The promise of novel treatments for severe chronic neutropenia

INTRODUCTION

Severe chronic neutropenia, i.e. absolute neutrophil count (ANC) less than 0.5 × 109/L, is a serious health problem because it predisposes patients to recurrent bacterial infections. Management radically changed with the discovery that granulocyte colony-stimulating factor (G-CSF) could be used to effectively treat most patients; therapy required regular subcutaneous injections. In the early days of G-CSF therapy, there were concerns that it might somehow overstimulate the bone marrow and cause myelodysplasia (MDS) or acute myeloid leukemia (AML). Detailed research records from the Severe Chronic Neutropenia International Registry (SCNIR) indicate that this is a relatively low-risk event. The research records suggest that certain patient groups are primarily at risk. Presently, allogeneic hematopoietic stem cell therapy serves as an alternate form of therapy.

AREAS COVERED

Due to these concerns and the desire for an easy-to-take oral alternative, several new treatments are under investigation. These treatments include neutrophil elastase inhibitors, SGLT-2 inhibitors, mavorixafor - an oral CXCR4 inhibitor, gene therapy, and gene editing.

EXPERT OPINION

All of these alternatives to G-CSF are promising. The risks, relative benefits, and costs are yet to be determined.

Disease Progression of WHIM Syndrome in an International Cohort of 66 Pediatric and Adult Patients

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome (WS) is a combined immunodeficiency caused by gain-of-function mutations in the C-X-C chemokine receptor type 4 (CXCR4) gene. We characterize a unique international cohort of 66 patients, including 57 (86%) cases previously unreported, with variable clinical phenotypes. Of 17 distinct CXCR4 genetic variants within our cohort, 11 were novel pathogenic variants affecting 15 individuals (23%). All variants affect the same CXCR4 region and impair CXCR4 internalization resulting in hyperactive signaling. The median age of diagnosis in our cohort (5.5 years) indicates WHIM syndrome can commonly present in childhood, although some patients are not diagnosed until adulthood. The prevalence and mean age of recognition and/or onset of clinical manifestations within our cohort were infections 88%/1.6 years, neutropenia 98%/3.8 years, lymphopenia 88%/5.0 years, and warts 40%/12.1 years. However, we report greater prevalence and variety of autoimmune complications of WHIM syndrome (21.2%) than reported previously. Patients with versus without family history of WHIM syndrome were diagnosed earlier (22%, average age 1.3 years versus 78%, average age 5 years, respectively). Patients with a family history of WHIM syndrome also received earlier treatment, experienced less hospitalization, and had less end-organ damage. This observation reinforces previous reports that early treatment for WHIM syndrome improves outcomes. Only one patient died; death was attributed to complications of hematopoietic stem cell transplantation. The variable expressivity of WHIM syndrome in pediatric patients delays their diagnosis and therapy. Early-onset bacterial infections with severe neutropenia and/or lymphopenia should prompt genetic testing for WHIM syndrome, even in the absence of warts.

Heterozygous variants of CLPB are a cause of severe congenital neutropenia

Severe congenital neutropenia is an inborn disorder of granulopoiesis. Approximately one third of cases do not have a known genetic cause. Exome sequencing of 104 persons with congenital neutropenia identified heterozygous missense variants of CLPB (caseinolytic peptidase B) in 5 severe congenital neutropenia cases, with 5 more cases identified through additional sequencing efforts or clinical sequencing. CLPB encodes an adenosine triphosphatase that is implicated in protein folding and mitochondrial function. Prior studies showed that biallelic mutations of CLPB are associated with a syndrome of 3-methylglutaconic aciduria, cataracts, neurologic disease, and variable neutropenia. However, 3-methylglutaconic aciduria was not observed and, other than neutropenia, these clinical features were uncommon in our series. Moreover, the CLPB variants are distinct, consisting of heterozygous variants that cluster near the adenosine triphosphate-binding pocket. Both genetic loss of CLPB and expression of CLPB variants result in impaired granulocytic differentiation of human hematopoietic progenitor cells and increased apoptosis. These CLPB variants associate with wild-type CLPB and inhibit its adenosine triphosphatase and disaggregase activity in a dominant-negative fashion. Finally, expression of CLPB variants is associated with impaired mitochondrial function but does not render cells more sensitive to endoplasmic reticulum stress. Together, these data show that heterozygous CLPB variants are a new and relatively common cause of congenital neutropenia and should be considered in the evaluation of patients with congenital neutropenia.

