Clinical practice guideline for activated phosphatidyl inositol 3-kinase-delta syndrome in Japan

Abstract Activated phosphatidyl inositol 3-kinase-delta syndrome (APDS) due to gain-of-function variant in the class IA PI3K catalytic subunit p110δ (responsible gene: PIK3CD) was described in 2013. The disease is characterized by recurrent airway infections and bronchiectasis. It is associated with hyper-IgM syndrome due to the defect of immunoglobulin class switch recombination and decreased CD27-positive memory B cells. Patients also suffered from immune dysregulations, such as lymphadenopathy, autoimmune cytopenia or enteropathy. T-cell dysfunction due to increased senescence is associated with a decrease in CD4-positive T lymphocytes and CD45RA-positive naive T lymphocytes, along with increased susceptibility to Epstein-Barr virus/cytomegalovirus infections. In 2014, loss-of-function (LOF) mutation of p85α (responsible gene: PIK3R1), a regulatory subunit of p110δ, was identified as a causative gene, followed in 2016 by the identification of the LOF mutation of PTEN, which dephosphorylates PIP3, leading to the differentiation of APDS1 (PIK3CD-GOF), APDS2 (PIK3R1-LOF) and APDS-L (PTEN-LOF). Since the pathophysiology of patients with APDS varies with a wide range of severity, it is crucial that patients receive appropriate treatment and management. Our research group created a disease outline and a diagnostic flow chart and summarized clinical information such as the severity classification of APDS and treatment options.


Introduction
Inborn errors of immunity (IEI) are a group of disorders that cause increased susceptibility to infections and immune dysregulation.To date, variants of almost 500 responsible genes have been shown to cause various IEI [1].Activated phosphatidyl inositol 3-kinase (PI3K)-delta syndrome (APDS) is an IEI for which the causative gene was discovered in 2013.It is reported as a disease caused by gain-offunction variants in the Class IA PI3K catalytic subunit p110d (responsible gene: PIK3CD) [2,3].The disease is characterized by recurrent airway infections and bronchiectasis beginning in childhood, and many patients present with lymphadenopathy, enteropathy and autoimmune diseases.Immunologically, aberrant antibody production (i.e., high IgM, low IgG, IgG2, low IgA) leading to hyper-IgM syndrome (HIGM) is observed, along with increased susceptibility to opportunistic infections, such as Epstein-Barr virus (EBV) or cytomegalovirus (CMV) infections.Flow cytometry analysis of peripheral blood lymphocytes shows Tcell dysfunction decrease in CD4-positive T lymphocytes and CD45RA-positive naive T lymphocytes, decrease in CD27-positive memory B cells and increase in CD24-high CD38-high CD10-positive transitional B cells [2,3].
Furthermore, in 2014, a loss-of-function variant of p85a (responsible gene: PIK3R1), a regulatory subunit of p110d, was identified in patients with symptoms similar to APDS [4,5].Disorders caused by gain-of-function variants in the PIK3CD gene are now categorized as APDS type 1 (APDS1), and those caused by loss-of-function variants in PIK3R1 are categorized as APDS type 2 (APDS2).In 2016, APDS-like condition caused by loss-of-function variants in PTEN was identified.Since PTEN catalyzes the dephosphorylation reaction of PIP3 and suppresses the AKT/mTOR/S6 pathway, loss-of-function mutations in PTEN cause overexpression of PIP3, resulting in a clinical condition similar to APDS [6].PTEN mutation was found in several phenotypically defined syndromes (Cowden syndrome 1, Lhermitte-Duclos disease, Macrocephaly/autism syndrome, PTEN Hamartoma tumor syndrome, Bannayan-Riley-Ruvalcaba Syndrome, etc.) described two patients with immunodeficiency and found the PTEN loss of function mutations [6].They had mental retardation with macrocephaly as seen in other PTEN mutated patients diagnosed as Macrocephaly/autism syndrome but they lacked the phenotype of hamartoma, the hallmark of Cowden syndrome.On the other hand, immunological phenotype was similar to recently described APDS.
Variants in either subunit cause APDS, with over 250 reported patients and many more not reported in the literature.APDS1 and APDS2 consist of approximately 75% and 25%, respectively [2][3][4][5][6].These variants lead to immunodeficiency with recurrent infections by multiple pathogens (bacterial, viral and other) affecting multiple organ systems.
In APDS, overexpression of PIP3 caused by constant activation of class IA PI3K induces overactivation of the PI3K signaling pathway, leading to hyperphosphorylation of the downstream AKT/mTOR/S6 [2][3][4][5].AKT is an important molecule that controls cell proliferation, differentiation, growth and metabolism, and hyperphosphorylation causes abnormal activation of lymphocytes and enlargement of lymphatic tissue [7].As PTEN suppresses PI3K, when this function is impaired by a PTEN gene variant, PI3K becomes dominant, causing a pathological condition similar to APDS [6].Based on the current knowledge experiences in Japan, we (the expert committee of IEI supported by a grant from the Ministry of Health, Labor and Welfare) made the Clinical Practice Guideline for APDS in Japan.

