Many congenital neutropenic conditions have been defined, but given their rarity, at a combined estimated prevalence of ten per million live births, no unifying classification schema has been established. Generally, conditions are considered in diagnostic clusters as those with (1) isolated neutropenia, (2) additional haematological defects, (3) extrahaematopoietic defects or (4) syndromic associations.34 As the molecular landscape underpinning these conditions is explored, one helpful approach is to separate these disorders according to their defective molecular pathways and disease mechanisms, summarised in figures 1 and 2.

Cyclic neutropenia (OMIM#162800)

Cyclic neutropenia (CN) is an autosomal dominant (AD) disorder that manifests with ANCs that fluctuate with regular periodicity.35 36

Genetic basis of disease

CN is caused by mutations in the neutrophil elastase (ELANE) gene (19p13.3).37 38 ELANE encodes a serine protease that localises to primary/azurophilic granules, intracellular membranes, cell surface and phagolysosomes. It degrades engulfed pathogens, digests surrounding connective tissue matrices and helps regulate the complement cascade and inflammatory responses.39–42

Clinical presentation and laboratory results

Patients are generally diagnosed in the first years of life, following recurrent infections, oral aphthae, gingivitis, tonsillitis, pharyngitis and/or periodontitis. The most common life-threatening illnesses include respiratory infections, spontaneous peritonitis, segmental bowel necrosis and septicaemia. There are no extrahaematopoietic features.

Peripheral blood and BM Findings

The periodicity of granulocytopenia varies. The average patient will experience episodes of profound neutropenia, ANC<200 cells/µL, approximately every 21 days; however, cycles between 11 and 52 days have been reported.37 While neutropenia drives the pathogenesis of the disease, other lineages may fluctuate, including platelets and occasionally reticulocytes.43

BM findings are related to ANC values. The severity of myeloid maturation arrest in the BM generally predicts the ANC in the peripheral blood (PB) several days later. For example, if a BM biopsy is taken at the time of, or shortly before, neutropenic episodes, the findings may mimic SCN. In contrast, the BM may demonstrate complete maturation if performed during or shortly before periods of near-normal ANCs.42 43

Treatment and risk of progression to MDS/AML

Patients with severe neutropenia are treated with G-CSF to increase ANC values and prevent serious infection. G-CSF may temper the underlying periodicity of neutropenic episodes.43 CN is not thought to confer an increased risk of MDS/leukaemia.44–46

Differential diagnosis

Some CN ELANE mutations may also be seen in SCN.46 The diagnosis is based on observation. Patients should have CBC with differential performed twice weekly for 6–8 weeks to characterise ANC fluctuations accurately. Autoimmune neutropenia may demonstrate cyclic oscillation of ANCs after steroid administration and should not be confused with CN. In one study, neutrophil-specific antibodies were found in 36% of patients with CN and SCN, and are not specific for immune neutropenias.47 48

Severe congenital neutropeniaGeneral features of SCN

The SCN category encompasses genetically heterogeneous germline conditions. General features are discussed, followed by specifics of each entity. Germline sequencing panels are necessary as each genetic driver is associated with different haematological manifestations and variable extrahaematological features.49 50 All variants are characterised by critically low ANC. Generally, patients present with recurrent fevers, oral ulcers, skin infections, especially omphalitis, and deep tissue abscesses within the first weeks or months of life. SCN 1, 2, 7, 8 and X do not demonstrate extrahaematopoietic manifestations. Other subtypes have syndromic associations. SCN is considered a premalignant condition, with an attendant risk for MDS and leukaemia relative to somatic mutational status, particularly CSF3R mutation and clinical sensitivity to G-CSF treatment.2

PB evaluation demonstrates marked neutropenia. The few neutrophils present may demonstrate bizarre nuclear morphology: pseudo-Pelger-Huet cells or unusually large, abnormally segmented neutrophils. Cytoplasmic granules may be reduced.50 There is often monocytosis and eosinophilia. CD4+ T cells and natural killer (NK) cells may be decreased and CD8+ T cells increased. Concurrent platelet abnormalities, anaemia of chronic disease or polyclonal hypergammaglobulinaemia are reported.51

SCN, type 1 (SCN1, ELANE, OMIM#: 202700)

SCN1, due to AD ELANE mutations, makes up 50%–60% of SCN cases.37 Genotype–phenotype correlations between ELANE mutations and clinical outcomes show that P139L, IVS4+5 G>A and S126L portend a better prognosis than C151Y and G214R in regard to the development of MDS/leukaemia, outcomes following stem cell transplantation and overall survival.52

BM examination demonstrates myeloid maturation arrest at the promyelocyte-to-myelocyte stage and essentially no mature forms (figure 3A,B). Promyelocytes are increased in number and may have abnormal cytomorphology: large size, irregular nuclei and cytoplasmic vacuoles. Monocytes, eosinophils and their precursors are increased. Atypical megakaryocytes may be present. Maturation may be seen in acute infection.

