Fragile X mutation

CGG expansion in 5' regulatory region of FMR1 gene on X chromosome

Xq27.3

Epigenetics

inherited changes in phenotype caused by mechanisms OTHER than changes in underlying DNA sequece

normal CGG repeat number

6-52

polymorphic

full mutation

repeat expansion >230 copies-->methylation/hypermethylation of CpG Island-->gene shut off

all males and half of females with full mutation are mentally retarded

premutation

copy # 50-230

not methylated, therefore FMR1 is transcriptionally active

no phenotypic abnormalities

Martin-Bell syndrome

triad of signs:

mental retardation

long face w large everted ears

macro-orchidism

Full mutation transmitted through male

unaffected sons

daughters have premutation

 Premutation transmitted through mother

daughters 50% normal, 50% premutation or full mutation (expansion when passed by mother)

sons: 50% normal, 50% premutation or full mutation (more likely than daughters to receive FULL mutation)

full mutation transmitted by mother

50% normal, 50% full mutation

premutation transmitted by father

sons: normal

daughters: usually show reduced number of repeats

Sherman paradox

aka DNA instability, dynamic mutations

males do not transmit more than 230 copies, females often transmit more copies than they have

premutations larger than 80 almost always expand to full when passed through mothers

*Var related to premutation size and carrier's gender

molecular diagnosis of Fragile X

Southern blot/PCR

restriction endonucleases (EcoR1, Eag 1, BssHII, PstI, XhoI) and probes (Pfxa3 & StB12.3)

methylation-sensitive restrictino endonucleases cut within CGG island

Southern blot results for Fragile X testing

normal F: 5.2 (inactive X), 2.8, 2.4

normal M: 2.8

premutation F: normal plus 2 additional bands

premutation M: slightly larger than 2.8

full mutation F: smear band in excess of 5.7

full mutation M: band larger than 5.2

therapy for Fragile X patients

none

treat behavioral/learning/OT problems