Results of a phase 2 trial of an oral CXCR4 antagonist, mavorixafor, for treatment of WHIM syndrome

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is a rare primary immunodeficiency caused by gain-of-function mutations in the CXCR4 gene. We report the safety, tolerability, pharmacokinetics, pharmacodynamics, and preliminary efficacy of mavorixafor from a phase 2 open-label dose-escalation and extension study in 8 adult patients with genetically confirmed WHIM syndrome. Mavorixafor is an oral small molecule selective antagonist of the CXCR4 receptor that increases mobilization and trafficking of white blood cells from the bone marrow. Patients received escalating doses of mavorixafor, up to 400 mg once daily. Five patients continued on the extension study for up to 28.6 months. Mavorixafor was well tolerated with no treatment-related serious adverse events. At a median follow-up of 16.5 months, we observed dose-dependent increases in absolute neutrophil count (ANC) and absolute lymphocyte count (ALC). At doses ≥300 mg/d, ANC was maintained at >500 cells per microliter for a median of 12.6 hours, and ALC was maintained at >1000 cells per microliter for up to 16.9 hours. Continued follow-up on the extension study resulted in a yearly infection rate that decreased from 4.63 events (95% confidence interval, 3.3-6.3) in the 12 months prior to the trial to 2.27 events (95% confidence interval, 1.4-3.5) for patients on effective doses. We observed an average 75% reduction in the number of cutaneous warts. This study demonstrates that mavorixafor, 400 mg once daily, mobilizes neutrophil and lymphocytes in adult patients with WHIM syndrome and provides preliminary evidence of clinical benefit for patients on long-term therapy. The trial was registered at www.clinicaltrials.gov as #NCT03005327.

Registries for study of nonmalignant hematological diseases: the example of the Severe Chronic Neutropenia International Registry

PURPOSE OF REVIEW

Registries provide 'real world' perspectives on the natural history and outcomes for many clinical conditions. The purpose of this review is to identify registries for nonmalignant hematological disease and to describe the operation of a successful long-term registry for patients with severe chronic neutropenia.

RECENT FINDINGS

There was an upswing in registries about 20 years ago, based on optimism about their utility to improve patient care. To show value, registries must define outcomes for populations of patients with specific medical conditions and the effects of treatment. This is challenging for many reasons. The Severe Chronic Neutropenia International Registry is an example of a successful registry. This report describes underlying reasons for its success.

SUMMARY

Registries are important to organize and analyze clinical information across geographic, ethnic and social boundaries. They are also challenging to organize, administer and support.

Elastase inhibitors as potential therapies for ELANE-associated neutropenia

Mutations in ELANE, the gene for neutrophil elastase (NE), a protease expressed early in neutrophil development, are the most frequent cause of cyclic (CyN) and severe congenital neutropenia (SCN). We hypothesized that inhibitors of NE, acting either by directly inhibiting enzymatic activity or as chaperones for the mutant protein, might be effective as therapy for CyN and SCN. We investigated β-lactam-based inhibitors of human NE (Merck Research Laboratories, Kenilworth, NJ, USA), focusing on 1 inhibitor called MK0339, a potent, orally absorbed agent that had been tested in clinical trials and shown to have a favorable safety profile. Because fresh, primary bone marrow cells are rarely available in sufficient quantities for research studies, we used 3 cellular models: patient-derived, induced pluripotent stem cells (iPSCs); HL60 cells transiently expressing mutant NE; and HL60 cells with regulated expression of the mutant enzyme. In all 3 models, the cells expressing the mutant enzyme had reduced survival as measured with annexin V and FACS. Coincubation with the inhibitors, particularly MK0339, promoted cell survival and increased formation of mature neutrophils. These studies suggest that cell-permeable inhibitors of neutrophil elastase show promise as novel therapies for ELANE-associated neutropenia.