Figure 3

Severe congenital neutropenia due to ELANE, G6PC3 and SRP54 mutations. (A, B) Bone marrow aspirate from SCN due to ELANE mutation shows maturation arrest at the promyelocyte to myelocyte stage with essentially no further maturation (A) and with increased eosinophils and eosinophil precursors (B). (C, D) SCN due to G6PC3 mutation in a patient on G-CSF therapy but with peripheral neutropenia. (C) Bone marrow core biopsy shows myeloid hyperplasia with complete maturation and non-paratrabecular clusters of neutrophils, suggestive of inappropriate neutrophil retention in the bone marrow. (D) Aspirate smear shows complete myeloid maturation with frequent neutrophils including very rare forms with cytomorphology reminiscent of WHIM syndrome (arrow) with branching along thin intranuclear chromatin strands. (E, F) SCN due to SRP54 mutation. Bone marrow aspirate from SCN due to SRP54 mutation shows maturation arrest at promyelocyte-to-myelocyte stage (E). Some promyelocytes demonstrate atypical features including cytoplasmic vacuolisation (F). G-CSF, granulocyte colony-stimulating factor; SCN, severe congenital neutropenia; WHIM, Warts, hypogammaglobulinaemia, infections and myelokathexis.

G-CSF treatment is highly effective. 95% of patients show an ANC increase beyond 1000 cells/µL. BMs become hypercellular with mild eosinophilia and show a left-shift with myeloid hyperplasia and an expanded paratrabecular cuff of immature myeloid cells with full-spectrum maturation.53 54 G-CSF induces morphological changes in myeloid cells, including abnormal nuclear lobation and hypergranular cytoplasm; CD34+ myeloblasts may be increased, together mimicking MDS. Dyserythropoiesis and overt dysmegakaryopoiesis are not typical, however, and should raise concern for MDS.

SCN, type 2 (SCN2, OMIM#: 613107)

SCN2 is linked to mutations in the transcriptional repressor zinc finger domain of growth factor independent 1 transcriptional repressor (GFI1). Dominant negative, loss of function GFI1 mutations result in overexpression of ELANE. PBs contain only rare neutrophils, sometimes with circulating immature myeloid cells and a mixed B-cell/T-cell lymphopenia.55 56 The BM findings of SCN2 are similar to SCN1.

Patients respond well to G-CSF. In some cases, a striking monocytosis is present post-G-CSF. SCN2 appears to be associated with low risk of MDS/leukaemia transformation. Only two cases of transformation have been seen, both in individuals carrying germline variants of ELANE in addition to GFI1, and with acquired mutations of CSF3R and RUNX1.57

SCN, type 3 (SCN3, Kostmann disease, OMIM#: 610738)

SCN3 is due to autosomal recessive (AR) defects in HCLS1-associated protein X-1 (HAX1) and accounts for approximately 30% of SCN cases. HAX1 encodes a regulator of mitochondrial membrane potential, essential in apoptosis.58–61 Most patients surviving past infancy have neurocognitive delays or epilepsy.61–63 Commonly reported features include short stature, splenomegaly, osteoporosis, delayed puberty and primary gonadal insufficiency in females.64 65

SCN3 is often associated with thrombocytosis, in contrast to the thrombocytopaenia typical of other variants.66 BMs are normocellular with the characteristic myeloid maturation arrest at the promyelocyte-to-myelocyte stage (figure 4).60 61 SCN3 carries a relatively increased risk for MDS/leukaemia beyond other subtypes (figure 4).60 67 Sparse data are available regarding transplant experiences in these patients.68

Figure 4

Severe congenital neutropenia due to HAX1 mutation. (A, B) Bone marrow from untreated patient shows myeloid maturation arrest at the promyelocyte-to-myelocyte stage (A) and increased eosinophils (B) in a normocellular marrow. (C) Bone marrow aspirate from a patient on G-CSF shows complete myeloid maturation. (D–F) MDS occurring in HAX1-mutated SCN. Megakaryocytes are small and dysplastic with hypolobated nuclei and separated nuclear lobes (D), highlighted on factor VIII immunostain (E) and on aspirate smear (F). G-CSF, granulocyte colony-stimulating factor; SCN, severe congenital neutropenia.

