A novel compound heterozygous HAX1 mutation in a Chinese patient with severe congenital neutropenia and chronic myelomonocytic leukemia transformation but without neurodevelopmental abnormalities

Severe congenital neutropenia (SCN) is a rare disease, comprising a heterogeneous group of inherited disorders. Clinical features include reduced absolute neutrophil counts from birth, increased susceptibility to recurrent and life-threatening infections, and a pre-leukemic predisposition.1

SCN exhibits different genetic modes of inheritance, including autosomal dominant, autosomal recessive, X-linked and sporadic forms. Specific genetic mutations have been described for the different types of SCN.2 Homozygous mutations in HAX1 are the pathogenic mutations in autosomal recessive SCN.3 However, compound heterozygous HAX1 mutations have also been identified in these patients.4–6 A correlation between genotype and phenotype was observed in these patients, suggesting that mutations affecting transcript variant 1 of HAX1 were associated with CN alone, whereas mutations affecting both transcripts caused CN and neurological symptoms.6–8 This finding implies isoform b of the HAX1 protein is critical for neuronal function, which has also been confirmed in an animal experiment.9 The present study describes the first Chinese autosomal recessive SCN patient with chronic myelomonocytic leukemia (CMML) transformation, who carries a novel compound heterozygous mutation in the HAX1 gene that affects both transcripts without neurodevelopmental abnormalities.

This 20-year old patient was referred to us in April 2009 for persistent neutropenia. Repeated full blood counts showed leukopenia and neutropenia without monocytosis. Bone marrow analysis revealed a maturation arrest at the promyelocyte/myelocyte stage with few mature neutrophils. No cytogenetic abnormalities were found and cranial magnetic resonance imaging was normal.

Past medical history revealed recurrent and refractory oral ulcers, chronic gingivitis and frequent bouts of fever of no obvious origin from soon after birth. He had never received G-CSF. By 17 years of age, the patient had splenomegaly, hepatomegaly, cervical lymphadenopathy and thickening of the walls of the ascending colon with a dilated lumen. Splenectomy and biopsies of the affected tissue were performed and showed non-specific inflammatory responses to an acute or chronic inflammatory stimulus.

The patient performed well at school and is now a skilled craftsman. No cognitive problems or neurodevelopmental delay were observed, and there was no relevant family history.

A diagnosis of SCN was made but no treatment was given as the patient felt well. During a routine follow-up visit in February 2010, CMML was diagnosed. This was confirmed by a bone marrow smear showing dysplastic erythrocytes and megakaryocytes, with persistent peripheral blood monocytosis (>1×10/L). No cytogenetic abnormalities, including Bcr/abl, PDGFRA or PDGFRB, were detected.10

Table 1.Demographic data and clinical information of patients with compound heterozygous HAX1 mutations.

To clarify the pathogenic mutation in this patient, genomic DNA was extracted from bone marrow samples before and after developing CMML. Sequencing analysis of the candidate genes proved that the patient had a novel compound heterozygous HAX1 mutation consisting of two frame-shift mutations which resulted in premature stop codons. One was c.430-1insG (exon 3) which has been described previously.5,7 The other mutation, c.655-9del5bp (exon 5), causes p.Pro219TrpfsX13; to the best of our knowledge this is a novel mutation.

Figure 1.Sequence chromatograms of the affected family. The filled symbol denotes the affected patient who is compound heterozygous for two mutations, each of them inherited from one of the parents. The parents, paternal grandfather, and the patient’s second elder sister are heterozygous carriers (half-filled symbol). Mutations are indicated with black arrows. Square symbols denote males, the circles denote females.

To define the inheritance pattern of this novel mutation, the HAX1 gene was sequenced in genomic DNA from peripheral blood of all other family members. The paternal grandfather, father and second elder sister of the proband were heterozygous carriers of the c.430-1insG mutation; the proband’s mother was a heterozygous carrier of the c.655-9del5bp mutation, and all carriers had a normal phenotype. The HAX1 protein was detected in the carriers of both types of HAX1 mutation by Western blotting, but the patient had a deficiency in HAX1 protein levels. Figure 1 shows the pedigree of the autosomal recessive inheritance pattern.

The co-relationship between genotype and phenotype in HAX1 mutations has been revealed by several research groups in SCN patients.6–8 This correlates with compound heterozygous HAX1 mutations reported previously (Table 1).4–6 Such a correlation was not the case in our current patient as there was no neurodevelopmental abnormality. A SCN patient with neurological symptoms due to homozygous HAX1 mutation of c.430-1insG has been described.7 Therefore, we speculate that the p.Pro219TrpfsX13 mutation produced by c.655-9del5bp could replace the role of the isoform b of HAX1 in neuronal function. This protein probably could not be detected in our patient by a monoclonal antibody to the HAX-1 amino acid 10–148 sequence due to the altered tertiary structure.

To understand more about SCN patients with compound heterozygous HAX1 mutations, we reviewed the patients reported in previous studies and summarized their characteristics in Table 1. They all presented typical clinical phenotypes of CN, as summarized.1 They could also show neurological manifestations but no epilepsy was recorded, while for SCN patients with homozygous HAX1 mutations common to transcript variants 1 and 2, all of these showed epilepsy except the mutation of c. 568 C>T.3,5–8,11

Although CMML may have developed in our patient independently, an increased risk of malignant transformation in patients with SCN has been well documented and our patient had wild genotypes of G-CSFR, KRAS, NRAS and no other cytogenetic abnormalities.12 Whole genome sequencing of this patient is being performed to identify the mutated genes that contributed to the malignant transformation.

Acknowledgments

The authors are indebted to the patient and his family for participating in this study and would like to thank Y Chen and L-J Cao for their assistance.

Footnotes

• S-LX, J-LL and J-YZ contributed equally to this paper.
• Funding: this work was supported by a grant from the government of Jiangsu Province (A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, PAPD2011).
• The information provided by the authors about contributions from persons listed as authors and in acknowledgments is available with the full text of this paper at www.haematologica.org.
• Financial and other disclosures provided by the authors using the ICMJE (www.icmje.org) Uniform Format for Disclosure of Competing Interests are also available at www.haematologica.org.