Endocrine Abstracts

Introduction: Blood chimerism is the condition characterized by the presence of cells from at least 2 zygote lineages in only the lympho-hematopoietic system. It is very rare in humans and is most commonly seen in monozygotic monochorionic twins through placental anastomoses.

Case report: A 12 years old young girl, one of dichorionic dizygotic twins, had a genetic test performed for paternity issues. CGH array showed 46XY genotype (90%) and another genotype at a low level. She had female external and internal genitalia with no evidence of virilisation. Her puberty staging was breast stage: 3–4, pubic hair stage 3 to 4, axillary hair stage 2, menstrual stage 0. There was no clitoromegaly. Her endocrinology has been normal (pubertal LH:5.3 U/L, FSH:3.0 U/L, oestradiol:130 pmol/l, normal prolactin:261 mIU/L, TSH:1.22 mIU/L, anti-mullerian hormone:31 pmol/L, inhibinB:77.1 pg/ml, low testosterone:<0.5 nmol/L, 17-OH-Progesterone:<1.3 nmol/L) with normal urine steroid profile. Her pelvic ultrasound showed normal anteverted uterus, endometrial stripe 6 mm and normal ovaries. Her investigations, therefore, excluded androgen insensitivity and inborn errors of steroidogenesis. Further tests included a skin biopsy from the patient and blood tests for genetics on her twin brother. The skin biopsy revealed female genotype which was the same as the low level genotype detected in her blood sample. There was very low level of male genotype (same as her blood and majority of brother’s genotype). There was low level of same XX genotype in her brother’s blood sample. At her follow up visit she attained menarche.

Conclusions: The results indicated haematologic chimerism due to twin-twin transfusion. Blood chimerism has been reported before in cases of dizygotic monochorionic twins, but there is only one previously described case of XY-DSD in dizygotic dichorionic twins. When an admixture of cells with 46,XX and 46,XY is detected, it should be determined whether the admixture is present in the entire body or limited to the blood. The risk for tumour formation and for passing Y chromosome to her oocytes is unknown and needs to be monitored.