Mosaic Tetraploidy and Transient Mutations in the GFI1 Gene in a Patient with Severe Chronic Neutropenia.

Severe congenital neutropenia (SCN), also known as Kostmann disease, is a rare disorder characterized by peripheral blood agranulocytosis and maturation arrest of neutrophils within the bone marrow at the promyelocytic stage. Patients usually present with an absolute neutrophil count below 0.2 x 10⁹/L and severe infections, with the typical onset of symptoms during infancy. Most patients respond to treatment with granulocyte colony-stimulating factor, but retain an increased risk for death from sepsis and for development of myelodysplastic syndrome and acute myeloid leukemia (MDS/AML). Several reports have described karyotype abnormalities associated with this syndrome, most typically clonal loss of chromosome 7 or abnormalities of chromosome 21 associated with transformation to MDS/AML. The etiology of these abnormalities and their role in progression to MDS/AML remains unclear. Here we present a case of a 15 year-old boy with SCN, lymphopenia, and persistent tetraploid mosaicism in the bone marrow and in subpopulations of peripheral blood leukocytes. His phenotype, including growth and development, was otherwise normal. The patient’s father, who had mild neutropenia and a history of monoclonal gammopathy, also showed a tetraploid subpopulation in his bone marrow. Flow cytometric analysis of the DNA content of CD45+ nucleated cells in the patient’s peripheral blood showed that 20% of CD13+ granulocytes, and 15% of CD14+ monocytes were tetraploid. The lifelong persistence of the mosaicism and the similar finding in the patient’s father suggest that it represents a congenital propensity towards acquiring tetraploidy. However, cytogenetic evaluation of the patient’s skin fibroblasts detected no tetraploidy, indicating failure of the putative tetraploid embryonic cells to contribute to that lineage. The fact that the patient’s father also had a population of tetraploid cells in his bone marrow and a history of neutropenia suggests a common underlying genetic cause for both conditions in this kindred.

Mutational analysis revealed that the ELA2 gene sequence was normal, but the GFI1 gene exhibited transient, simultaneous appearance of two novel mutations: a missense substitution in exon 1 and a single base change in the promoter region, at a putative binding site for the myeloid-specific transcription factor MZF1. The GFI1 mutations were repeatedly identified in a DNA sample obtained in 2003, but not in 2001, 2002, or 2006 samples. Sequencing of DNA obtained from buccal swabs of the patient and his parents also revealed no GFI1 mutation. The transient appearance may represent the emergence and subsequent loss of a clone of hematopoietic stem or progenitor cells with GFI1 mutations. This hypothesis is supported by a recent report demonstrating that GFI1 mutation results in premature apoptosis of myeloid cells, similar to apoptosis-inducing mutations in the neutrophil elastase gene ELA2. The fact that GFI1 mutations were identified in a DNA sample from one year, but not from others, raises the possibility that GFI1 mutations reported in other neutropenic patients may also be transient. We speculate that an underlying genetic defect, inherited in an autosomal dominant pattern, leads to both disordered mitosis and leukopenia in this kindred.