Methods
To create an APDS clinical practice guideline based on Minds, we utilized a literature search system to conduct a literature review of past reports on APDS, summarized clinical and laboratory findings and formulated a clinical practice guideline.We conducted a search of articles published on PubMed before 16 December 2020 and selected 15 articles of significance as references.The search terms and results are as follows: 1. 'Activated PI3K-delta syndrome'-61 results 2. 'Activated PI3K-delta syndrome 'AND 'PIK3CD'-37 results

Differential diagnosis
It has been reported that a considerable number of cases of APDS are included in cases clinically diagnosed as HIGM or common variable immunodeficiency (CVID).The possibility of APDS should be considered in patients with HIGM or CVID where the causative gene has not been identified.

Severity classification
Cases with increased susceptibility to infections due to antibody deficiency are considered 'severe', and regular immunoglobulin supplementation and administration of prophylactic antibacterial agents are essential.
Cases that require treatment for immune dysregulation, lymphoid organ hyperplasia or regular monitoring of complications are also considered 'severe'.
Mutation-positive family members with no or mild symptoms can be considered 'mild'.

Complications
Hepatosplenomegaly, lymphoid hyperplasia and bronchiectasis are common complications seen in all types of APDS [8,9].Failure to thrive and mild mental retardation have been reported in approximately 50% of patients with APDS2.Additionally, malignant tumors (especially B-cell lymphoma), autoimmune diseases (i.e., cytopenia), bronchiectasis and chronic diarrhea are observed [9].Mild mental retardation and macrocephaly are observed in patients with APDS-L [6].

Diagnosis
In patients with recurrent sinusitis, lower airway infection and hepatosplenomegaly or lymphoid hyperplasia, APDS can be determined by immunological evaluations, such as low levels of IgG, IgG2, IgA and/or high IgM, and findings from peripheral blood flow cytometry, such as decreased CD4-positive T cells, decreased CD45RA-positive naive T cells, increased TFH, decreased memory B cells and increased transitional B cells.Macrocephaly and mental retardation should also be considered complications.
Definitive diagnosis is made by genetic testing and confirming increased phosphorylation of AKT and S6 proteins in activated T lymphocytes or B lymphocytes of the patient.
Diagnosis flow chart is shown in Figure 2.

Treatment
For immunocompromised patients with antibody deficiency, regular immunoglobulin supplementation and administration of prophylactic antibacterial drugs such as trimethoprim/sulfamethoxazole and low-dose macrolides are strongly recommended.Antiviral prophylaxis by acyclovir or ganciclovir and regular monitoring of EBV/CMV infections should also be implemented.Allogeneic hematopoietic cell transplantation (HCT) can be considered in cases with T-cell dysfunction or dysregulation, massive lymphoid organ hyperplasia or malignant lymphoma.In some cases, immunosuppressive therapy (i.e., rituximab, sirolimus) may be necessary for autoimmune diseases [8][9][10][11][12][13].
In recent years, there have been reports of cases in which mTOR inhibitors and selective p110d inhibitors were effective against lymphoid hyperplasia or immune dysregulation in patients with APDS.Several studies have reported that rituximab is effective for the treatment of lymphoid hyperplasia, such as lymphadenopathy and hepatosplenomegaly, often seen in patients with APDS.However, rituximab should be cautiously considered, because it can cause persistent, often permanent, B-cell depletion.In recent years, mTOR inhibitors (sirolimus) [14] and selective p110d inhibitors (i.e., leniolisib) [15] have also been reported to be effective.mTOR inhibitors have been shown to reduce hepatosplenomegaly and lymphadenopathy, increase the naive Tcell fraction and restore T-cell proliferation and IL-2 secretion [14].Maccari et al. [14] reported that of 25 patients, eight achieved complete remission and 11 achieved partial remission.However, relapse after completion of treatment and side effects of longterm use have also been reported.
On the other hand, selective p110d inhibitors are attracting attention as treatments that can reduce side effects and achieve higher efficacy.Rao et al. [15] reported that in six patients with APDS, after 12 weeks of treatment with leniolisib (oral medication), reduced lymphadenopathy (mean 40%) and splenomegaly (mean 39%) were observed in all patients.Furthermore, normalization of transitional B cells and serum IgM level was observed.The results from further research on this drug are anticipated.
HCT has also been performed in some cases to replace abnormal lymphocytes that cause persistent activation of PI3Kd [10,11,16].However, considerable difficulty in management after HCT has been reported [11].Further prospective studies to improve the prognosis of APDS patients are necessary.

Considerations in clinical practice
1.While APDS is an autosomal dominant disorder, it can present with various clinical symptoms and laboratory findings even within the same family.

Prognosis and challenges in adulthood
The incidence of malignant tumors in APDS has been reported to be 13% for APDS1 and 28% for APDS2, with a particularly high incidence of B-cell lymphoma.In addition, 16% of patients with APDS die from lymphoma-related complications.Appropriate observation and treatment are directly linked to an improved prognosis [13].Okano et al. [11] reported that in a study of 23 patients with APDS1, nine of whom received HCT, the OS at 30 years was 86.1%, and the event-free survival was 39.6%.Complications such as infections or lymphatic tissue enlargement were observed frequently in childhood and adolescence.In particular, a major challenge is the limited treatment options that are effective in treating lymphatic tissue enlargement.Elkeim et al. [9] reported that in 36 patients with APDS2, the median survival was 18 years (range 3-56 years), and of the five deaths, four were caused by malignant lymphoma.

2 .
Patients with APDS2 have a high probability of developing malignant tumors (especially B-cell lymphoma) as a complication.Appropriate patient management and assessment by imaging tests is needed.