SCN, type 4 (SCN4, OMIM#: 612541)

SCN4 is due to biallelic loss-of-function mutations in glucose-6-phosphatase catalytic subunit 3 (G6PC3).69 70 Severe G6PC3 deficiency (Dursun syndrome) is characterised by extrahaematopoietic manifestations and additional cytopenias. SCN4 presents early in the neonatal period, often with sepsis or episodes of pharyngitis, otitis, bronchitis and pneumonia. Most patients demonstrate atrial septal defects and prominent superficial veins across the trunk and limbs. Other cardiac structural malformations and urogenital defects may be seen.70 Facial dysmorphology with microcephaly, cleft palate, short stature, skeletal abnormalities, sensorineural hearing loss and myopathy are reported.70 71

PB may show iron-deficiency anaemia, lymphopenia and thrombocytopaenia. The neutropenia associated with SCN4 is notable for transient reversals with severe infection, akin to Warts, hypogammaglobulinaemia, infections and myelokathexis (WHIM) syndrome.72 BM examinations have revealed variable phenotypes. One group reported abundant marrow neutrophils consistent with myelokathexis and increased CXCR4 expression on neutrophils (figure 3C,D), while others describe paucity of BM neutrophils and dysmegakaryopoiesis.69 72 73 Patients with SCN4 respond well to G-CSF and may have a lower risk of transforming to MDS/leukaemia.73

SCN, type 5 (SCN5, OMIM#: 615285)

SCN5 results from AR mutations in vacuolar protein sorting 45 homolog (VPS45). VPS45 controls endolysosomal vesicle maturation and mutations disrupt cargo delivery, including G-CSFR. SCN5 manifests with hepatosplenomegaly, nephromegaly, diarrhoea and poor weight gain.74–76 A subset of patients have neurological abnormalities: developmental delay, vision and hearing loss, a thin corpus callosum, dysrhythmias and skeletal abnormalities with osteosclerosis.75 77

The ANC is often low, between 100 and 500 cells/µL, with poor G-CSF response. Thrombocytopaenia and anaemia may be present. BMs, off G-CSF, demonstrate hypercellularity with trilineage haematopoiesis and full-spectrum myeloid maturation, significant reticulin and collagen fibrosis (figure 5). Neutrophils are characteristically present in paratrabecular spaces, and fibrotic areas contain hypolobated, hypogranular dyspoietic, proapoptotic forms.74 Extramedullary haematopoiesis may be seen. No cytogenetic anomalies classically associated with MDS are reported. BM transplantation has met variable success, highlighting the need for early diagnosis and rapid transplantation.74 77

Figure 5

Severe congenital neutropenia due to VPS45 mutation. (A) Bone marrow biopsy shows complete myeloid maturation with inappropriate localisation of neutrophils adjacent to bone trabeculae and abnormal neutrophil nuclear lobation. (B) Reticulin stain shows a marked increase in reticulin fibrosis. Images courtesy of Thierry Vilboux, May Christine Malicdan and William Gahl.

SCN, type 6 (SCN6, OMIM#: 616022)

SCN6 is attributed to AR defects in Jagunal homolog 1 (JAGN1). JAGN1 is an ER-transmembrane protein involved in the early secretory pathway essential to G-CSFR-mediated signalling, neutrophil extracellular traps and survival of mature granulocytes.78 79 SCN6 patients demonstrate facial dysmorphism, short stature, skeletal abnormalities including hip dysplasia, amelogenesis imperfecta, osteoporosis, scoliosis, gastrointestinal issues like pyloric stenosis, pancreatic insufficiency, pancolitis and coarctation of aorta.78 79

PB findings include an ANC<100–1000/µL, sometimes accompanied by a T-cell/B-cell lymphopenia. BMs are generally normocellular with myeloid maturation arrest. Some have reported neutrophils with hypogranulation or nuclear dyspoiesis. Responses to G-CSF administration are mixed, with some patients responding poorly and others unable to tolerate the increased doses required due to bone pain.78 79

SCN, type 7 (SCN7, OMIM#: 617014)

SCN7 is ascribed to biallelic loss-of-function defects in CSF 3 receptor gene (CSF3R), which encodes GCSF-R, a cytokine receptor regulating early myeloid cell proliferation and differentiation.80–83 BM examination may demonstrate full-spectrum myeloid maturation with subtle left-shift and no overt morphological atypia. Response to G-CSF administration is mixed. Patients may respond to GM-CSF.80 83

