Background
Hemophagocytic Lymphohistiocytosis (HLH) is a rare, complex and life-threatening syndrome which is characterized by hyper-inflammation due to over-activation of lymphocytes that releases secretory cytokines in the body [
1]. The actual epidemiology of HLH is difficult; however, overall estimated prevalence of HLH in patients younger than 18 years from different ethnic groups is approximately 1 per 100,000 [
2]. HLH is classified in two groups, primary or genetic form (P-HLH) where genetic defect is an underlying cause. P-HLH is normally inherited in an autosomal recessive pattern and mostly observed in childhood leading to early death. Secondary or acquired HLH is triggered due to viral infection, autoimmune/rheumatologic, malignant or metabolic conditions and is seen across all age groups [
3].
Primary-HLH is further classified in two groups; Familial HLH (FHL) and Lymphoproliferative disorder [
4]. Worldwide incidence of FHL is not known. However, extensive studies on FHL are carried out in Sweden and are shown to occur at an estimated incidence of 1 in 50,000 live births [
5]. FHL syndromes are sub-classified into FHL-1 through FHL-5, depending upon the gene/s involved and alterations in the functional protein. FHL has an autosomal recessive inheritance and is more commonly seen in the consanguineous families [
6,
7]. The prominent feature of hemophagocytosis is seen during early disease course. It very rarely occurs in secondary HLH or might be observed late after the disease has progressed [
1].
The diagnosis of primary and/or secondary HLH is carried out – either on clinical presentations or molecular findings or both as suggested by the Histiocyte Society [
8].
Nearly 58% FHL cases are reported to harbour mutations in
PRF1 gene (located at 10q22.1) that alters the normal function of perforin [
8]; whereas 40–60% of all X-linked lymphoproliferative disorders harbour mutation in SLAM-associated protein (SAP) regulating gene
SH2D1A (located at Xq25) [
9]. One hundred eight disease-causing mutations in the
PRF1 gene and 99 disease causing mutations in the
SH2D1A gene have been reported so far in the literature [
10,
11] including 14 HLH cases from India attributed to mutations in the
PRF1 gene [
12‐
14]. We report here two cases of HLH with the mutation in
PRF1 and
SH2D1A gene each.
To the best of our knowledge the molecular studies on mutation in SH2D1A gene is reported for the first time from India. Both the cases were enrolled for the study as per the institutional ethics committee guidelines in accordance with Helsinki declaration.
Discussion and conclusion
Literature survey for mutations in exon-2 of
PRF1 and intron-1 of
SH2D1A genes are presented in Additional file
2. PRF gene mutations (
N = 33/108; Additional file
2) span over multiple ethnicities [
10]. They are mostly observed in Asian and European population and less commonly seen in American and other populations [
10,
11]. Nonsense mutation (p.Trp34Ter) is mostly seen in Turkish population [
16,
17], frameshift mutation (c.1090_1091delCT and c.50delT) is more commonly observed in Japanese and African-American population [
17,
18] whereas missense mutation (p.Ala91Val and p.Trp129Ser) is seen in Italian and Indian population respectively [
12,
13,
19]. Most of the missense mutations commonly seen in HLH are observed under the umbrella of
PRF1 gene. Nonetheless, such ethnic predilection is not reported for
SH2D1A gene mutation [
11].
In our study, proband P1 has shown to have aforesaid disease-causing mutation (p.Trp129Ser) in
PRF1 gene. This gene encodes for perforin protein, located at 10q22.1 region and is obligatory for granule release mechanism of Natural Killer cells (NK-cell) and cytotoxic T lymphocytes (CTL) by forming pores into plasma membrane of the target cells and for passive diffusion of granzyme B for an apoptotic event [
13]. This protein is expressed up to 0–3% on CD8+ cells in the children from birth till 1 year of age whereas it is expressed on CD56+ cells from 1 to 30% in lymphocytes from 1 year to 15 years of age [
18]. In our case P1, missense mutation p.Trp129Ser in
PRF1 gene is likely to have caused a gain in glycosylation site of protein structure that result in loss of the binding stability with intracellular calcium ions [
21]. This renders perforin protein to become non-functional through changes in conformation leading to defective NK cell activity which has been observed in the majority of FHL patients [
12‐
22]. Though present study lacks the functional evidence of defective NK cell activity, Schneider et al. have classified FHL patients into four sub-types based on the level of NK cell activities [
23]. On the other hand, none of the cases with homozygous nonsense mutation in
PRF1 gene have shown the evidence of different NK cell activity irrespective of the FHL sub-types. This indicates that
PRF1 mutation is likely to play an important role in the persistence of deficient NK cell activity in FHL cases [
24].
