Monozygotic Twins with Discordant Sex

Sex Ratios Provide Evidence for Monozygotic Twinning in the Ring-Tailed Lemur,Lemur catta

Monozygotic (MZ) twinning is generally considered to be rare in species other than human. We inspected sex ratios in European zoo-bred ring-tailed lemurs (Lemur catta), revealing a significant excess of same-sex twins. Of 94 pairs, 60 (64%) were either both males or both females (p= .004). Application of the Weinberg differential rule argues that 27% of all twins in this species are MZ pairs. In this protected species, where twinning is commonplace (~50% of newborns are twins), the probable existence of frequent MZ twinning has ramifications for breeding programs aimed to maximize genetic diversity, and suggests that twin studies in a species other than human could have potential as a medical research tool.

Monozygotic twins discordant for trisomy 18

We report on the pre- and postnatal cytogenetic, molecular genetic and clinical findings in monochorionic-diamniotic twins discordant for trisomy 18. Structural anomalies were identified in one of the twins on prenatal ultrasound examination at 20 weeks' gestation and sampling of amniotic fluid from both sacs was performed for karyotyping. This revealed trisomy 18 in the twin with abnormalities and a normal karyotype in the other twin. Elective Cesarean section was performed at 31 + 5 weeks and the aneuploid twin died shortly after delivery. The surviving twin showed low-grade mosaicism for trisomy 18 on postnatal analysis but has shown normal development. For prenatal diagnosis in monochorionic-diamniotic twin pregnancy the sampling of both amniotic sacs is recommended, especially if one twin has structural anomalies on ultrasound scan.

Identical but not the same: the value of discordant monozygotic twins in genetic research

Monozygotic (MZ) twins show remarkable resemblance in many aspects of behavior, health, and disease. Until recently, MZ twins were usually called "genetically identical"; however, evidence for genetic and epigenetic differences within rare MZ twin pairs has accumulated. Here, we summarize the literature on MZ twins discordant for Mendelian inherited disorders and chromosomal abnormalities. A systematic literature search for English articles on discordant MZ twin pairs was performed in Web of Science and PubMed. A total number of 2,016 publications were retrieved and reviewed and 439 reports were retained. Discordant MZ twin pairs are informative in respect to variability of phenotypic expression, pathogenetic mechanisms, epigenetics, and post-zygotic mutagenesis and may serve as a model for research on genetic defects. The analysis of single discordant MZ twin pairs may represent an elegant approach to identify genes in inherited disorders.

Difficult diagnosis and management of an heterokaryotypic monochorionic twin pregnancy with discordant fetal sex and 45,X/47,XYY karyotypes

We report twins for whom ultrasound examinations revealed a Turner syndrome in the female fetus and a normal male fetus. A selective pregnancy termination was decided on the female fetus with hydrops. The death of both twins called in question the chorionic diagnosis. Amniotic fluid cytogenetic analysis revealed a 45,X karyotype in the female twin and a 47,XYY karyotype in the male twin. Molecular cytogenetic analysis on genital and renal cells showed different levels of 45,X/47,XYY mosaicism in both twins; molecular analysis on the amniocytes showed monozygosity. Monozygotic twins with discordant sex are very rare. This study showed the difficult diagnosis and management of a monochorionic twin pregnancy with discordant fetal sex.

Monozygotic twins concordant for blood karyotype, but phenotypically discordant: A case of “mosaic chimerism”

We report on 23 years old discordant monozygotic (MZ) twins, one with minor anomalies and mental delay, the other one being normal. Both had 46,XX,dup(11)(p12p15)/46,XX mosaicism in blood, with a similar proportion of abnormal cells (respectively, 16% and 17%). However, interphase fluorescence in situ hybridization (FISH) analysis performed on buccal smear and urinary sediment using specific probes located at the duplicated region showed that mosaicism was only present in the abnormal twin, with 68% abnormal cells. We hypothesize that the postzygotic chromosomal rearrangement may have occurred early in one embryo after the twinning event, and the blood mosaicism observed in both twins would have resulted from blood exchanges via placental anastomoses. This hypothesis of chimerism is strongly supported by twin-to-twin transfusion syndrome observed during fetal life of our twins. This case and those previously reported lead us to suggest that blood is particularly unsuitable for cytogenetic investigations of twins.

