Sex-discordant monochorionic dizygotic twins: a case report

Gender-Discordant Monochorionic-Diamniotic Twins Both With 45,X/46,X, Idic(Y) Mosaicism and a Novel Deletion Within the TBC1D5 Gene

“…Placental vascular anastomoses can allow blood chimerism through mixing of fetal lymphocytes among twins, leading to pseudomosaicism. 5,18 The proportion of mosaic cells within the peripheral blood may not necessarily reflect the true proportion as seen in other tissues. 18 Thus, the true degree of mosaicism in our case cannot be confirmed due to the confounder of potential pseudomosaicism within blood.…”

“…5,18 The proportion of mosaic cells within the peripheral blood may not necessarily reflect the true proportion as seen in other tissues. 18 Thus, the true degree of mosaicism in our case cannot be confirmed due to the confounder of potential pseudomosaicism within blood. The disadvantage of using peripheral blood is further highlighted by previous studies, which have suggested that the phenotype expressed by 45,X/46,XY patients is not related to the proportion of mosaicism in peripheral blood but rather gonadal tissues.…”

“…19,20 While gonadal tissues have been implied to correlate best with phenotype, skin fibroblasts and buccal tissues have also been utilized for FISH evaluation in other studies without the potential confounder of blood chimerism. 18,20,21 However, obtaining gonadal tissue is not feasible in this instance, especially since the male twin may not require gonadal biopsy for management. The etiology of mosaicism in our case may be explained by 2 factors: the presence of an isodicentric Y chromosome and the possible occurrence of anaphase lag.…”

“…Since placental examination revealed monochorionic diamniotic status in the context of sex discordant babies, further evaluation of the genotype of the babies was undertaken via evaluation of blood lymphocytes for karyotype, fluorescence in situ hybridization (FISH), and microarray studies. Twin A, girl showed a 45,X[2]/46,X,idic(Y)(q11.223pter) [18] karyotype and Twin B, male demonstrated a 45,X[4]/46,X, idic(Y)(q11.223) [16] karyotype (Figure 3(A) and (C)).…”

“…The results suggested an isodicentric Y chromosome fused in mirror image at Yq11.223. This isodicentric Y chromosome included the entire short [18] and a single X with a 2 centromeric Y/SRY signal pattern in 70% of interphase nuclei along with a single X/CEPX signal and no Y in 30% of interphase nuclei, respectively, while male infant (C and D) shows 45,X[4]/46,X,idic (Y)(q11.223) [16] karyotype and a single X with a 2 centromeric Y/SRY signal pattern in 80% of interphase nuclei along with a single X/CEPX signal and no Y in 20% of interphase nuclei, respectively, thereby confirming 45,X/46XY mosaicism in twins. Green and red arrows indicate the CEPX and LSI SRY signals, respectively.…”