SCN, type 8 (SCN8, OMIM#: 618752)

SCN8 is due to AD mutations in signal recognition peptide 54 (SRP54). SRP54 is the signal recognition particle subunit, which recognises signal peptides of nascent proteins destined for the ER.84 85 Most mutations are missense variants affecting GTPase activity, resulting in protein secretion defects and autophagy. SCN8 manifests two phenotypes. Most patients have neutropenia with no extrahaematological abnormalities.86–88 Another set mimics Shwachman-Diamond syndrome (SDS), with neurodevelopmental delays, skeletal dysplasia and exocrine pancreatic dysfunction.85 89

BM examination may show normal haematopoiesis or myeloid maturation arrest; immature forms may show cytoplasmic vacuolisation (figure 3E,F). Treatment with G-CSF appears to be effective but may require high-dose treatments.90 Two cases of transformation to acute myeloid and lymphoblastic leukaemia have been reported with RUNX1 and CSF3R mutations.89 91 Patients appear to do well poststem cell transplantation.85 92

SCN, type 9 (SCN9, OMIM#: 619813)

SCN9 results from defects in caseinolytic peptidase B homolog protein (CLPB). CLPB is a mitochondrial ATPase and chaperone refoldase implicated in mitochondrial cristae maintenance and apoptosis.93–95 Germline biallelic CLPB mutations, usually loss-of-function, cause CLPB syndrome in which congenital neutropenia is associated with 3-methylglutaconic aciduria and cataracts, with a variable spectrum of neurodevelopmental issues.94–98 In contrast, AD CLPB mutations, causing defective ATP binding, have been reported as a cause of SCN with variable (no to severe) 3-methylglutaconic aciduria and more variable neurological manifestations.97 In both conditions, BMs show myeloid maturation arrest at the promyelocyte-to-myelocyte stage. Myeloid malignancy has been reported in patients with CLPB mutations in both the biallelic and AD state. Patients typically, but not always, respond to G-CSF.

X-linked SCN (SCNX, OMIM#: 300299)

SCNX is due to missense mutations in Wiskott-Aldrich syndrome (WAS), which result in the constitutive activation of the GTPase domain, driving actin polymerization.99 100 WAS serves as a regulator of cytoskeletal actin polymerization. Despite the profound neutropenia in SCNX, patients have an attenuated susceptibility to severe infection. For this reason, SCNX is often diagnosed later in life, around the second decade.

Patients have monocytopenia and lymphocytopenia. Platelets are normal or slightly decreased, with large forms in some patients. NK cells may be decreased. BMs typically demonstrate classic myeloid maturation arrest without other abnormalities, but transient MDS-like changes have been reported.

Although SCNX responds well to G-CSF, the ANC and predisposition to infection do not correlate well. Reserving therapy for symptomatic episodes may be indicated. Transformation to myeloid malignancy has been reported in two cases. One developed MDS with excess blasts, which fell after the discontinuation of G-CSF, and another developed refractory acute leukaemia with monosomy 7.101

Other types of SCN

Pathogenic mutations in additional genes such as SEC6A1A and TCIRG1 have been identified as drivers of very rare types of SCN.102

Acquired somatic mutations in SCN

SCN is a preneoplastic syndrome. The estimated risk of MDS/leukaemia is 22% over 10 years.1–3 Patients on prolonged treatment or high dosages of G-CSF are at higher risk for malignancy, approximately 40% at 10 years.1 2 Several cytogenetic and molecular factors affect the behaviour of disease.103 Patients with SCN due to ELANE, HAX1, G6PC3 and JAGN1 mutations respond well to treatment with G-CSF. However, somatic CSF3R mutations occur in over 30% of SCN patients, and up to 80% of SCN patients with MDS/leukaemia, and are associated with a poor response to G-CSF and a higher transformation risk.104–106 ELANE mutations result in misfolded proteins that accumulate in the endoplasmic reticulum and trigger the unfolded protein response (UPR) and apoptosis. Somatic CSF3R mutations are hypothesised to inhibit the UPR, allowing myeloid cells to proliferate and differentiate.104 107 Tracking CSF3R mutational status on routine BM biopsies can help identify at risk patients and planning for transplantation. However, the intervals between the development of CSF3R clones and transformation vary months to years. Clones appear, disappear or persist without developing into overt malignancy.105–108 Discontinuation of G-CSF after the rise of CSF3R mutations may lead to the decline of mutant clones to undetectable levels; reinitiating G-CSF may be associated with rapid re-expansion of the clone.