An update on the diagnosis and treatment of chronic idiopathic neutropenia

PURPOSE OF REVIEW

Neutropenia lasting for at least for 3 months and not attributable to drugs or a specific genetic, infectious, inflammatory, autoimmune or malignant cause is called chronic idiopathic neutropenia (CIN). CIN and autoimmune neutropenia (AIN) are very similar and overlapping conditions. The clinical consequences depend upon the severity of neutropenia, but it is not considered a premalignant condition.

RECENT FINDINGS

Long-term observational studies in children indicate that the disease often lasts for 3-5 years in children, then spontaneously remits, but it rarely remits in adult cases. The value of antineutrophil antibody testing in both children and adults is uncertain. Most recent data suggest that CIN and AIN are immune-mediated diseases, but there are no new clinical or genetic tests to aid in diagnosis. Treatment with granulocyte colony stimulating factor (G-CSF) is effective to increase blood neutrophils in almost all cases; this treatment is reserved, however, for patients with both neutropenia and evidence of recurrent fevers, inflammatory symptoms and infections. There is little or no evidence to indicate that G-CSF treatment predisposes to myeloid malignancies in this population.

SUMMARY

It is important to recognize CIN and AIN, the most common causes of chronic neutropenia in both children and adults. If the neutropenia is not severe, that is more than 0.5 × 10/l, most patients can be observed and not treated prophylactically with antibiotics or a growth factor. When neutropenia is severe, treatment with G-CSF is often beneficial.

TCIRG1-associated congenital neutropenia

Severe congenital neutropenia (SCN) is a rare hematopoietic disorder, with estimated incidence of 1 in 200,000 individuals of European descent, many cases of which are inherited in an autosomal dominant pattern. Despite the fact that several causal genes have been identified, the genetic basis for >30% of cases remains unknown. We report a five-generation family segregating a novel single nucleotide variant (SNV) in TCIRG1. There is perfect cosegregation of the SNV with congenital neutropenia in this family; all 11 affected, but none of the unaffected, individuals carry this novel SNV. Western blot analysis show reduced levels of TCIRG1 protein in affected individuals, compared to healthy controls. Two unrelated patients with SCN, identified by independent investigators, are heterozygous for different, rare, highly conserved, coding variants in TCIRG1.

Variable clinical presentation of Shwachman-Diamond syndrome: update from the North American Shwachman-Diamond Syndrome Registry

OBJECTIVES

To investigate the range of clinical presentations for Shwachman-Diamond syndrome (SDS) with the long-term goal of improving diagnosis.

STUDY DESIGN

We reviewed the North American Shwachman-Diamond Syndrome Registry. Genetic reports of biallelic Shwachman-Bodian-Diamond syndrome mutations confirming the diagnosis of SDS were available for 37 patients.

RESULTS

Neutropenia was the most common hematologic abnormality at presentation (30/37, 81%); however, only 51% (19/37) of patients presented with the classic combination of neutropenia and steatorrhea. Absence of pancreatic lipomatosis on ultrasound or computed tomography scan, normal fecal elastase levels, and normal skeletal survey do not rule out the diagnosis of SDS. SDS was diagnosed in 2 asymptomatic siblings of SDS probands. Twenty-four of 37 patients (65%) had congenital anomalies.

CONCLUSION

Our cohort reveals a broad range of clinical presentation for SDS. Clues to the underlying diagnosis of SDS included cytopenias with a hypocellular marrow, congenital anomalies, family history, and myelodysplasia with clonal abnormalities frequently found in SDS. Reliance on classic clinical criteria for SDS would miss or delay diagnosis of a significant subset of patients with SDS.

Cyclic neutropenia and severe congenital neutropenia in patients with a shared ELANE mutation and paternal haplotype: evidence for phenotype determination by modifying genes

BACKGROUND

Cyclic neutropenia (CN) and severe congenital neutropenia (SCN) are disorders of neutrophil production that differ markedly in disease severity. Mutations of the ELANE gene (the symbol recently replacing ELA2) are considered largely responsible for most cases of CN and SCN, but specific mutations are typically associated with one or the other.

