Severe prothrombin deficiency caused by prothrombin-Edmonton (R-4Q) combined with a previously undetected deletion
A Y K Wong1, J Hewitt, B J Clarke
1Centre for Blood Research and Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC, Canada.
Insights
A male patient with severe bleeding inherited a prothrombin gene mutation from his mother and a deletion from his father. These genetic defects cause his prothrombin deficiency and bleeding disorder.
Area of Science:
- Genetics
- Molecular Biology
- Hematology
Background:
- Investigated a male patient with life-threatening bleeding episodes and significantly low plasma prothrombin activity.
- Family coagulation tests revealed reduced prothrombin levels in the patient compared to his parents.
Observation:
- A novel heterozygous point mutation (g.1755 G > A, prothrombin-Edmonton) was identified in the patient and his mother, causing an Arg-4 to Gln (R-4Q) substitution in the prothrombin propeptide.
- Analysis of the patient's liver sample revealed two mRNA transcripts, differing by the presence or absence of exon 11.
- A deletion (g.10435_10809del) was confirmed in the paternal allele of the prothrombin gene.
Findings:
- The patient possesses a maternally inherited point mutation (R-4Q) and a paternally inherited deletion.
- The R-4Q mutation likely leads to under-carboxylation and impaired propeptide cleavage of prothrombin.
- The deletion results in a truncated prothrombin lacking 50 amino acids, potentially affecting protein folding, secretion, stability, and activity.
Implications:
- The combined effects of the point mutation and deletion explain the patient's severe prothrombin deficiency.
- This study elucidates the molecular basis of a rare inherited bleeding disorder.
- Understanding these genetic defects provides insights into prothrombin function and regulation.
Background:
During infancy, a male patient experienced several life-threatening bleeding episodes. Standard coagulation tests revealed that the patient's plasma prothrombin activity was 8%, while his father's and mother's levels were 74% and 62%, respectively.
Objectives:
A molecular genetic approach was used to determine the molecular basis of prothrombin deficiency within the family.
Patient/Methods:
Prothrombin genomic DNA fragments were amplified by using the polymerase chain reaction (PCR). In addition, liver cDNA fragments were amplified from the patient by using reverse transcription (RT) and PCR. The nucleotide sequences of the DNA fragments were determined.
Results:
A novel, heterozygous point mutation (g.1755 G > A, named prothrombin-Edmonton) was detected in the patient and his mother, resulting in the mutation of Arg-4 in the prothrombin propeptide to Gln (R-4Q). RT-PCR analysis of the patient's liver sample demonstrated the presence of two mRNA transcripts that differed by the presence or absence of exon 11. Real-time PCR analysis on genomic DNA and cDNA confirmed a deletion (g.10435_10809del) in the paternal allele.
Conclusions:
The patient has a maternally-inherited point mutation (R-4Q) and a paternally-inherited deletion. By analogy with the previously reported factor IX San Dimas, the R-4Q mutation probably causes under-carboxylation of prothrombin and poor cleavage of the propeptide in the hepatocyte. The deletion probably results in a polypeptide that lacks 50 amino acids from the protease domain; this is likely to impair folding, secretion, stability and/or activity of the truncated prothrombin. The two mutations combine to give the prothrombin deficiency observed in the patient.
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