Monosomy 7, trisomy 21 and trisomy 8 are described in SCN patients who developed MDS/leukaemia and may be events in a graded leukemogenic transformation.5 107–109 Somatic RUNX1 mutations are common and typically appear after CSF3R mutation, especially after treatment with high cumulative doses of G-CSF and are proposed to be an intermediary step in the progression to leukaemia.105 107 109 110 Activating RAS gene mutations have also been associated with malignant transformation, independently of CSF3R mutations.108 109 111 112

GATA2 deficiency (immunodeficiency 21; OMIM#: 614172)Genetic basis of disease

GATA2 is a zinc-finger transcription factor essential to the proliferation and survival of haematopoietic progenitors.113 114 The disorder is an AD condition due to inherited or de novo mutations in GATA2.

Clinical presentation

The first descriptions of GATA2 deficiency were those of monocytopenia and mycobacterial infection syndrome (MonoMAC), and dendritic cell, monocyte, B lymphocyte and NK lymphocyte deficiency (DCML). The disorder is associated with a broad spectrum of clinical manifestations including neutropenia and an increased risk of MDS/leukaemia. Some patients have clinical profiles mimicking SCN.113 115 116 Null mutations segregate with lymphedema and higher risk for severe infections.114 115 Patients have susceptibility to disseminated nontuberculous mycobacterial, papillomavirus, opportunistic fungal infections and pulmonary alveolar proteinosis.114–116 Emberger syndrome constitutes panniculitis, erythema nodosum, thrombi and predisposition to miscarriage and preterm labour.114–116 Age at diagnosis varies, with cases reported in the literature between <1 and 80 years, and a median age of 32 years at diagnosis.115 116

PB and BM findings

Cytopenias include decreased to absent B-cell, NK-cell, dendritic cells and monocytes. Neutropenia is identified in approximately 50% of patients. Anaemia has been reported. There is generally little or no effect on T cells.117 118 BMs are typically hypocellular. In cytopenic patients, dyspoietic changes including dysmegakaryopoiesis are often present (figure 6). Overall, the lifetime risk of MDS in GATA2 deficiency is 90%, with a median onset of 30 years old.117 119 120 In GATA2 deficiency with MDS, megakaryocyte dysplasia is especially prominent, including small and large hypolobated forms, and those with separate peripherally distributed nuclear lobes. In patients with MDS, erythroid and myeloid dysplasia and reticulin fibrosis are also common.

Figure 6

GATA2 deficiency. (A) Bone marrow biopsy from a patient with GATA2 deficiency and cytopenias showing characteristic dysmegakaryopoiesis including megakaryocytes with separated nuclear lobes including some with peripherally located nuclear lobes imparting a ‘wreath-like’ appearance. (B) GATA2 deficiency with overtly dysplastic megakaryocyte proliferation. Megakaryocytes include forms with separated and simplified nuclear lobes. (C) Aspirate smear showing dysplastic megakaryocytes with separated nuclear lobes. (D) Aspirate smear from GATA2 deficiency with myelodysplasia including hypolobated myeloids. (E, F) GATA2 deficiency with evolving acute myeloid leukaemia. The biopsy shows myeloid hyperplasia and left-shifted maturation with increased blasts, as well as small hypolobated megakaryocytes (E). Immunohistochemistry for CD34 shows increased myeloblasts (F).

Flow cytometry is useful in differentiating GATA2 deficiency from aplastic anaemia. In GATA2 deficiency, haematogones, monocytes, mature B and NK cells are decreased.117 The CD4:CD8 ratio is commonly inverted with increased numbers of CD57+large granular T cells. In approximately 50% of patients, plasma cells express CD56.

Treatment and risk of progression to MDS/AML

Patients with GATA2 deficiency typically progress through an intermediate MDS phase before AML.115 117 However, disease progression is unpredictable. The time from first diagnosis to onset of AML may be as short as 4 months, making MDS an especially critical marker of disease progression.