PROCEDURE

We performed ELANE genotyping on all individuals and paternal sperm in an SCN kindred with eight SCN progeny of a sperm donor and six different mothers.

RESULTS

One patient with CN had the same S97L ELANE mutation as seven patients with the SCN phenotype. The mutant allele was detected in the donor's spermatozoa, representing 18% of the ELANE gene pool, but not in DNA from his lymphocytes, neutrophils, or buccal mucosa, indicating gonadal mosaicism.

CONCLUSIONS

The coexistence of CN and SCN phenotypes in this kindred with a shared paternal haplotype strongly suggests both a role for modifying genes in determination of congenital neutropenia disease phenotypes, and the classification of CN and SCN within a spectrum of phenotypes expressing varying degrees of the same disease process.

Distinct patterns of mutations occurring in de novo AML versus AML arising in the setting of severe congenital neutropenia

Severe congenital neutropenia (SCN) is an inborn disorder of granulopoiesis. Like most other bone marrow failure syndromes, it is associated with a marked propensity to transform into a myelodysplastic syndrome (MDS) or acute leukemia, with a cumulative rate of transformation to MDS/leukemia that exceeds 20%. The genetic (and/or epigenetic) changes that contribute to malignant transformation in SCN are largely unknown. In this study, we performed mutational profiling of 14 genes previously implicated in leukemogenesis using 14 MDS/leukemia samples from patients with SCN. We used high-throughput exon-based resequencing of whole-genome-amplified genomic DNA with a semiautomated method to detect mutations. The sensitivity and specificity of the sequencing pipeline was validated by determining the frequency of mutations in these 14 genes using 188 de novo AML samples. As expected, mutations of tyrosine kinase genes (FLT3, KIT, and JAK2) were common in de novo AML, with a cumulative frequency of 30%. In contrast, no mutations in these genes were detected in the SCN samples; instead, mutations of CSF3R, encoding the G-CSF receptor, were common. These data support the hypothesis that mutations of CSF3R may provide the "activated tyrosine kinase signal" that is thought to be important for leukemogenesis.

Strong evidence for autosomal dominant inheritance of severe congenital neutropenia associated with ELA2 mutations

OBJECTIVE

To investigate cases of severe congenital neutropenia (SCN) to ascertain SCN inheritance after determining that the same sperm donor was used by 4 different families to impregnate mothers.

STUDY DESIGN

Because the donor sperm was not available, alternative methods were used to determine whether the sperm donor transmitted SCN. DNA isolated from leukocytes was used to sequence the ELA2 gene in the affected children and their mothers. ELA2 was amplified by polymerase chain reaction (PCR), and the product was sequenced. PCR was also performed with genomic DNA from the mothers and affected children using a set of 22 microsatellite PCR primers on chromosomes 14 and 19 to establish linkage to the paternal allele.

RESULTS

None of the mothers had a mutation in ELA2, but all 5 affected children had the same mutation affecting the fourth exon at site S97L. Linkage mapping analysis confirmed that all affected children had the same paternal allele on chromosome 19, which contains ELA2.

CONCLUSIONS

Our findings indicate that the father provided consistent haplotypes leading to the expression of SCN in all affected children, supporting an autosomal dominant inheritance in which ELA2 mutations occur.

Severe chronic neutropenia: treatment and follow-up of patients in the Severe Chronic Neutropenia International Registry