Katano et al. had demonstrated accumulation of an uncleared precursor form of perforin and absence of an active mature form of perforin in patients with
PRF1 missense mutation [
25]. This indicates that missense mutation of
PRF1 gene causes conformational changes in the protein and inhibits the proteolytic cleavage of perforin precursor. The immune competence of patients with FHL depends on functional analysis of antigen-specific cytotoxic T-lymphocytes (CTLs). Cytotoxic formation in patient with missense mutation in
PRF1 gene remains intact than those having nonsense mutation [
24]. Nonetheless, functional analysis of antigen-specific CTLs may yield better insight into the immune competence of patients.
In proband P2, disease-causing hemizygous variant (c.138-3C>G) in intron-1 of
SH2D1A gene was detected. The same variant was reported earlier by Sayos et al. in a 6-year-old Italian boy [
26]. This gene encodes SAP protein, located on Xq25 position [
9]. It has a role as a key regulator in normal immune function of all three; NK-T (Natural killer T) cells, T-cells and B-cells. Increased SAP expression is seen in thymus-derived lymphocytes. A SLAM-SAP interaction initiates the lymphocyte activation [
26,
27]. Variant c.138-3C>G affects the splice site of exon-2, causing frameshift termination of SAP and leads to dysfunctional form. This alters the SH2 domain that plays a crucial role in binding with SLAM molecule. Thus, dysfunctional SAP protein induces the signal transduction of T-lymphocytes and display defects in their regulation. This defect is likely to cause the impaired production of interferon-gamma by defective Helper T-cell (CD4+ T-cell) leading to an increase in viral replication [
26].
A positive Epstein Barr virus test in the proband point towards a failure of immune system in protecting against EBV infection, due to an elimination of EBV-infected B lymphocytes by defective Helper T cells [
28] which is likely to be due to defect in NK cell activation and NK cell-mediated cytotoxicity. The model proposed by Parolini et al. [
29] suggested that altered function of a 2B4 molecule (SLAM family member) affects the ability to control EBV infection in the body. The authors also showed that in the absence of SAP/2B4-mediated recognition, CD48 play a crucial role in the inhibition of NK cell-mediated killing of EBV-infected B-cell. Benoit et al. [
30] also suggested that SAP/2B4 signal alteration leads to immune dysfunction. Immune system failure in the killing of EBV-Infected cells leads to prolonged activation of cytotoxic cells and results in cytokine secretion which activates macrophages. It decreases blood cell count and haemoglobin level which leads to hemophagocytosis in bone marrow and deranged liver functions which has been observed in the case under report. This indicates that the gene variant affects the immune function but after the viral infection, XLP becomes life-threatening as seen in the present case.
XLP1 patients with splice site variants are reported to have two years average median age of onset and predicted survival of 3 years [
31] which is in accordance with our observation. This indicates that lack of SAP resulting as a consequence of variants leads to a severe activation of lymphocytes and have a profound effect on the patients. It was also noted that XLP patients who have a nonsense and deletion mutations have a decreased SAP whereas missense and splice site mutations have absent or decreased SAP expression [
9].
The patients who are either not treated with specific therapies or undergoing transplantation die mostly due to severe pancytopenia that deregulates immune function. Moreover, it is reported that with fungal, bacterial or viral infection the survival of patients without treatment is < 2 months [
32] as seen in both these cases.
In conclusion, our study reports mutation in PRF1 and SH2D1A gene respectively that are known to be associated with HLH. Detailed molecular analysis has helped the families with precise genetic counselling and prenatal diagnosis during subsequent pregnancy. It is also recommended that male patients presenting with EBV-associated HLH need to be screened for XLP for early diagnosis and therapeutic implication if any.