocation with 18p11.32p11.21 Tetrasomy and 9q34.3 Trisomy in a Fetus Revealed by Cell-Free Fetal DNA (cffDNA) Testing: Cytogenetic and Cytogenomic Characterization in Prenatal Diagnosis.

BACKGROUND/OBJECTIVE: Balanced reciprocal translocations are structural chromosomal anomalies that involve a mutual exchange of segments between two non-homologous chromosomes with a consequent 50-80% risk of conceiving fetuses with unbalanced chromosomal anomalies. This study describes a 37-year-old woman, at 13 + 5 weeks of gestation, who is a balanced reciprocal translocation 46,XX,t(9;18)(q34;q11.2) carrier, with a high-risk non-invasive prenatal screening test, NIPT, for chromosome 18 aneuploidy.

METHODS

The highlighted aneuploidy was characterized with cytogenetic, FISH and SNP-array techniques.

RESULTS

Cytogenetic analysis, performed on flask-cultured amniocytes, indicated a 48,XX,+2mar karyotype on 50 metaphases. SNP array analysis showed a 15.3 Mb duplication of chromosome 18p (arr[hg19]18p11.32-p11.21(12,842-15,303,932)x4), consistent with a partial tetrasomy 18p, and a 926 kbp duplication of chromosome 9q (arr[GRCh37]9q34.3(140,118,286-141,044,489)x3), consistent with partial trisomy 9q. FISH analysis with a 9q34.3 probe was performed on flask-cultured amniocytes' metaphases, highlighting the presence of a third signal on one of the two marker chromosomes (18p11.32-p11.21).

CONCLUSIONS

The evidence of such partial aneuploidies suggests that different mutational events may be possible at meiotic segregation or probably post-meiotic segregation. The results obtained highlight the high sensitivity of the screening test, NIPT, with massive parallel sequencing, and the usefulness of cytogenetics, cytogenomics and molecular biology techniques, in synergy, to characterize and confirm positive NIPT results.