Manuela Germeshausen

HAX1 deficiency causes autosomal recessive severe congenital neutropenia (Kostmann disease)

Autosomal recessive severe congenital neutropenia (SCN) constitutes a primary immunodeficiency syndrome associated with increased apoptosis in myeloid cells, yet the underlying genetic defect remains unknown. Using a positional cloning approach and candidate gene evaluation, we identified a recurrent homozygous germline mutation in HAX1 in three pedigrees. After further molecular screening of individuals with SCN, we identified 19 additional affected individuals with homozygous HAX1 mutations, including three belonging to the original pedigree described by Kostmann. HAX1 encodes the mitochondrial protein HAX1, which has been assigned functions in signal transduction and cytoskeletal control. Here, we show that HAX1 is critical for maintaining the inner mitochondrial membrane potential and protecting against apoptosis in myeloid cells. Our findings suggest that HAX1 is a major regulator of myeloid homeostasis and underline the significance of genetic control of apoptosis in neutrophil development.

Clinical implications of ELA2‐, HAX1‐, and G‐CSF‐receptor (CSF3R) mutations in severe congenital neutropenia

Congenital Neutropenia (CN) is a heterogeneous bone marrow failure syndrome characterized by a maturation arrest of myelopoiesis at the level of the promyelocyte/myelocyte stage with peripheral blood absolute neutrophil counts below 0·5 × 109/l. There are two major subtypes of CN as judged by inheritance: an autosomal dominant subtype, e.g. defined by neutrophil elastase mutations (approximately 60% of patients) and an autosomal recessive subtype (approximately 30% of patients), both presenting with the same clinical and morphological phenotype. Different mutations have been described (e.g. HAX1, p14 etc) in autosomal recessive CN, with HAX1 mutations in the majority of these patients. CN in common is considered as a preleukemic syndrome, since the cumulative incidence for leukemia is more than 25% after 20 years of observation. Leukemias occur in both, the autosomal dominant and recessive subtypes of CN. The individual risk for each genetic subtype needs to be further evaluated. Numbers of patients tested for the underlying genetic defect are still limited. Acquired G‐CSFR (CSF3R) mutations are detected in approximately 80% of CN patients who developed acute myeloid leukemia independent of the ELA2 or HAX1 genetic subtype, suggesting that these mutations are involved in leukemogenesis. As the majority of patients benefit from G‐CSF administration, HSCT should be restricted to non‐responders and patients with leukaemic transformation.

Congenital amegakaryocytic thrombocytopenia: a retrospective clinical analysis of 20 patients.

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare bone marrow failure syndrome characterised by isolated thrombocytopenia because of ineffective megakaryocytopoiesis at birth. In the last 10 years, we collected data from 20 patients diagnosed with CAMT based on a severe thrombocytopenia since birth and absent or markedly decreased numbers of megakaryocytes in the bone marrow. Fanconis anaemia and thrombocytopenia absent radii syndrome were ruled out for all patients. We retrospectively compared the clinical courses, laboratory findings and treatment outcome. Development of pancytopenia was observed in 14 of the patients, only one patient presented with an isolated thrombocytopenia over a period of over 14 years. One boy died from bleeding complications. We defined two groups of patients according to the course of platelet counts during the first year of life, which also differed in the course of development of pancytopenia. Physical anomalies in addition to haematopoiesis were found in a number of patients: two children presented with cardiac defects, six with growth abnormalities, and four with retardation of psychomotor development. Fifteen patients were treated with haematopoietic stem cell transplantation, four of whom died of transplantation‐related events.

Congenital amegakaryocytic thrombocytopenia: clinical presentation, diagnosis, and treatment.

Congenital amegakaryocytic thrombocytopenia (CAMT, MIM #604498) is a rare inherited bone marrow failure syndrome presenting as isolated hypomegakaryocytic thrombocytopenia at birth without other characteristic physical anomalies. Most of the patients develop a severe aplastic anemia and trilineage cytopenia during the first years of life and hematopoietic stem cell transplantation is the only curative treatment. In most of the cases the disease is caused by homozygous or compound heterozygous mutations in the gene MPL encoding the receptor for the hematopoietic growth factor thrombopoietin. The present review summarizes clinical and laboratory data for 96 patients with CAMT, reported since 1990.

