LAUSR

Zygosity is a description that reflects the type of conception, that is, whether a twin pair arise from one or two zygotes. Monozygotic (MZ) twins, developing from a single zygote, are assumed to be genetically identical. Most of the time MZ twins are expected to be concordant for a genetic condition, including chromosomal abnormalities and Mendelian disorders. However, an increasing number of studies have found that MZ twins with discordant clinical phenotypes can have different genomes. Considering that MZ twins have identical genomes at the time of fertilization, the discordant variation must have arisen in the postzygotic stage. In this study, we report on a unique pair of MZ twins with discordant rhabdomyomas identified by prenatal ultrasound, in which a postzygotic TSC2 variant exclusively in the affected twin leads to somatic mosaicism. A twin pregnancy occurred in a 27-year-old multigravida, nulliparous woman who conceived spontaneously. Family history was unremarkable. Sonographic examination disclosed a set of monochorionic, diamniotic (MCDA) pregnancy with normal nuchal translucency in both twins at 12 weeks (Figure 1A). Aneuploidy screening using maternal cell-free DNA testing was negative. The pregnancy was then kept under ultrasound surveillance every 2 weeks at other clinic. The woman was referred to our center due to presentation of cardiac masses in one twin at 22 weeks. Fetal echocardiography demonstrated two intracardiac masses in twin A with one located at the apex of left ventricle and one in the interventricular septum (Figure 1B). The two masses measured 3.7 × 3.6 mm and 6.2 × 4.0 mm, respectively. Both masses showed a hyperechogenic, homogeneous aspect, and the diagnosis of cardiac rhabdomyomas was established. Both cardiac size and function were normal. No associated intracardiac and extracardic anomalies were noted. Fetal anatomy scan and echocardiography were both normal in twin B. The possibility of tuberous sclerosis complex (TSC) in twin A was discussed with the parents. Considering the MCDA pregnancy suggestive of MZ twins, twin B was also likely to have TSC if the diagnosis was confirmed in the co-twin. Therefore, dual amniocentesis for investigation of TSC1/TSC2 was recommended. The microarray showed normal results in both twins. Tuberous sclerosis 2-gene panel (TSC1/TSC2) based on next-generation sequencing (NGS) was used with uncultured fluid cells. A heterozygous missense variant, NM_000548:c.1790A > G (p.H597R) of TSC2 was identified in twin A, with a level of about 15% variant fraction, but was not confirmed by Sanger sequencing due to its low fraction. The p.H597R, a recurrent variant reported previously in patients with TSC, was not present in amniocytes of twin B and blood samples of both parents with the same NGS panel. Unfortunately, on the third day after amniocentesis, the woman reported liquor escaping from her vagina. Per speculum examination showed liquor escaping out through the cervix. The parents had no choice but to terminate the whole pregnancy after expectant in-hospital treatment for 3 days. Placental examination confirmed to be monochorionic and diamniotic. Physical examination of the two male fetuses showed no apparent abnormalities. Autopsy was declined. Fetal skin tissues from both twins were sent for targeted variant (c.1790A > G) testing (ultra-high sensitivity) by NGS (longrange PCR and deep sequencing). The read numbers was 9855× and 6748×, respectively in twin A and twin B. The results showed that twin A carried a mosaic variant with a fraction level of 13.35%, and that twin B was negative. The mosaicism in this affected fetus can be designated as A1B2C1D1E2F2: a somatic mosaicism (A1), according to a new six-attribute classification of genetic mosaicism proposed by Martínez-Glez et al. In addition, short tandem repeat polymorphism marker analysis also confirmed the monozygotic pregnancy. TSC is a neurocutaneous disorder characterized by multisystem hamartomas, most commonly in skin, brain, kidney, lung, and heart, appearing at different ages. Although per se benign and rarely complicating cardiac function, a fetal identification of cardiac rhabdomyomas may be of great clinical relevance. Rhabdomyoma is a well-established prenatal marker for TSC, which may be complicated by varying neurological outcomes. The incidence rate of TSC is 51% to 86% in patients Li Zhen and Jie Guo contributed equally to this study. Received: 27 August 2020 Revised: 2 October 2020 Accepted: 17 October 2020