Severe chronic neutropenia (SCN) is defined as an absolute neutrophil (ANC) of less than 0.5 x 10(9)/L, lasting for months or years. Congenital, cyclic, and idiopathic neutropenia are principal categories of SCN. Since 1994, the Severe Chronic Neutropenia International Registry (SCNIR) has collected data to monitor the clinical course, treatments, and disease outcomes for SCN patients. This report summarizes data for 853 patients, almost all treated with daily or alternate-day recombinant human granulocyte colony-stimulating factor (G-CSF or Filgrastim). G-CSF treatment increased the ANC overall from 0.34 x 10(9)/L +/- 0.018 pre-treatment to 3.70 x 10(9)/L +/- 0.18 during the first year of treatment. For most patients, the responses were durable with patients remaining on the same dose of G-CSF for many years. Long-term hematological observations showed stable mean leukocyte and neutrophil counts and gradually increasing hemoglobin levels. Thrombocytopenia developed in 4% of patients. As of January 1, 2000, myelodysplasia (MDS) or acute myelogenous leukemia (AML) has occurred in 35 of 387 patients with congenital neutropenia with a cumulative risk of 13% after 8 years of G-CSF treatment. This event occurred without a predictable relationship to the duration or dose of G-CSF treatment. No patients with cyclic or idiopathic neutropenia developed MDS or AML. Other important adverse events included hepatomegaly, osteoporosis, vasculitis, glomerulonephritis, and deaths in 4 of 14 cases requiring splenectomy. Growth and development and the outcome of pregnancy appeared to be unaffected by G-CSF treatment. These data indicate that congenital, cyclic, and idiopathic neutropenia can be effectively treated with long-term G-CSF. The risk of leukemia, osteoporosis, other potentially adverse events, and pregnancy outcome need to be further evaluated with continuing long-term observations.

Cyclic neutropenia

Cyclic neutropenia is a rare hematologic disorder, characterized by repetitive episodes of fever, mouth ulcers, and infections attributable to recurrent severe neutropenia. Fluctuations in blood cells are due to oscillatory production of cells by the bone marrow. Recent genetic, molecular, and cellular studies have shown that autosomal-dominant cyclic neutropenia and sporadic cases of this disease are due to a mutation in the gene for neutrophil elastase (ELA2), located at 19p13.3. This enzyme is synthesized in neutrophil precursors early in the process of primary granule formation. It is currently presumed that the mutant neutrophil elastase functions aberrantly within the cells to accelerate apoptosis of the precursors, resulting in effective and oscillatory production. Cyclic neutropenia is effectively treated with granulocyte colony-stimulating factor (G-CSF), usually at doses of 1 to 5 microg/kg/d (median dose, 2.5 microg/kg/d). Long-term, daily, or alternate-day administration reduces fever, mouth ulcers, and other inflammatory events associated with this disorder. Leukemic transformation is not a recognized risk for cyclic neutropenia, with or without treatment with G-CSF.

Mutations in the gene encoding neutrophil elastase in congenital and cyclic neutropenia

Congenital neutropenia and cyclic neutropenia are disorders of neutrophil production predisposing patients to recurrent bacterial infections. Recently the locus for autosomal dominant cyclic neutropenia was mapped to chromosome 19p13.3, and this disease is now attributable to mutations of the gene encoding neutrophil elastase (the ELA2 gene). The authors hypothesized that congenital neutropenia is also due to mutations of neutrophil elastase. Patients with congenital neutropenia, cyclic neutropenia, or Shwachman-Diamond syndrome were referred to the Severe Chronic Neutropenia International Registry. Referring physicians provided hematologic and clinical data. Mutational analysis was performed by sequencing polymerase chain reaction (PCR)-amplified genomic DNA for each of the 5 exons of the neutrophil ELA2 gene and 20 bases of the flanking regions. RNA from bone marrow mononuclear cells was used to determine if the affected patients expressed both the normal and the abnormal transcript. Twenty-two of 25 patients with congenital neutropenia had 18 different heterozygous mutations. Four of 4 patients with cyclic neutropenia and 0 of 3 patients with Shwachman-Diamond syndrome had mutations. For 5 patients with congenital neutropenia having mutations predicted to alter RNA splicing or transcript structure, reverse transcriptase-PCR showed expression of both normal and abnormal transcripts. In cyclic neutropenia, the mutations appeared to cluster near the active site of the molecule, whereas the opposite face was predominantly affected by the mutations found in congenital neutropenia. This study indicates that mutations of the gene encoding neutrophil elastase are probably the most common cause for severe congenital neutropenia as well as the cause for sporadic and autosomal dominant cyclic neutropenia.

