Year: 2005
Type: article
Abstract: Haemophilia B is a recessive X-linked disorder with an incidence of 1:30 000 male births. Affected homozygous females are extremely rare, but have been reported (Shetty et al, 2001). Carrier females are usually asymptomatic, but in rare cases can show phenotypic expression. In the majority of female haemophilia B cases, disease manifestation has been ascribed to skewed X-inactivation, with the affected factor IX allele on the active X. In some cases the underlying cause of the skewed X-inactivation has been elucidated, for example, cytogenetically visible aberrations of the X chromosome, but in other cases both X chromosomes appear cytogenetically normal. In another X-linked condition, Duchenne muscular dystrophy, isodisomy for an affected allele has been shown (Quan et al, 1997). We have investigated a young female patient who was presented at the age 14 months with a spontaneous haematoma in the thigh. Her only previous history had been a tendency to bruise, and there was no known family history of a bleeding disorder. Factor studies showed a factor IX level of <1% (normal range 55–163%). Mutation analysis revealed only the c.791C > A p.Thr264Asn missense mutation, which was also present in her mother who had a normal factor IX level (nomenclature as recommended by http://www.hgvs.org/mutnomen, reference sequence Genbank accession number NM_000133.2 using the A of the ATG initiation codon as nucleotide +1). The mother had a second child, a boy who also had a factor IX level of <1% and also carried the c.791C > Ap.Thr264Asn sequence variation. X-inactivation studies were undertaken for the child as a possible explanation for the disease phenotype. The family was uninformative at the commonly used HUMARA locus. As a potential alternative for X-inactivation studies, the FRAXA site was investigated. The patient was found to have a single allele, the size of which corresponded to 21 CGG repeats, which she shared with her mother who had two alleles of 21 and 30 repeats, but the patient did not display the paternal allele of 25 repeat units. A similar situation was observed at the FRAXE locus, with the patient displaying one of the two maternal alleles, but not the paternal allele. The possibility of a deletion of part of the paternal X chromosome was investigated. As the patient was heterozygous for the factor IX mutation, no deletion had occurred here (Xq27.1-q27.2). Probes were generated to four sites on the X chromosome between the factor IX gene and the FRAXA site to determine by Southern blot whether a deletion had occurred. None of the probes (at Xq27, Xq27.2, Xq27.3) or the probe at the FRAXA site indicated a deletion in the patient. Microsatellite analysis was undertaken to further investigate the origin of the X chromosomes. The patient exhibited both maternal and paternal alleles at microsatellites at Xq11.2–13 and Xq21.1. The presence of paternal DNA at these loci as well as at the factor IX gene (shown by heterozygous presence of the normal gene sequence) ruled out maternal isodisomy of the entire X chromosome (i.e. two copies of the same X chromosome from the mother). An explanation that is consistent with the data is that the patient has segmental isodisomy for the X chromosome, having inherited one complete X chromsome from her mother (harbouring a factor IX mutation), while the other ‘paternal’ X chromosome actually consists of maternal X chromsome from Xq27.3 down (Fig 1A). (A) Schematic showing the origin of X chromosomes in a female haemophilia B patient. (B) X inactivation studies at the GRIA3 locus. After HpaII digestion of the patient DNA, it can be seen that the paternal allele (A) is amplifiable, and therefore methylated and inactive. The maternal allele (B) carrying the factor IX mutation has negligible amplification, indicating it is unmethylated and therefore active. The second maternal allele (C) is also shown. The patient would in this case still have a normal paternal factor IX sequence, so to account for the disease phenotype,X-inactivation studies were again pursued using the GRIA3 locus (at Xq25–26) (Gecz et al, 1999) (see Fig 1B). It can be seen that the modified paternal chromosome with a maternal qter is completely inactivated, suggesting that the modification has rendered the chromosome non-viable. This has allowed complete expression of the maternal X harbouring the factor IX mutation and hence the haemophilia phenotype. This is the first report of segmental isodisomy of the X chromosome.
Source: British Journal of Haematology
Authors: L. N. Sellner, P. J. Price
Institution: Princess Margaret Hospital for Children
Language: English
Subfield: Hematology
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W2012242799