RT Journal Article
SR Electronic
T1 Monozygotic twin brothers with the fragile X syndrome: different CGG repeats and different mental capacities
JF Journal of Medical Genetics
JO J Med Genet
FD BMJ Publishing Group Ltd
SP 253
OP 257
DO 10.1136/jmg.36.3.253
VO 36
IS 3
A1 Helderman-van den Enden, A T J M
A1 Maaswinkel-Mooij, P D
A1 Hoogendoorn, E
A1 Willemsen, R
A1 Maat-Kievit, J A
A1 Losekoot, M
A1 Oostra, B A
YR 1999
UL http://jmg.bmj.com/content/36/3/253.abstract
AB Little is known about the mechanism of CGG instability and the time frame of instability early in embryonic development in the fragile X syndrome. Discordant monozygotic twin brothers with the fragile X syndrome could give us insight into the time frame of the instability. We describe monochorionic diamniotic twin brothers with the fragile X syndrome who had different CGG repeats and different mental capacities, whereas the normal mother had a premutation. The more retarded brother had a full mutation in all his cells and no FMR-1 protein expression in lymphocytes, whereas the less retarded brother had 50%/50% mosaicism for a premutation and full mutation and FMR-1 protein expression in 26% of his lymphocytes. The differences in repeat size could have arisen either before or after the time of splitting. The time of splitting in this type of twin is around day 6-7. Given the high percentage of mosaicism, we hypothesise that the instability started before the time of splitting at day 6-7.