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M. Germeshausen and M, Ballmaier
of both alleles from independently isolated genomic DNAs in five patients and an unbalanced distribution of both alleles in one patient.Two of these six patients were from families with other members affected by CAMT- MPL: CAMT139 was heterozygously affected by the missense mutation c.1390A>G, homozygously detected in her sister CAMT138. Both sisters had similar clinical and laboratory findings. Both parents were heterozygous carrier of the mutation without any hematological prob- lems. Thrombopoietin plasma levels were high in both sisters but not in the parents. However, in contrast to both parents who demonstrated a balanced distribution of both alleles the wild-type allele in CAMT139 was reproducibly markedly underrepresented (approximatly 20%), arguing for somatic mosaicism. Patient CAMT065 heterozygously harbored the c.127C>T nonsense mutation, which was homozygously found in his cousins CAMT036 and CAMT018. Besides these familial cases we identified four other patients with heterozygous MPL mutations. In one of these patients (CAMT129) we found CD110 expression on early hematopoietic progenitors comparable to that from patients with a predicted complete loss of the receptor (Figure 2). In patient CAMT73 we found a novel non- sense mutation in exon 7 together with a synonymous substitution c.585T>C (p.Pro195=). Although synony- mous mutations can significantly influence protein levels via changes in translation efficiency,31 both codons are nearly equally used in human genes, and the mutation has no predicted effect on splicing.
Discussion
This report summarizes the results of a long term study on the largest cohort of patients with CAMT-MPL caused by biallelic mutations in MPL. We limited our cohort to this group of patients (i) to provide a reliable definition of the clinical picture of CAMT-MPL, (ii) to define the effects of the MPL/THPO system in humans, and (ii) to allow for evi- dence based treatment recommendations.
CAMT has been used in the past to describe an IBMFS with no characteristic malformations presenting as isolated thrombocytopenia at birth progressing to a general bone marrow failure.13,32 However, large differences in the report- ed percentages for MPL mutations, for the development of aplastic anemia and leukemia, and for somatic malforma- tions reveal differences in the definition of this disease.29,33-35 This together with misleading combinations of findings from the pre-molecular era involves the risk of mistreat- ment e.g., HSCT of patients with CAMT due to THPO mutations.
The most severe clinical problems for patients with CAMT-MPL are (i) - so far underestimated - pre- and peri- natal bleedings and the resulting long-term consequences thereof, and (ii) the development of aplastic anemia in the later course of the disease.
Severe bleedings, especially intracranial bleedings, occur mainly pre- or perinatally but much less frequently after the first weeks of life despite partly very low platelet counts. Specific functional deficits in neonatal platelets like a decreased P-Selectin expression and reduced platelet activa- tion and secretion36-38 could be a possible explanation for the high bleeding tendency pre- or perinatally in combination with the thrombocytopenia. Furthermore, both life-span
and thrombin dependent activation of platelet GPIIb/IIIa are markedly reduced in neonatal Mpl-/- mice compared to adult Mpl-/- mice.37 Our results indicate a possible functional impairment of platelets also in human fetuses and new- borns with MPL defect which is in contrast to the assump- tion of a normal function of Mpl-/- platelets.39
Development of aplastic anemia due to exhaustion of three lineage hematopoiesis is a characteristic feature of CAMT-MPL and reveals the essential role of MPL for the maintenance of hematopoietic stem cells:40 almost all patients inevitably develop a fatal bone marrow failure. In our study we observed only one patient with an isolated thrombocytopenia until adulthood. In the literature one fur- ther patient is described with stable thrombocytopenia in the period of record.41
Half of the patients in our cohort exhibit non-hematopoi- etic abnormalities. This is in contrast to the characterization of CAMT as an IBMF with no physical anomalies (OMIM). Most of the non-hematopoietic abnormalities seen in our cohort are related to the brain and the eye. For neurological abnormalities, which have been reported for other CAMT- MPL patients it has been argued, that these could be a direct consequence of the roles of thrombopoietin and MPL in the brain.42-45 However, the high correlation between structural abnormalities in the brain and intracranial bleedings argues for a secondary effect of thrombocytopenia. Indeed, most of these structural abnormalities observed in our cohort have also been reported as a consequence of intracranial bleedings,46-48 even strabismus and nystagmus.49 This is fur- ther supported by the observation that higher incidences of ocular anomalies have also been described for other BMFS going along with thrombocytopenia (Fanconi anemia, den- dritic cells) but not for those with normal platelet counts (Diamond Blackfan anemia, Shwachman Diamond syn- drome).50 Previous reports of other non-hematological abnormalities refer to CAMT patients with unreported or wild-type MPL genotype.14,51 Our data suggests that the pri- mary effects of MPL deficiency are restricted to the hematopoietic system - most of the non-hematopoietic symptoms seem to be secondary to the thrombocytopenia or bone marrow failure. For other symptoms, especially those observed only in single cases or in highly consan- guineous families52 we suppose that they emerged coinci- dentally.
Although CAMT is regarded to be a preleukemic syn- drome in most of the recent reviews, only weak evidence for this assumption exists. One single patient with CAMT and confirmed MPL mutation has been reported to develop a pre-B acute lymphoblastic leukemia.41 Increased accumu- lation of chromosomal aberrations, however, has been observed in our and previous studies.53 The exhaustion of hematopoietic stem cells due to MPL deficiency may be the reason for both, the acquisition of pre-leukemic cellular alterations due to increased hematopoietic stress, but also for early development of aplastic anemia leading to death or replacement of the hematopoietic system by means of HSCT, thereby preventing the development of overt leukemia. The debate about CAMT-MPL as a preleukemic syndrome therefore might be of less relevance.
Genotype-phenotype correlations in CAMT-MPL have led us to our concept of CAMT I and II groups: a complete loss of MPL function results in persistently low platelet counts and a fast progression into pancytopenia in CAMT I patients whereas a residual function of the receptor leads to a milder course with a transient increase of platelet counts
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haematologica | 2021; 106(9)