PB0677 - Low level mosaicism in a boy with mild haemophilia A excludes his mother as a carrier.

Background: Haemophilia A is an X-linked bleeding disorder caused by variants in F8. Due to the possibility of mosaicism, mothers of sporadic cases of haemophilia cannot be excluded as carriers, even if the causative variant is absent in their somatic cells. This can lead to difficult conversations with these mothers explaining that while not confirmed as a carrier, the possibility of her having a second affected son cannot be excluded.

Aims: The definitive resolution of carrier status.

Methods: A 2 year old boy with sporadic mild haemophilia A (FVIII 15 IU/dL) was referred for analysis of F8 to identify the causative variant. His mother was subsequently analysed for the F8 variant identified in her son. Analysis was performed by PCR amplification and Sanger sequencing of the 26 exons and immediate flanking regions of F8. An allele specific assay was designed to allow the amplification of either the wild-type or variant sequence.

Results: The affected child was initially shown to be hemizygous for a c.1733_1738dupTAGATC in exon 11 of F8. This duplication predicts the in-frame insertion of two amino acids in to the Factor VIII protein, p.(Asp579_Gln580insLeuAsp). Analysis of the mother showed her to have only the normal sequence at this position. Review of the sequence data for the affected child showed the presence of a low level of normal sequence under the variant sequence. An allele specific assay demonstrated that both wild-type and variant sequences were present, confirming mosaicism in the child.

Conclusion(s): The demonstration of low level mosaicism in the affected child allowed the conclusion that the pathogenic variant had arisen post fertilisation and had not been inherited from the maternal germ cell. This in turn allowed the mother to be informed that she did not carry the variant and that subsequent pregnancies would be unaffected by this variant.