DMD caused by a large pericentric X chromosome inversion

1. DMD caused by a large pericentric X chromosome inversion

2. DMD • Most frequently occurring muscle disease • X-linked recessive disease • 1 in 3500 males • Severely disabling and life shortening • 2.4mb, 79 exons, encodes 427kDa protein • DMD gene codes for the protein dystrophin, an important structural component within muscle tissue

3. Mutations • Two forms • DMD patients protein virtually absence (<3%) • BMD protein levels 10-40% (normal, truncated protein) • 65%-85% partial deletions, 5-10% of partial duplications, remaining point mutations • MLPA analysis • Point mutation analysis offered by Guys • DMD out of frame, BMD in frame • Hotspots 2-20/44-53

4. Patient Background • Proband was referred with a clinical diagnosis of DMD CK? Immunoblotting loss of DMD gene • Unaffected Sibling • Molecular analysis No del/dup by MLPA No point mutation detected at Guy’s • The mother is pregnant therefore linkage is needed in the event of a male pregnancy.

5. DYS I 2 DYS II 2 5n3 1 STR 44 1 STR 45 1 STR50 1 STR H1 1 DYS I 2 1 DYS II 2 1 5n3 1 2 STR 44 1 2 STR 45 1 2 STR50 1 2 STR H1 1 2 DYS I 1 DYS II 1 5n3 2 STR 44 2 STR 45 2 STR50 2 STR H1 2 DYS I 2 DYS II 2 5n3 1 STR 44 1 STR 45 1 STR50 1 STR H1 1

6. In the meantime…… Mother is seen in in clinical genetics; Pseudohypertrophy of the calves Is she a manifesting carrier of DMD? Full karyotype initiated

7. Mother Proband DMD Xp21 p21.2 q23 “apparently balanced pericentric inversion of one X chromosome, at breakpoints p21.2 and q23” “Chromosome analysis shows a male karyotype with an apparently balanced pericentric inversion of the X chromosome, with breakpoints at p21.2 and q23. This inversion appears to be the same as that seen in his mother. He has therefore inherited this inversion from his mother”.

8. Possible pathogenic mechanisms Gene translocated to a region of repressive chromatin and silenced DMD Breakpoint in the promoter region Breakpoint in an intron leading to a truncated protein (exonic seq intact due to pt mut analysis) RNA studies on muscle biospy ongoing

9. Prenatal • CVS taken 1/9/08: • Female – no further testing requested • Carrier 46,X,inv(X)(p21.2q23)mat • X inactivation studies • Typically skewing -> normal X active: asymptomatic carrier. • Skewing not complete: mild DMD features/Risk of cardiomyopathy • Linkage could be used to back up cytogenetic findings in the event of any future male pregnancy Further risk • Risk of unbalanced offspring 0.8-10% depending on the size of the inversion.

10. Reproductive risk of carrying the inverted X (absence of Recombination):

11. C Crossing over within a Pericentric inversion loop. B C D E B A Normal X Inverted X E D A Normal X Inverted X Risk of having unbalanced offspring due to Recombination A A SHOX DMD gene XIC ? POF B C C B D D E E E Normal X D A A B C B A C B C B D D C D E E E A Normal X Inverted X Rec1: Dup(Xp)&Del(Xq) Rec 2: Dup(Xq)&Del(Xp)

12. Mother Father Outcome Rec 1 Normal X ♀ unlikely to have DMD may have POF. Absence of D & E Rec 1 Normal Y ♂ unlikely to be viable Rec 2 Normal X ♀ may have DMD (other X-linked recessive) =unlikely, Turner features =likely due to loss of SHOX Rec 2 Normal Y ♂ unlikely to be viable Reproductive risk of unbalanced offspring due to Recombination: SHOX DMD gene XIC ? POF Normal X Recombination events loss of genetic material

13. Summary • DMD case that was not detectable molecularly • Interaction between cytogenetic & molecular testing • Inverted X has is a significant risk to unbalanced offspring, reduced fertility and recurrent miscarriage. • Acknowledgements • Chris Campbell • Rebecca Watts