Novel mutations in the CYBB gene causing X-linked chronic granulomatous disease: a case report of 2 patients

Chronic granulomatous disease (CGD) is an inherited primary immunodeficiency leading to recurrent infections, autoimmunity and chronic inflammation. It is caused by a mutation in any of the 5 structural subunits of the NADPH complex in neutrophilic granulocytes and monocytes, leading to defective reactive oxygen species production necessary for killing bacteria and fungi (Segal et al. 2000). The incidence of CGD varies around the world with estimates from 1:200 000 in the United States, 1:250 000 in Europe, 1:1 000 000 in Italy and 1:70 000 in the Israeli Arab population (Winkelstein et al. 2000; Martire et al. 2008; van den Berg et al. 2009; Wolach et al. 2017). While the vast majority of cases are X-linked in nature, rates of autosomal recessive CGD tend to predominate in countries with a high prevalence of consanguinity (van den Berg et al. 2009; Fattahi et al. 2011; Köker et al. 2013; Wolach et al. 2017).

The NADPH oxidase complex is made up of membrane-bound and cytosolic proteins that work together upon phagocyte activation. The 2 cell membrane proteins are catalytic and include glycoprotein gp91^phox and non-glycosylated protein p22^phox. Together they form a heterodimer known as cytochrome b₅₅₈. The 3 cytosolic proteins include p47^phox, p67^phox and p40^phox. When phagocytes are activated the cytosolic proteins translocate to cytochrome b₅₅₈ and recruit Rac 1/2 causing a conformational change in gp91^phox. This allows NADPH in the cytosol to donate an electron and form superoxide ions which generate further reactive oxygen species and are detrimental to phagocytosed organisms.

The most commonly affected NAPDH subunit in CGD is the transmembrane protein glycoprotein gp91^phox, encoded by CYBB on the X chromosome, accounting for about two-thirds of CGD cases. Mutations in NCF1 (p47^phox), CYBA (p22^phox) and NCF2 (p67^phox) are autosomal recessive and account for approximately 20%, 5%, and 5% of cases, respectively (Winkelstein et al. 2000; Jones et al. 2008; Martire et al. 2008; van den Berg et al. 2009; Kuhns et al. 2010). A single case report of an NCF4 (p40^phox) mutation causing CGD has been reported (Matute et al. 2009).

Infections tend to occur in the skin, lymph nodes, lungs and liver and are generally caused by catalase-positive organisms (e.g., Aspergillus, Nocardia, Staphylococcus aureus, Burkholderia cepacia, Serratia marcescens and Salmonella) (Segal et al. 2000; Winkelstein et al. 2000; Jones et al. 2008; Martire et al. 2008; van den Berg et al. 2009; Marciano et al. 2015). The rate of invasive fungal infections is higher in CGD than any other primary immunodeficiency with up to 20%–40% of patients affected, primarily occurring in the chest wall and the lungs. The most commonly isolated fungi include Aspergillus fumigatus and A. nidulans (Beauté et al. 2011; Blumental et al. 2011; Falcone and Holland 2012; Marciano et al. 2015). Patients with CGD can also suffer from immune dysregulation with the gastrointestinal tract (diarrhea, oral ulcers, anal fistulae, vomiting, anorexia, abdominal pain), lungs (dyspnea, granulomata, micronodules, pleural thickening, lymphadenopathy, interstitial lung disease, fibrosis), urogenital tract (hydronephrosis, cystitis, granulomatous orchitis) and eye involvement (chorioretinitis, uveitis, ocular granuloma. Patients are also at risk for autoimmune conditions such as lupus, vasculitis and dermatomyositis (Magnani et al. 2014).

Most patients with CGD are diagnosed before the age of 5, however, it can present at any age from infancy to late adulthood (Jones et al. 2008; Martire et al. 2008; van den Berg et al. 2009). X-linked CGD tends to lead to both an earlier presentation and mortality (Winkelstein et al. 2000; van den Berg et al. 2009). With better disease recognition and management there has been a significant improvement in life expectancy with recent studies reporting a survival rate of as high as 90% at 10 years of age (Kuhns et al. 2010). Previous mortality rates had been reported as high as 5% annually for X-linked CGD and 2% annually for autosomal recessive CGD (Winkelstein et al. 2000).

Improvements in mortality are thought to be related to the use of oral antifungal medications, earlier disease recognition, better antibiotic prophylaxis, the use of interferon-gamma and treatment with hematopoietic stem cell transplantation (HSCT) (Kuhns et al. 2010). Studies have shown mixed results for the use of interferon-gamma supplementation for infection prophylaxis and its use varies worldwide (The International Chronic Granulomatous Disease Cooperative Study Group 1991; Marciano et al. 2004; Martire et al. 2008). The only curative treatment for CGD remains allogenic HSCT, with the potential to prevent inflammatory and infectious complications altogether. Many issues remain however, including graft failure, complications from infections developed prior to transplant and organ dysfunction (Arnold and Heimall 2017). Unfortunately, the median age of death continues to be around age 30–40 with worsening quality of life over time (Jones et al. 2008; Martire et al. 2008; van den Berg et al. 2009).

We report on 2 patients with novel mutations in the CYBB gene causing X-linked CGD who have undergone HSCT. To our knowledge these mutations have not been previously described in the literature.

CGD is a primary immunodeficiency caused by a mutation in any of the 5 genes that encode the NADPH oxidase system subunits. CGD can present from infancy to adulthood which is reflective of the heterogenous nature of the disease. Patients with X-linked CGD were previously thought to present earlier, have a more aggressive disease course and earlier age of death than patients with autosomal recessive CGD. More recent publications, however, suggest that certain autosomal recessive forms may also have a relatively severe course (Vignesh et al. 2017). Residual NADPH activity has been thought to be reflective of the clinical course and likelihood of survival for a patient (Roos and de Boer 2014).

Given the wide spectrum of disease, variability in clinical manifestations and different genes that can be implicated in CGD, a better understanding of the association between the underlying genetic mutations and phenotypic manifestations is important.