Clinical and functional spectrum of RAC2-related immunodeficiency

ABSTRACT

Mutations in the small Rho-family guanosine triphosphate hydrolase RAC2, critical for actin cytoskeleton remodeling and intracellular signal transduction, are associated with neonatal severe combined immunodeficiency (SCID), infantile neutrophilic disorder resembling leukocyte adhesion deficiency (LAD), and later-onset combined immune deficiency (CID). We investigated 54 patients (23 previously reported) from 37 families yielding 15 novel RAC2 missense mutations, including one present only in homozygosity. Data were collected from referring physicians and literature reports with updated clinical information. Patients were grouped by presentation: neonatal SCID (n = 5), infantile LAD-like disease (n = 5), or CID (n = 44). Disease correlated to RAC2 activity: constitutively active RAS-like mutations caused neonatal SCID, dominant-negative mutations caused LAD-like disease, whereas dominant-activating mutations caused CID. Significant T- and B-lymphopenia with low immunoglobulins were seen in most patients; myeloid abnormalities included neutropenia, altered oxidative burst, impaired neutrophil migration, and visible neutrophil macropinosomes. Among 42 patients with CID with clinical data, upper and lower respiratory infections and viral infections were common. Twenty-three distinct RAC2 mutations, including 15 novel variants, were identified. Using heterologous expression systems, we assessed downstream effector functions including superoxide production, p21-activated kinase 1 binding, AKT activation, and protein stability. Confocal microscopy showed altered actin assembly evidenced by membrane ruffling and macropinosomes. Altered protein localization and aggregation were observed. All tested RAC2 mutant proteins exhibited aberrant function; no single assay was sufficient to determine functional consequence. Most mutants produced elevated superoxide; mutations unable to support superoxide formation were associated with bacterial infections. RAC2 mutations cause a spectrum of immune dysfunction, ranging from early onset SCID to later-onset combined immunodeficiencies depending on RAC2 activity. This trial was registered at www.clinicaltrials.gov as #NCT00001355 and #NCT00001467.

A disease-linked lncRNA mutation in RNase MRP inhibits ribosome synthesis

RMRP encodes a non-coding RNA forming the core of the RNase MRP ribonucleoprotein complex. Mutations cause Cartilage Hair Hypoplasia (CHH), characterized by skeletal abnormalities and impaired T cell activation. Yeast RNase MRP cleaves a specific site in the pre-ribosomal RNA (pre-rRNA) during ribosome synthesis. CRISPR-mediated disruption of RMRP in human cells lines caused growth arrest, with pre-rRNA accumulation. Here, we analyzed disease-relevant primary cells, showing that mutations in RMRP impair mouse T cell activation and delay pre-rRNA processing. Patient-derived human fibroblasts with CHH-linked mutations showed similar pre-rRNA processing delay. Human cells engineered with the most common CHH mutation (70AG in RMRP) show specifically impaired pre-rRNA processing, resulting in reduced mature rRNA and a reduced ratio of cytosolic to mitochondrial ribosomes. Moreover, the 70AG mutation caused a reduction in intact RNase MRP complexes. Together, these results indicate that CHH is a ribosomopathy.

Current and Future Therapeutic Approaches for Thymic Stromal Cell Defects

Inborn errors of thymic stromal cell development and function lead to impaired T-cell development resulting in a susceptibility to opportunistic infections and autoimmunity. In their most severe form, congenital athymia, these disorders are life-threatening if left untreated. Athymia is rare and is typically associated with complete DiGeorge syndrome, which has multiple genetic and environmental etiologies. It is also found in rare cases of T-cell lymphopenia due to Nude SCID and Otofaciocervical Syndrome type 2, or in the context of genetically undefined defects. This group of disorders cannot be corrected by hematopoietic stem cell transplantation, but upon timely recognition as thymic defects, can successfully be treated by thymus transplantation using cultured postnatal thymic tissue with the generation of naïve T-cells showing a diverse repertoire. Mortality after this treatment usually occurs before immune reconstitution and is mainly associated with infections most often acquired pre-transplantation. In this review, we will discuss the current approaches to the diagnosis and management of thymic stromal cell defects, in particular those resulting in athymia. We will discuss the impact of the expanding implementation of newborn screening for T-cell lymphopenia, in combination with next generation sequencing, as well as the role of novel diagnostic tools distinguishing between hematopoietic and thymic stromal cell defects in facilitating the early consideration for thymus transplantation of an increasing number of patients and disorders. Immune reconstitution after the current treatment is usually incomplete with relatively common inflammatory and autoimmune complications, emphasizing the importance for improving strategies for thymus replacement therapy by optimizing the current use of postnatal thymus tissue and developing new approaches using engineered thymus tissue.

RAC2 and primary human immune deficiencies

RAC2 is a GTPase that is exclusively expressed in hematopoietic cells. Animal models have suggested important roles for RAC2 in the biology of different cell types, such as neutrophils and lymphocytes. Primary immunodeficiencies represent "experimentum naturae" and offer priceless insight on the function of the human immune system. Mutations in RAC2 have been identified in a small number of patients giving rise to different forms of primary immunodeficiencies ranging from granulocyte defects caused by dominant negative mutations to combined immunodeficiency due to dominant activating mutations. This review will focus on the clinical and immunologic phenotype of patients with germline mutations in RAC2.