Patients with normal BM morphology generally do not have MDS-related mutations. Cytopenic patients often have some degree of dysmorphology, such as atypical megakaryocytes. Studies attempting to establish progression patterns have discovered recurrent cytogenetic alterations and somatic mutations. When MDS features are present somatic mutations in ASXL1, STAG2 and DNMT3A are common, with ASXL1 and STAG2 mutations correlating with MDS transformation and conferring a poorer survival probability in some studies.121–123 Less commonly identified mutations include: RUNX1, IDH2, TP53, SETBP1, NRAS, IKZF1, CRLF2, MLL and TET.124 Cytogenetic changes, including monosomy 7, trisomy 8, and trisomy 1q, are common. Monosomy 7 may be associated with poor outcomes, but trisomy 8 does not appear to have any prognostic value.118–121 Identification of a paediatric patient with MDS and monosomy 7 should prompt testing for GATA2 deficiency and SAMD9/SAMD9L diseases, among other monosomy 7 predisposition syndromes.124 Currently, the timing of transplantation varies between providers; expectations ought to be discussed at the time of first diagnosis.125 126

WHIM syndrome (OMIM#: 193670)Genetic basis of disease

Warts, hypogammaglobulinaemia, infections and myelokathexis (WHIM) syndrome is a germline disorder caused by gain-of-function mutations in the CXC chemokine receptor 4 (CXCR4) gene.127–129 Interaction between CXCR4 and CXCL12 functions to retain leucocytes within the BM. In WHIM syndrome, nonsense and frameshift mutations in the C-terminal region result in impaired CXCR4 receptor internalisation and abnormal retention of neutrophils, lymphocytes and monocytes in the BM, associated with peripheral cytopenias.

Clinical presentation and laboratory results

WHIM syndrome has an estimated incidence of 1 in 4 million people.129 Patients present with severe neutropenia and recurrent infections in early childhood but counts may normalise during infections, masking the condition. The severity of symptoms, warts and hypogammaglobulinaemia, are variable. Warts may not develop until late childhood or adulthood. The complete WHIM phenotype is present in less than 25% of cases, and diagnosis of WHIM syndrome is often delayed into adulthood despite presentation in childhood.130

PB and BM findings

Patients have severe neutropenia and leucopenia. Mononcytopenia, B-cell and T-cell lymphopenia are common.127 128 PB and BM eosinophil counts are normal but eosinophils often have vacuolisation, irregular granulation and nuclear lobation abnormalities. In PB, residual neutrophils are predominantly normal. Still, careful examination may reveal a subset with features of myelokathexis: hyperdense chromatin and long, thin, exaggerated, redundant chromatin strands separating nuclear lobes (figure 7).

Figure 7

WHIM syndrome. (A–C) Bone marrow aspirate smears demonstrating the characteristic neutrophil myelokathexis morphology including hypercondensed chromatin, long thin chromatin strands separating nuclear lobes and variable cytoplasmic vacuolisation. (D) Core biopsy showing myeloid hyperplasia and non-paratrabecular aggregates of neutrophils with neutrophil apoptosis. In the setting of peripheral neutropenia these findings are consistent with inappropriate neutrophil retention in the marrow. WHIM, warts, hypogammaglobulinaemia, infections and myelokathexis.

The BM demonstrates abnormal retention of neutrophils. Complete maturation of the myeloid lineage should prompt consideration of WHIM syndrome. The BM may be hypercellular or normocellular and often demonstrates myeloid hyperplasia. The proportion of neutrophils with myelokathexis in the BM is variable, in some cases may be <50%, and reflects several factors, including specific CXCR4 mutation and functional defect. Neutrophil vacuolisation is common and apoptotic neutrophils may be increased. On core biopsy, non-paratrabecular clusters of neutrophils reminiscent of microabscesses may be seen (figure 7).

Treatment and risk of progression to MDS/AML

Patients typically respond to G-CSF therapy but myelofibrosis may be seen with long-term treatment. There have been promising responses to CXCR4 antagonists.127 128 WHIM syndrome patients have a strong predisposition to HPV-driven squamous cell carcinomas and may develop EBV-driven lymphomas but MDS/leukaemia has not been reported.130 131

Differential diagnosis

Myelokathexis and increased CXCR4 expression have also been described in G6PC3 deficiency (SCN4) but neutrophil morphology is less striking. Germline biallelic CXCR2 loss-of-function mutations result in severe neutropenia with recurrent infection, gingivitis and oral ulceration, variable myelokathexis and less frequent warts and hypogammaglobulinaemia than WHIM syndrome.131

Shwachman-Diamond syndrome 1 (OMIM#: 260400)Genetic basis of disease

SDS is a syndrome of pancreatic exocrine insufficiency and BM dysfunction, resulting predominantly from AR SBDS ribosome maturation factor mutations.132 133 SBDS, in association with elongation factor-like GTPase 1 (EFL1), dissociates eukaryotic initiation factor 6 (eIF6) from the 60S ribosomal subunit once it has left the nucleus and reached the cytoplasmic compartment, permitting its association with the 40S subunit and formation of the 80S ribosome.132 In SDS, unassembled ribosomal subunits accumulate and drive an increasingly poor translation efficiency, which induce cellular stress triggering FAS/FASL-mediated senescence of cells and BM failure.133 134 There is no genotype–phenotype correlation in clinical or haematological measures.