Growth of heterokaryotic monozygotic twins discordant for Ullrich-Turner syndrome during the first years of life

The rare observation of different karyotypes in monozygotic (MZ) twins, i.e., heterokaryotic monozygosity, occurs due to chromosomal aberration in one of the twins after separation of the embryos. We report on the differences of heterokaryotic MZ Turkish twins who are discordant for Ullrich-Turner syndrome. Chromosomal analyses from peripheral lymphocytes revealed a 45,X/46,XX mosaicism in both twins. FISH analyses of buccal smears showed 99% of nuclei 45,X in twin A and 98% of nuclei 46,XX in twin B. These results are consistent with a non-mosaic 45,X and 46,XX karyotype, respectively. The girls showed a different growth pattern in the first years. As their genotype should be identical except for the number of X chromosomes, the difference in phenotype may be a pure result of loss of one X chromosome in the affected girl. Special interest is set on the spontaneous and growth hormone induced growth of the twins.

Mechanisms for differences in monozygous twins

Monozygous (MZ) twins are often described as being physically and genetically identical. Clinical determination of zygosity relies on the assumption that any physical differences between a pair of twins imply they are dizygous. Most twin research relies on the assumption that dizygous twins share approximately 50% of the same genes, whereas monozygous twins share 100%. There is, however, increasing evidence to challenge both these assumptions. In this review, we describe a number of intrauterine effects and genetic mechanisms that may result in phenotypic, genotypic, and epigenetic differences between monozygous twins. Newer molecular techniques are resulting in such differences being increasingly commonly recognised. The potential for differences in monozygotic twin pairs is an important consideration for both clinicians and researchers involved in twin work.

Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex

The presence of a monozygotic twin gestation with discordant sex of the twins is a very rare constellation, which is referred to as heterokaryotypic monozygotic pregnancy. This constellation can develop either due to a chromosomal aberration after twinning or is - as in the following case - due to a mitotic error before twinning and an unequal distribution of mosaicism in both embryos. So far the diagnosis of heterokaryotypic monozygotic pregnancy has always been made postnatally, with only one exception (Gonsoulin et al., 1990). In this case we suspected the presence of monozygotic twins ultrasonically because of the chorionic and amniotic membrane characteristics. Surprisingly the sex of the fetuses was discrepant. As one of them had hydrops and a structural heart defect, we carried out an amniocentesis, which revealed mosaicism [45,X/46,X,i(Y)(p10)] of both fetuses. The female fetus with a predominant 45,X set of chromosomes and the typical intrauterine signs of the Ullrich-Turner syndrome (massive hygroma colli, hydrops fetalis and multiple cardiac defects) died during the 25th week of gestation due to cardiac decompensation. The other fetus appeared to be male with a predominance of a 46,X,i(Y)(p10) set of chromosomes and was born a few days after the intrauterine death of the hydropic fetus. In conclusion, our observation shows that ultrasonic evidence of discordant fetal sex in twins does not necessarily exclude monozygosity.

Discordant monozygotic twins with the Schimmelpenning-Feuerstein-Mims syndrome

The Schimmelpenning-Feuerstein-Mims syndrome (SFM), characterized by linear nevus sebaceous and ocular and neurologic abnormalities, is a sporadic condition without known familial cases or etiology. We report the occurrence of SFM in only one of two monozygotic (MZ) twins. After considering a variety of possible causative mechanisms, we suggest that a postzygotic dominant lethal mutation in mosaic form may best explain SFM and the discordancy for SFM in these MZ twins.

Some causes of genotypic and phenotypic discordance in monozygotic twin pairs

The use of the adjective "identical" rather than monozygotic leads to misunderstandings about the biology of monozygotic twinning. Most monozygotic twin pairs are not identical; there may be major discordance for birth weight, genetic disease, and congenital anomalies. These indicate that postzygotic events may lead to the formation of two or more cell clones in the inner cell mass and early embryo that actually stimulate the monozygotic twinning event. There is also evidence that there may be unequal allocation of numbers of cells to the monozygotic twins; this may have widespread implications for the cascade of developmental events during embryogenesis, formation, and vascularization of the placenta. Large-scale zygosity testing at birth could be the template for analysis of twin outcomes and their biologic causes.

Infant with mos45,x/46,XY/47,XYY/48,XYYY: genetic and clinical findings

We describe an infant with mos45,X/46,XY/47,XYY/48,XYYY who presented with ambiguous genitalia. Her phenotype was also remarkable for minor ear and eye anomalies and coarctation of the aorta with bicuspid aortic valve. Laparoscopy revealed bilateral Fallopian tubes and a left infantile testis with epididymis. Chromosomal analyses of blood, skin, aorta, right Fallopian tube, and left gonadal tissue showed mos45,X/46,XY/47,XYY/48,XYYY. The 46,XY cell line was identified with routine trypsin-Giemsa banding only in cultured cells from an aortic biopsy. Fluorescence in-situ hybridization (FISH) was utilized to identify the presence of 46,XY cells in other tissues. The clinical manifestations of this patient are discussed and compared with those of similar cases of Y chromosome aneuploidy. To our knowledge, this is the first report of a patient with this unusual karyotype.