A high-throughput sequencing test for diagnosing inherited bleeding, thrombotic, and platelet disorders

Inherited bleeding, thrombotic, and platelet disorders (BPDs) are diseases that affect ∼300 individuals per million births. With the exception of hemophilia and von Willebrand disease patients, a molecular analysis for patients with a BPD is often unavailable. Many specialized tests are usually required to reach a putative diagnosis and they are typically performed in a step-wise manner to control costs. This approach causes delays and a conclusive molecular diagnosis is often never reached, which can compromise treatment and impede rapid identification of affected relatives. To address this unmet diagnostic need, we designed a high-throughput sequencing platform targeting 63 genes relevant for BPDs. The platform can call single nucleotide variants, short insertions/deletions, and large copy number variants (though not inversions) which are subjected to automated filtering for diagnostic prioritization, resulting in an average of 5.34 candidate variants per individual. We sequenced 159 and 137 samples, respectively, from cases with and without previously known causal variants. Among the latter group, 61 cases had clinical and laboratory phenotypes indicative of a particular molecular etiology, whereas the remainder had an a priori highly uncertain etiology. All previously detected variants were recapitulated and, when the etiology was suspected but unknown or uncertain, a molecular diagnosis was reached in 56 of 61 and only 8 of 76 cases, respectively. The latter category highlights the need for further research into novel causes of BPDs. The ThromboGenomics platform thus provides an affordable DNA-based test to diagnose patients suspected of having a known inherited BPD.

Advances in the understanding of congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (MIM #604498) is an extremely rare inherited bone marrow failure syndrome, usually presenting as a severe thrombocytopenia at birth due to ineffective megakaryocytopoiesis and no characteristic physical anomalies. Usually the isolated thrombocytopenia progresses to pancytopenia during the first years of life. The only curative therapy to date is haematopoietic stem cell transplantation. Most of the cases of congenital amegakaryocytic thrombocytopenia are caused by defective expression or function of the thrombopoietin receptor due to homozygous or compound heterozygous mutations in the gene MPL. The essential roles of thrombopoietin as a lineage specific regulator of platelet production and as a regulator of haematopoietic stem cell function are reflected in the haematological defects seen in affected individuals.

Severe congenital neutropenia: inheritance and pathophysiology.

Purpose of reviewSevere congenital neutropenia is a heterogeneous disorder of hematopoiesis characterized by a maturation arrest of granulopoiesis at the level of promyelocytes with peripheral blood absolute neutrophil counts below 0.5 × 109/l. In this review we summarize our current knowledge on inheritance and pathophysiolgy of congenital neutropenia. Recent findingsThere are two major subtypes of congenital neutropenia as judged by inheritance: autosomal dominant trait defined by neutrophil elastase mutations consisting of 60% of patients and autosomal recessive trait comprising approximately 30% of patients. This genetic heterogeneity suggests that several pathologic mechanisms may lead to the same phenotype due to downregulation of common myeloid transcription factors. Lymphoid enhancer-binding factor 1 is the most promising candidate, as its abrogation together with downregulation of lymphoid enhancer-binding factor 1 target genes is compatible with this phenotype. Congenital neutropenia is considered as a preleukemic syndrome, since after 10 years of observation the cumulative incidence for leukemia is 21%. Acquired granulocyte colony-stimulating factor receptor mutations are detected in approximately 80% of congenital neutropenia patients who developed acute myeloid leukemia. SummaryCongenital neutropenia is a congenital disorder of hematopoiesis inherited by autosomal dominant or recessive traits. Downregulation of lymphoid enhancer-binding factor 1 is involved in the pathophysiology of all congenital neutropenia patients. Congenital neutropenia patients with acquired granulocyte colony-stimulating factor receptor mutations define a group with high risk for development of leukemia.