Clinical presentation and laboratory results

SDS is associated with the triad of neutropenia (with or without additional cytopenias), exocrine pancreatic dysfunction and skeletal abnormalities. Patients typically present in infancy with failure to thrive. Skeletal abnormalities typically onset between 1 and 3 years old with metaphyseal chondrodysplasia, especially in the hips, dysostosis, epiphysial dysplasia and dwarfism. Additional findings include liver disease, renal tubular defects, insulin-dependent diabetes and neurocognitive delays. Bacterial sepsis, meningitis and viral infections are observed in multiple SDS registries.134–137

PB and BM findings

At the time of diagnosis, haematological parameters are variable. Neutropenia will eventually present in all patients with SDS but may be intermittent and is usually less severe than SCN. G-CSF treatment is required in about 30% of cases.134–138 Neutropenia is an isolated finding in 12% of patients and is associated with anaemia (typically without compensatory reticulocytosis) and thrombocytopaenia in 66% and 24% of cases, respectively.135 Persistent red cell count macrocytosis or pancytopenia may be present. Fetal haemoglobin is transiently elevated in up to 80% of cases, but persistently elevated levels are not uncommon. Specific findings associated with a high pretest probability for the diagnosis of SDS versus other inherited BM failure syndromes include milder, isolated neutropenia with a high RCC mean corpuscular volume or haemoglobin F levels, more severe anaemia and absence of thrombocytopaenia.

BM hypocellularity presents early in life, with only 35.5% of patients under 1 year old having normocellular BM in one study.134 135 137 Cellularity declines with age, at which point all patients demonstrate at least intermittently hypocellular marrow. Cellularity may transiently rise into the normocellular range with trilineage haematopoiesis, but with poor correlation to PB counts. At baseline, mild dysmyelopoiesis with neutrophil nuclear hypolobation and less frequently hypogranularity are present, features that do not indicate development of MDS (figure 8).137 138 Dyserythropoiesis and dysmegakaryopoiesis are not typical at baseline and should raise concern for progression to MDS (figure 8).

Figure 8

Shwachman-Diamond syndrome (SDS). (A, B) SDS at baseline with bone marrow biopsy showing marked marrow hypocellularity (A) and aspirate smear showing a subset of hypolobated neutrophils, a commonly seen finding not indicative of myelodysplasia. (C, D) SDS with myelodysplasia. Bone marrow biopsies often show increased cellularity from baseline (C). Increased dysplastic small hypolobated megakaryocytes are present (C). Another case with aspirate smears demonstrating significant dyserythropoiesis (D). (E, F) SDS with evolving acute myeloid leukaemia. Bone marrow biopsy is cellular with left-shifted myelopoiesis with increased primitive forms and incomplete maturation (E). Immunohistochemistry for CD34 shows increased myeloblasts (F).

Treatment and risk of progression to MDS/AML

Patients with SDS have a high risk of developing BM failure or MDS/leukaemia early in life, complications which account for approximately 85% of deaths. The French and Italian SDS Registries estimate the cumulative risk for severe cytopenias over 20 years at 10%–24.3%. The Italian SDS Registry showed that the cumulative incidences of severe neutropenia, thrombocytopaenia and anaemia at 30 years old were 59.9%, 66.8% and 20.2%, respectively, and that the 20-year cumulative incidence of MDS/leukaemia was about 10%.134 135

At the time of diagnosis, approximately half of the patients have a normal karyotype. Clonal haematopoiesis, with mutations in genes or chromosomal changes that alleviate the intrinsic defects of the underlying ribosomopathy (adaptive clones with no risk of MDS) are common.139 140 These include acquired EIF6 mutations and del(20q), which includes the EIF6 locus, both of which result in reduced eIF6 expression, leaving a greater 60S subunit fraction available for 40S subunit binding, thus improving the efficiency of ribosome assembly and protein synthesis. Iso(7q) is another common adaptive clone and results in an additional copy of SBDS, increasing SBDS protein levels and similarly improving ribosome assembly.