Myelodysplasia syndrome and acute myeloid leukemia in patients with congenital neutropenia receiving G-CSF therapy

Granulocyte colony-stimulating factor (G-CSF) has had a major impact on management of "severe chronic neutropenia," a collective term referring to congenital, idiopathic, or cyclic neutropenia. Almost all patients respond to G-CSF with increased neutrophils, reduced infections, and improved survival. Some responders with congenital neutropenia have developed myelodysplastic syndrome and acute myeloblastic leukemia (MDS/AML), which raises the question of the role of G-CSF in pathogenesis. The Severe Chronic Neutropenia International Registry (SCNIR), Seattle, WA, has data on 696 neutropenic patients, including 352 patients with congenital neutropenia, treated with G-CSF from 1987 to present. Treatment and patient demographic data were analyzed. The 352 congenital patients were observed for a mean of 6 years (range, 0.1-11 years) while being treated. Of these patients, 31 developed MDS/AML, for a crude rate of malignant transformation of nearly 9%. None of the 344 patients with idiopathic or cyclic neutropenia developed MDS/AML. Transformation was associated with acquired marrow cytogenetic clonal changes: 18 patients developed a partial or complete loss of chromosome 7, and 9 patients manifested abnormalities of chromosome 21 (usually trisomy 21). For each yearly treatment interval, the annual rate of MDS/AML development was less than 2%. No significant relationships between age at onset of MDS/AML and patient gender, G-CSF dose, or treatment duration were found (P >.15). In addition to the 31 patients who developed MDS/AML, the SCNIR also has data on 9 additional neutropenic patients whose bone marrow studies show cytogenetic clonal changes but the patients are without transformation to MDS/AML. Although our data does not support a cause-and-effect relationship between development of MDS/AML and G-CSF therapy or other patient demographics, we cannot exclude a direct contribution of G-CSF in the pathogenesis of MDS/AML. This issue is unclear because MDS/AML was not seen in cyclic or idiopathic neutropenia. Improved survival of congenital neutropenia patients receiving G-CSF therapy may allow time for the expression of the leukemic predisposition that characterizes the natural history of these disorders. However, other factors related to G-CSF may also be operative in the setting of congenital neutropenia. (Blood. 2000;96:429-436)

Stem cell transplantation in patients with severe congenital neutropenia without evidence of leukemic transformation

Severe congenital neutropenia (CN) (Kostmann syndrome) is a hematologic disorder characterized by a maturation arrest of myelopoiesis at the promyelocyte/myelocyte stage of development. This arrest results in severe neutropenia leading to absolute neutrophil counts (ANC) below 0.2 x 10(9)/L associated with severe bacterial infections from early infancy. Data on over 300 patients with CN collected by the Severe Chronic Neutropenia International Registry (SCNIR) beginning in 1994 indicate that more than 90% of these patients respond to recombinant human granulocyte-colony stimulating factor (r-HuG-CSF) treatment with an ANC greater than 1. 0 x 10(9)/L. For patients who are refractory to r-HuG-CSF treatment and continue to have severe and often life-threatening bacterial infections, hematopoietic stem cell transplantation is the only currently available treatment. We report on a total of 11 patients with CN reported to the SCNIR who underwent transplantation for reasons other than malignant transformation between 1976 and 1998. Of these patients, 8 were nonresponders or showed only partial response to r-HuG-CSF treatment with ongoing infections. Results from these patients suggest that transplantation of stem cells from an HLA-identical sibling is beneficial for patients refractory to r-HuG-CSF. (Blood. 2000;95:1195-1198)

Quality of life of patients with severe chronic neutropenia receiving long-term treatment with granulocyte colony-stimulating factor

OBJECTIVE

To evaluate the impact of long-term granulocyte colony-stimulating factor (GCSF) treatment on quality of life of patients with congenital, cyclic, or idiopathic neutropenia. STUDY DESIGN, INTERVENTION, AND OUTCOME MEASURES: Twenty-one patients receiving daily subcutaneous GCSF responded to retrospective questions about disease-related symptoms and physical, psychological, and social functioning before and with GCSF therapy.

RESULTS

Statistically significant improvement occurred in energy, emotional reactions, social isolation, functional ability, life satisfaction, decreased hospital admissions, and school attendance.

CONCLUSION

GCSF greatly improves quality of life in patients with severe chronic neutropenia.