HAX1 Associated Congenital Neutropenia: A 25 Year Long-Term Analysis of the European Scnir Cohort

Background: Congenital neutropenia (CN) is a group of rare inborn genetic defects including a variety of different gene mutations and different patterns of inheritance. While mutations in the ELANE gene comprise the largest cohort of autosomal dominant CN, HAX1 gene mutations make up the largest cohort of autosomal recessive CN in Europe.

Independent of the underlying genetic subtypes, CN patients benefit from G-CSF maintenance treatment, which improved the life expectancy and quality of life significantly. By prolonging the life expectancy, a significant risk of secondary leukemia development has been unmasked for most genetic CN subtypes including HAX1 .

There are two HAX1 splice isoforms and genotype-phenotype studies have shown that HAX1 mutations affecting both isoforms (mainly p.Q190X in the original patients in Sweden and p.R86X in Japanese patients) cause severe CN with neurologic symptoms. Mutations affecting only one isoform (mainly p.W44X, found in consanguineous families of Turkic or Arabic origin) lead to severe congenital neutropenia without neurologic involvement.

Methods: Here we report on the genotype - phenotype correlation and outcome of 48 patients revealing different HAX1 mutations documented by the European Branch of the SCNIR since 1994.

Results: HAX1 mutations have been identified in 48 of the 458 patients with CN enrolled by the European Branch of the SCNIR, especially in consanguineous pedigrees. 45 patients reveal homozygous mutations in one transcript variant (p.W44X), which is the major subtype in patients of Turkic or Arabic origin. In 3 patients originated from Sweden both isoforms are affected (p.Q190X).

At diagnosis, all HAX1 patients present with severe neutropenia in CBCs, bone marrow shows a maturation arrest at the promyelocyte/myelocyte stage of granulopoiesis. Most, but not all, patients have received long-term treatment with G-CSF. Patients respond well to G-CSF treatment with a median G-CSF dose of 4.25 µg/kg/day compared to 4.38 µg/kg/day for the total CN cohort of the SCNIR Europe.

Age distribution is 2-38 years (median 12.5 years), 46 of 48 patients are alive, 1 patient expired from sepsis at age 2 years after developing hemophagocytic lymphohistiocytosis (HLH) while on G-CSF treatment (2 µg/kg/day), 1 patient expired at age 13 years from transplant related morbidity given for leukemia.

7 of the 48 patients (14 %) developed secondary myelodysplastic syndrome or acute leukemia (age at diagnosis 0.9-12.11 years, median 7.1 years). 6 of 7 patients carry mutation p.W44X, 1 patient carries p.Q190X. All 7 patients were treated with allogenic stem cell transplantation (SCT). In addition, 2 patients (age 0.9 and 18.1 years) received SCT for other reasons (prior to G-CSF availability and to cure the primary disease). None of the patients with HAX1 mutations required SCT for G-CSF non - responsiveness.

Because of the association of higher G-CSF doses with greater risk of leukemic transformation in CN patients, we examined the relationship between the median G-CSF dose and leukemia development in this cohort and found that patients who developed leukemia also required higher G-CSF doses, as compared to non-leukemic patients (3.8 µg/kg/day in non-leukemic patients vs. 7.05 µg/kg/day in patients with leukemia).

Next-generation ultra-deep-sequencing of the critical region of CSF3R of patients cDNA revealed that almost 50% of HAX1 -associated CN patients acquired single or multiple CSF3R mutations. This frequency is much higher than in other CN patient groups.

Digenic mutations (HAX1 plus ELANE or G6PC3) were identified in additional 3 patients who presented with severe neutropenia and a variety of additional symptoms and organ involvements.

Conclusions:

All patients with HAX1 mutations present with profound neutropenia, but respond well to long-term G-CSF treatment. Secondary leukemia is the major reason for SCT in CN patients with HAX1 mutations. The high frequency of CSF3R mutations is unique in this subgroup of CN patients and might lead to a higher risk of leukemia development in the future. Neurologic symptoms are associated with specific HAX1 genotype. Registry and biobanks are extremely valuable for understanding the consequences of rare hematological diseases such as congenital neutropenia.