Pharmacometric Analysis of IgPro10 in Japanese and Non-Japanese Patients With Primary Immunodeficiency

PURPOSE

Immunoglobulin (Ig) G replacement therapy, administered intravenously (IVIG) or subcutaneously (SCIG), is the standard treatment in patients with primary immunodeficiencies (PID). We aimed to characterize the pharmacokinetic (PK) characteristics of serum IgG following administration of IgPro10 every 3 or 4 weeks in Japanese patients with PID, and compare with PK in non-Japanese patients. A previously developed population PK (PPK) model was validated, and predicted parameters were compared with the results from the clinical study.

METHODS

The previously developed PPK model, containing IgG concentration data from 5 non-Japanese studies, was supplemented with data from 3 Japanese studies of IgPro10 or IgPro20 to compare the IgG PK parameters between Japanese and non-Japanese patients. The model was externally validated by simulating IgG concentration-time profiles in Japanese patients to predict serum IgG PK characteristics and to compare them with observed Japanese PK data from Study IgPro10_3004.

FINDINGS

The analysis included 4502 serum IgG concentration values (from 34 Japanese and 168 non-Japanese patients). PPK estimates from the current analysis demonstrated a clearance (CL) of 0.139 L/d, central volume (V2) of 4.01 L, inter-compartmental clearance (Q) of 0.30 L/d, and peripheral volume of 3.51 L. These results were consistent with those from the previously published PPK model, with similar bootstrap means and 95% CIs. Goodness-of-fit criteria indicated that the final PPK model was consistent with observed data, with no systemic bias in model prediction. Prediction-corrected visual predictive checks confirmed a good description of data on both SCIG and IVIG. PK parameters were equivalent between Japanese and non-Japanese patients. Body weight was determined to be a significant covariate on both CL and V2. Simulated and observed AUC and maximum and minimum serum IgG concentrations were similar, with 90% CIs overlapping between simulated and observed IgG concentrations in Japanese patients.

IMPLICATIONS

PK parameter estimates of serum IgG were similar between Japanese and non-Japanese patients with PID. The PPK model, updated with Japanese data, was consistent with the previously published PPK model and could accurately predict both individual and population serum IgG concentration-time profiles following IgPro10 IV infusions every 3 or 4 weeks. EudraCT identifier: 2016-001631-12.

Primary immunodeficiencies: novel genes and unusual presentations

Recent advances in genomics have greatly expanded the spectrum of primary immune deficiencies (PIDs). Along with the identification of pathogenic variants in novel genes, distinct phenotypes have been associated with different variants in the same gene. Although PIDs have been historically defined based on increased susceptibility to infections, immune dysregulation has emerged as a frequent and in some cases, predominant phenotype. Autoimmune cytopenias with onset in childhood, lasting longer than 12 months, and affecting multiple lineages should raise the suspicion of a possible PID with monogenic origin. Characterization of the various molecular and cellular mechanisms responsible for these unusual manifestations of PIDs, although at times resource intensive, may allow for targeted intervention in many of them.

Persistent epigenetic memory impedes rescue of the telomeric phenotype in human ICF iPSCs following DNMT3B correction

DNA methyltransferase 3B (DNMT3B) is the major DNMT that methylates mammalian genomes during early development. Mutations in human DNMT3B disrupt genome-wide DNA methylation patterns and result in ICF syndrome type 1 (ICF1). To study whether normal DNA methylation patterns may be restored in ICF1 cells, we corrected DNMT3B mutations in induced pluripotent stem cells from ICF1 patients. Focusing on repetitive regions, we show that in contrast to pericentromeric repeats, which reacquire normal methylation, the majority of subtelomeres acquire only partial DNA methylation and, accordingly, the ICF1 telomeric phenotype persists. Subtelomeres resistant to de novo methylation were characterized by abnormally high H3K4 trimethylation (H3K4me3), and short-term reduction of H3K4me3 by pharmacological intervention partially restored subtelomeric DNA methylation. These findings demonstrate that the abnormal epigenetic landscape established in ICF1 cells restricts the recruitment of DNMT3B, and suggest that rescue of epigenetic diseases with genome-wide disruptions will demand further manipulation beyond mutation correction.

Secondary C1q Deficiency in Activated PI3Kδ Syndrome Type 2

Monogenic forms of vasculitis are rare but increasingly recognized. Furthermore, genetic immunodeficiency is increasingly associated with inflammatory immune dysregulatory features, including vasculitis. This case report describes a child of non-consanguineous parents who presented with chronic digital vasculitis early in life, is of short stature, has facial dysmorphia, immunodeficiency (low serum IgA, high serum IgM), recurrent bacterial infections, lymphoproliferation, absence of detectable serum C1q, and low classical complement pathway activity. We identified a previously reported de novo heterozygous pathogenic splice mutation in PIK3R1 (c.1425 + 1G > A), resulting in the skipping of exon 11 of the p85α subunit of phosphatidylinositol 3-kinase and causing activated PI3Kδ syndrome type II (APDS2). This explained the phenotype, with the exception of digital vasculitis and C1q deficiency, which have never been described in association with APDS2. No mutations were identified in C1QA, B, or C, their promoter regions, or in any other complement component. Functional studies indicated normal monocytic C1q production and release, suggesting that the observed C1q deficiency was caused by peripheral consumption of C1q. Since C1q deficiency has never been associated with APDS2, we assessed C1q levels in two unrelated patients with genetically confirmed APDS2 and confirmed C1q deficiency in those two cases as well. This observation suggests C1q deficiency to be an inherent but previously unrecognized feature of APDS2. We speculate that the consumption of C1q is driven by increased apoptotic bodies derived from immune cellular senescence, combined with elevated IgM production (both inherent features of APDS2). Secondary C1q deficiency in APDS2 may further contribute to immunodeficiency and could also be associated with inflammatory immune dysregulatory phenotypes, such as the digital vasculitis observed in our case.