Familial thrombocytosis caused by the novel germ-line mutation p.Pro106Leu in the MPL gene

Familial thrombosis (FT) has been described as a rare autosomal‐dominant disorder, mostly caused by activating mutations of the thrombopoietin gene (THPO). Other cases of FT have been linked to one of two different germline mutations in the myeloproliferative leukaemia virus oncogene gene (MPL), which codes for the thrombopoietin receptor MPL. We studied an Arab family with two siblings with severe thrombocytosis by linkage analysis and obtained evidence for linkage to MPL. Sequencing revealed homozygosity for the novel MPL germline mutation p.Pro106Leu (c.317C > T) in the two siblings. Subsequently, homozygosity for p.Pro106Leu was identified in six further FT patients from three other Arab families. Of 18 heterozygous carriers, 14 had normal platelet counts, while four had mild thrombocytosis. Strong support for association of the novel MPL mutation p.Pro106Leu with development of familial thrombocytosis has been obtained. Overall, p.Pro106Leu was absent on 386 alleles of 193 healthy German controls and present on 14 of 426 alleles (3·3%) of 213 unrelated Arabs, which was statistically significantly different (P < 0·001, Fisher’s exact test). We assume that p.Pro106Leu is a frequent MPL mutation in the Arab population, leading to severe thrombocytosis in homozygotes and occasionally to mild thrombocytosis in heterozygotes. In the families described the mode of inheritance could be regarded as autosomal‐recessive with possible mild heterozygote manifestation rather than autosomal‐dominant with high penetrance as usually seen in FT.

Digenic mutations in severe congenital neutropenia

Severe congenital neutropenia a clinically and genetically heterogeneous disorder. Mutations in different genes have been described as causative for severe neutropenia, e.g. ELANE, HAX1 and G6PC3. Although congenital neutropenia is considered to be a group of monogenic disorders, the phenotypic heterogeneity even within the yet defined genetic subtypes points to additional genetic and/or epigenetic influences on the disease phenotype. We describe congenital neutropenia patients with mutations in two candidate genes each, including 6 novel mutations. Two of them had a heterozygous ELANE mutation combined with a homozygous mutation in G6PC3 or HAX1, respectively. The other 2 patients combined homozygous or compound heterozygous mutations in G6PC3 or HAX1 with a heterozygous mutation in the respective other gene. Our results suggest that digenicity may underlie this disorder of myelopoiesis at least in some congenital neutropenia patients.

G-csf receptor mutations in patients with congenital neutropenia

Purpose of reviewIn this review, we summarize our current knowledge on the acquisition of granulocyte-colony stimulating factor receptor (G-CSFR) gene (CSF3R) mutations in patients with congenital neutropenia and their role in leukemogenesis. Congenital neutropenia is a heterogeneous disorder of hematopoiesis characterized by a maturation arrest of granulopoiesis at the level of promyelocytes with peripheral blood absolute neutrophil counts below 0.5 × 109/l. Recent findingsThere are two major subtypes of congenital neutropenia as judged by inheritance, comprising the majority of congenital neutropenia patients: autosomal dominant trait defined by neutrophil elastase mutations consisting of 60% of patients and autosomal recessive trait comprising approximately 30% of patients, both presenting with the same clinical and morphological phenotype. Congenital neutropenia is considered as a preleukemic syndrome, as the cumulative incidence for leukemia is more than 25% after 20 years of observation. Acquired CSF3R mutations are detected in approximately 80% of congenital neutropenia patients who developed acute myeloid leukemia suggesting that these mutations are involved in leukemogenesis. One possible pathomechanism causing leukemia is that clones of cells harboring acquired CSF3R mutations have a growth advantage over wild type cells in vivo during granulocyte-colony stimulating factor treatment due to activation of STAT5 and ß-catenin, both known to be involved in leukemogenesis. SummaryCongenital neutropenia patients with acquired CSF3R mutations define a group with high risk for development of leukemia.