The haematological surveillance of patients with SDS includes PB studies every 3–6 months and a BM examination at least yearly, more often for patients previously identified as high risk for transformation. High-risk features include progressively ineffective haematopoiesis (increasing marrow cellularity with worsening cytopenias), dyspoietic BM morphology (especially if worsening or within the erythroid or megakaryocytic lineages), expanding or biallelic TP53 mutations, additionally acquired cytogenetic abnormalities (especially del(17p), monosomy 7, del(7q)) or complex karyotype.138 Acquired alterations in TP53 are common and have been associated with increased risk for leukaemic transformation, particularly if biallelic.141 142 Mutations in other genes, including PRPF8, CSNK1A1, ASXL1 and IDH1, have also been reported in myeloid neoplasms in SDS.

The overall survival of patients at 10 and 20 years old are estimated at 95.7% and 87.4%, respectively.135–137 The median survival is estimated at 38.2 years, with causes of death primarily attributed to MDS or leukaemia, followed by severe BM failure requiring hematopoietic stem cell transplant.143 144 Allogeneic stem cell transplantation is the only curative option for BM failure and myeloid malignancies. The outcomes for patients with BM failure are remarkably better than those for MDS or acute leukaemias. Reported outcomes of leukaemia occurring in SDS are exceedingly poor due to refractory disease, chemotherapeutic toxicity and graft failure with high relapse rates, highlighting the importance of early transplantation before the development of a high-grade myeloid neoplasm.143 144

Differential diagnosis

Approximately 10%–20% of patients with SDS clinical phenotypes test negative for SBDS mutations. Some of these cases have been attributed to germline EFL1, DNAJC21 and SRP54 mutations.145

Chediak-Higashi, Hermansky-Pudlak, Griscelli

Chediak-Higashi (CHS, OMIM#: 214500), Hermansky-Pudlak, type 2 (HPS2, OMIM#: 608233 and Griscelli syndrome, type 2 (GS2, OMIM#: 607624) are three distinct AR disorders due to mutations in genes involved in organelle biogenesis and intracellular vesicle transport.

Genetic basis of disease

CHS is caused by mutations in lysosomal trafficking regulator (LYST), encoding an intracellular adapter protein that regulates endosome/lysosome fusion and exocytic vesicle trafficking.146 HPS2 is caused by mutations in beta-1 subunit of adaptor-related protein complex 3 (AP3B1).147 148 The AP-3 complex cannot form without incorporating the beta-1 subunit. As a result, the trafficking through the trans-Golgi and lysosomes is compromised. GS2 is caused by Ras-associated protein 27A (RAB27A) mutations.149–151 RAB27A also drives vesicular fusion and exocytosis. Mutations in the genes described above result in the dysfunction of lysosome-relate organelles and a set of clinically similar conditions. Dysfunctional melanosome trafficking between melanocytes to keratinocytes, reduced enzymatic content and exocytosis of neutrophil azurophilic granules, and absence of platelet dense bodies result in oculocutaneous albinism, dysfunction of the innate immune system and bleeding diathesis, respectively.

Clinical presentation and laboratory results

Oculocutaneous albinism is usually striking. Patients have light skin and silver-white hair. CHS and GS patients demonstrate irregular melanin clumping within the medulla and otherwise a translucence of the hair shaft. HPS patients show reduced pigmentation without clumping.152 Cases of CHS without attendant albinism are rare.153 Ocular issues are associated with CHS and HPS2. Nystagmus is the most frequent finding, often present at birth.

The degree of neutropenia associated with these conditions varies but can be severe. Patients have a high risk of chronic periodontitis, recurrent dermal and pulmonary infections, mainly due to staphylococcal, streptococcal, gram-negative bacteria and Candida or Aspergillus species.154 CHS and HPS2 are also associated with lymphopenia and reduced cytolytic activity of T-cells and NK cells.155–158 GS2 patients are susceptible to recurrent pyogenic infections. HPS platelet counts are typically within normal limits, but patients have prolonged bleeding times due to severely reduced dense granules and an abnormal secondary wave in aggregation studies.159 Prothrombin time and partial thromboplastin times are normal. Bleeding is generally mild. It can, however, be life-threatening in a trauma or operative setting. CHS may be associated with neurological disorders, evident by early adulthood, including intellectual delays, sensorimotor neuropathies, Parkinsonism and spastic paraplegia.160 161 While neurological defects have not been associated with GS2, leucocyte infiltration of the brain can drive impairments and signal the development of hemophagocytic lymphohistiocytosis (HLH).162 163