Commitment to X inactivation precedes the twinning event in monochorionic MZ twins.
To gain insight into the timing of twinning, we have examined a closely related event, X-chromosome inactivation, in female MZ twin pairs. X-inactivation patterns in peripheral blood and buccal mucosa were compared between monochorionic MZ (MC-MZ) and dichorionic MZ (DC-MZ) twins. Overall, the MC-MZ twins displayed highly similar X-inactivation patterns, whereas DC-MZ twins frequently differed in their X-inactivation patterns, when both tissues were tested. Previous experimental data suggest that commitment to X inactivation occurs when there are 10-20 cells in the embryo. Simulation of embryo splitting after commitment to X inactivation suggests that MC-MZ twinning occurs three or four rounds of replication after X inactivation, whereas a DC-MZ twinning event occurs earlier, before or around the time of X inactivation. Finally, the overall degree of skewing in the MZ twins was not significantly different from that observed in singletons. This indicates that X inactivation does not play a direct role in the twinning process, and it further suggests that extreme unequal splitting is not a common mechanism of twin formation.
Division of Biology and Human Genetics, Department of Medicine, North Shore University Hospital, Manhasset, NY 11-30, USA.
J
J
Monteiro
C
C
Derom
R
R
Vlietinck
N
N
Kohn
M
M
Lesser
P K
PK
Gregersen
D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D002823:Chorion; D004247:DNA / Q000097:blood / Q000737:chemistry; D005260:Female; D005312:Fetal Blood; D006801:Humans; D007223:Infant; D009061:Mouth Mucosa / Q000166:cytology; D011247:Pregnancy; D015183:Restriction Mapping; D014429:Twins, Dizygotic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics*; D014960:X Chromosome
D004247:DNA