Blood, 1 June 2001, Vol. 97, No. 11, pp. 3675-3676
CORRESPONDENCE
To the editor:
Screening for c-mpl mutations in patients with
congenital amegakaryocytic thrombocytopenia identifies a polymorphism
Congenital amegakaryocytic thrombocytopenia (CAMT) is an
uncommon disorder, characterized by an isolated thrombocytopenia and
the almost complete absence of megakaryocytes in the bone marrow.
Several studies have indicated that the origin of CAMT is an intrinsic
stem cell defect.1-3 Recently, we and others have
demonstrated the presence of mutations in the thrombopoietin-receptor gene, c-mpl, as a possible cause of CAMT.4-7
Although some mutations directly predict loss of Mpl function, it has
not been established that others, notably those that lead to an amino
acid substitution, also directly predict this loss.
To exclude that the mutations we found in our patients represent
non-disease-related polymorphisms, we screened 50 healthy donors (100 alleles) for the presence of the different mutations by
either sequence analysis or allele-specific restriction
analysis.4 None of the healthy donors were carriers of our
reported CAMT-associated mutations. In one new CAMT patient, 3 heterozygous mutations were observed: a G-to-C substitution at
nucleotide 305 in exon 3, predicting an arginine-to-proline
substitution at codon 102; a G-to-A transition at position 340, also in
exon 3, leading to valine-to-methionine replacement at codon 114 (Mpl-114V/M); and a G-to-C substitution in the fifth nucleotide of
intron 3, which leads to loss of the splice site 3' of exon 3. Screening of 50 healthy donors revealed that 4 were heterozygous
carriers of the G340A mutation. The other mutations were not observed
in this population. The c-mpl-340A gene thus seems to have a
frequency of 0.04 in a white Dutch population. Functional
studies should reveal whether this Mpl-114V/M polymorphism influences
the function of Mpl.
Recently, Ballmaier et al7 reported
c-mpl mutations in another series of patients with
CAMT. One of their patients was a homozygous carrier for 2 different point mutations. One mutation predicted a stopcodon in exon
3. The second mutation was the G340A mutation, which we also found in
healthy donors. Therefore, we propose that the first mutation plays a
role in the development of CAMT in this patient. The G340A may not be
involved in CAMT, and its presence in 2 CAMT patients may be incidental.
In conclusion, mutations that predict amino-acid substitutions found by
genetic screening of patients with CAMT can be due to polymorphisms of
the c-mpl gene. The relation of such mutations to
disease should be proven by functional studies with the mutated protein.
Sonja van den Oudenrijn, Masja de Haas, and Albert E. G. Kr. von dem
Borne
Correspondence: Albert E.G.Kr. von dem Borne, Department of
Hematology, Academic Medical Centre, Meibergdreef 9, Amsterdam, 1105 AZ, The Netherlands
References
1.
Muraoka K, Ishii E, Tsuji K, et al.
Defective response to thrombopoietin and impaired expression of c-mpl mRNA of bone marrow cells in congenital amegakaryocytic thrombocytopenia.
Br J Haematol.
1997;96:287-292[CrossRef][Medline]
[Order article via Infotrieve].
2.
Freedman MH, Estrov Z.
Congenital amegakaryocytic thrombocytopenia: an intrinsic hematopoietic stem cell defect.
Am J Pedriatr Hematol Oncol.
1990;12:225-230[Medline]
[Order article via Infotrieve].
3.
Scarlett JD, Williams NT, McKellar WJ.
Acquired amegakaryocytic thrombocytopaenia in a child.
J Paediatr Child Health.
1992;28:263-266[Medline]
[Order article via Infotrieve].
4.
van den Oudenrijn S, Bruin M, Folman CC, et al.
Mutations in the thrombopoietin receptor, Mpl, in children with congenital amegakaryocytic thrombocytopenia.
Br J Haematol.
2000;110:441-448[CrossRef][Medline]
[Order article via Infotrieve].
