Congenital amegakaryocytic thrombocytopenia in three siblings: molecular analysis of atypical clinical presentation.
OBJECTIVE
An 11-year-old girl, presenting with fatigue and bruising, was found to be profoundly pancytopenic. Bone marrow exam and clinical evaluation were consistent with aplastic anemia. Family members were studied as potential stem cell donors, revealing that both younger siblings displayed significant thrombocytopenia, whereas both parents had normal blood counts. We evaluated this pedigree to understand the unusually late presentation of congenital amegakaryocytic thrombocytopenia (CAMT).


METHODS
The coding region and the intron/exon junctions of MPL were sequenced from each family member. Vectors representing each of the mutations were constructed and tested for the ability to support growth of Baf3/Mpl(mutant) cells.


RESULTS
All three siblings had elevated thrombopoietin levels. Analysis of genomic DNA demonstrated that each parent had mutations/polymorphisms in a single MPL allele and that each child was a compound heterozygote, having inherited both abnormal alleles. The maternal allele encoded a mutation of the donor splice-junction at the exon-3/intron-3 boundary. A mini-gene construct encoding normal vs mutant versions of the intron-3 donor-site demonstrated that physiologic splicing was significantly reduced in the mutant construct.


CONCLUSIONS
Mutations that incompletely eliminate Mpl expression/function may result in delayed diagnosis of CAMT and confusion with aplastic anemia.
Gandhi
Manish J
MJ
Puget Sound Blood Center, Seattle, WAash., USA; Division of Hematology, University of Washington School of Medicine, Seattle, WA 98104, USA. manishg@psbc.org
Pendergrass
Thomas W
TW
Cummings
Carrie C
CC
Ihara
Kenji
K
Blau
C Anthony
CA
Drachman
Jonathan G
JG
D000483:Alleles; D000741:Anemia, Aplastic / Q000097:blood / Q000151:congenital / Q000235:genetics / Q000473:pathology; D001772:Blood Cell Count; D002460:Cell Line; D002648:Child; D004252:DNA Mutational Analysis / Q000379:methods; D005091:Exons / Q000235:genetics; D005260:Female; D015870:Gene Expression; D005822:Genetic Vectors / Q000235:genetics; D006801:Humans; D007438:Introns / Q000235:genetics; D008533:Megakaryocytes / Q000378:metabolism / Q000473:pathology; D009154:Mutation / Q000235:genetics*; D010375:Pedigree; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D012326:RNA Splicing / Q000235:genetics*; D018121:Receptors, Cytokine / Q000235:genetics*; D053628:Receptors, Thrombopoietin; D035781:Siblings; D013921:Thrombocytopenia / Q000097:blood / Q000151:congenital / Q000235:genetics* / Q000473:pathology; D013926:Thrombopoietin / Q000097:blood
D011518:Proto-Oncogene Proteins; D018121:Receptors, Cytokine; D053628:Receptors, Thrombopoietin; C082218:MPL protein, human; D013926:Thrombopoietin