5.
Ihara K, Ishii E, Eguchi M, et al.
Identification of mutations in the c-mpl gene in congenital amegakaryocytic thrombocytopenia.
Proc Natl Acad Sci U S A.
1999;96:3132-3136[Abstract/Free Full Text].
6.
Tonelli R, Scardovi AL, Pession A, et al.
Compound heterozygosity for two different amino-acid substitution mutations in the thrombopoietin receptor (c-mpl gene) in congenital amegakaryocytic thrombocytopenia (CAMT).
Hum Genet.
2000;107:225-233[CrossRef][Medline]
[Order article via Infotrieve].
7.
Ballmaier M, Germeshausen M, Schulze H, et al.
c-mpl mutations are the cause of congenital amegakaryocytic thrombocytopenia.
Blood.
2001;97:139-146[Abstract/Free Full Text].
Response:
Screening for c-mpl mutations in patients with
congenital amegakaryocytic thrombocytopenia identifies a polymorphism
We agree with the hypothesis of van den Oudenrijn et al that a
mutation with a frequency of 0.04 cannot be the cause for the rare
disease congenital amegakaryocytic thrombocytopenia (CAMT). However, it
remains to be investigated whether the detected polymorphism has an
effect on the function of the c-Mpl protein and therefore can influence
megakaryopoiesis and platelet formation in homozygous carriers. The
presence of this mutation in healthy donors alone is not sufficient to
exclude a correlation with diseases of megakaryopoiesis: all parents of
patients with CAMT who bear c-mpl mutations in one allele
have a normal hematopoiesis and are "healthy" donors.
As we found a homozygous nonsense mutation (C268T) upstream of the
mutation G340A, we also were convinced that this C268T mutation was the
cause for CAMT in this patient.1 Accordingly, we
subsequently discussed this patient along with other patients bearing
nonsense or frameshift mutations in the c-mpl gene and therefore are predicted to have a complete loss of c-Mpl function. As
the patient CAMT-6 in our manuscript is homozygous for both mutations,
which were inherited from her unrelated heterozygous parents, most
likely the C268T mutation arose on the genetic background of the rare
G340A polymorphism. This hypothesis can be tested easily with linkage
analyses using polymorphic and informative markers.
Interestingly, the letter of van den Oudenrijn et al points to another
fact concerning the prevalence of c-mpl mutations. Another
patient is presented here bearing the previously reported (by Oudenrijn
et al and us) G305C mutation predicted to lead to an
arginine-to-proline substitution at amino acid position 102. This
confirms our proposition that the mutation seems to occur more
frequently in patients with CAMT, at least in Western Europe. In the
combined group of 5 patients described by Oudenrijn et al;2 the 5 of our patients, who were from Western Europe;
and the newly described patient in the above letter, the G305C mutation was found homo- or heterozygously in 4 different patients,
corresponding to a frequency of 0.35 in Western European patients with
CAMT. An initial screening for this mutation in newly diagnosed
patients with CAMT therefore might be helpful. Thus far, besides the
potential G340A polymorphism and the nonsense mutation A43X, which
could be detected in 2 of our patients, the G304C mutation is the most frequent mutation described in patients with CAMT.
Matthias Ballmaier, Manuela Germeshausen, and Karl Welte
Correspondence: Matthias Ballmaier, Abt Padiatrische Hamatologie
und Onkologie, Medizinische Hochschule Hannover, Carl-Neuberg-Strasse
1, Hannover, 30625, Germany
References
1.
Ballmaier M, Germeshausen M, Schulze H, et al.
c-mpl mutations are the cause of congenital amegakaryocytic thrombocytopenia.
Blood.
2001;97:139-146.
2.
van den Oudenrijn S, Bruin M, Folman CC, et al.
Mutations in the thrombopoietin receptor, Mpl, in children with congenital amegakaryocytic thrombocytopenia.
Br J Haematol.
2000;110:441-448.
