Submission Details

Agammaglobulinemia is a primary immunodeficiency characterized by low or absent serum immunoglobulins and circulating B cells. Affected individuals present an early onset of severe infections. The most common form of agammaglobulinemia is X-linked agammaglobulinemia which is caused by mutations in the BTK gene and accounts for 85 to 95% of male patients. Autosomal recessive agammaglobulinemia is a genetically heterogeneous disorder and accounts for up to 15% of patients with the characteristic signs of agammaglobulinemia (PMID: 17709424).

Mutations in the BRWD1 gene were first reported in association with autosomal dominant agammaglobulinemia with incomplete penetrance (MONDO:0015977) in 2018 (PMID: 30250168). Mutations in this gene have also been reported in association to Primary Ciliary Dyskinesia (MONDO:0016575) in 2021 (PMID: 33389130).

Per criteria outlined by the ClinGen Lumping and Splitting Working Group and due to the differences in molecular mechanism and inheritance patterns, and the obvious phenotypic variability, the ClinGen Motile Ciliopathy GCEP working group estimated that Agammaglobulinemia (MONDO:0015977) and Primary Ciliary Dyskinesia (MONDO: 0016575) represent distinct diseases and decided to split the entities.

BRWD1 is a protein containing 8 WD repeats and 2 bromodomains. While WD repeats have diverse functions (PMID: 11814058), the bromodomains are particularly involved in histone recognition (PMID: 22464331). The gene encoding the BRWD1 protein is located in the Down Syndrome critical region of chromosome 21 which shows conserved synteny with the mouse chromosome 16.

Evidence from 3 probands reported in only one publication (PMID: 30250168) were included in this curation. Heterozygous variants were identified in the three patients (1 frameshift, 1 splice and 1 missense) and two among the three patients’ mothers were asymptomatic carriers. The authors suggested “autosomal dominant with incomplete penetrance” mode of inheritance. With this publication being the only report of BRWD1 mutations in association with Agammaglobulinemia, we reached only 1 point for the genetic evidence of this curation.

BRWD1 is expressed in the tissues of the Immune System (bone marrow, spleen, lymph node, etc.) manifesting a similar expression pattern of other genes associated with Agammaglobulinemia (e.g., BTK, BLNK) (PMID: 25613900).

Abundant experimental evidence supports the nuclear localization of BRWD1 in cells from different tissues and notably in B cell progenitors of wild-type mice (PMID: 26301565, PMID: 25547156, PMID: 12889071). *BRWD1 *demonstrates an expression pattern similar to that of Igk throughout B cell development. Besides immunostaining, coimmunoprecipitation showed that BRWD1 associates with BRG1 which is a component of a chromatin remodeling complex (PMID: 12889071). This biochemical evidence suggests that BRWD1 binds (through its bromodomains) to chromatin in B cell progenitors to regulate the Igk chain recombination.

Brwd1 mutated mice were generated through Ethylnitrosourea (ENU) mutagenesis. In these mice, the generated splice mutation causes the skipping of the exon 10 and leads to the expression of a truncated BRWD1 protein (PMID: 26301565). The Brwd1 mutant mice showed diminished frequencies of later-stage B cell progenitors, a reduction in the IgM+ B cell population, small spleens, decreased numbers of total splenocytes, but no defects in other hematopoietic lineages and thymocytes (PMID: 26301565).

Compared to wild-type mice, RNA-seq results show that mutant mice have a clustering of upregulated and downregulated genes in small pre-B cells, which suggests that BRWD1 orchestrates transcriptional programs of late B-cell development (PMID: 26301565). Semi-quantitative PCR analysis of Igk rearrangement in small pre-B cells from wild- type and Brwd1 mutant mice shows a diminished Vκ-Jκ recombination. This highlights the role of BRWD1 in Igk recombination (PMID: 26301565). ChIP-qPCR analysis of the Igk region in Small pre-B cells shows that H4K16ac (acetylation at the 16th lysine residue of the histone H4 protein) is almost absent in Brwd1 mutant mice compared to a robust expression in wild-type. This suggests that BRWD1 binds to Igk and facilitates chromatin decompaction (PMID: 26301565).

In summary, there is limited evidence to support this gene-disease relationship. Although more evidence is needed to support a causal role, no convincing evidence has emerged that contradicts the gene-disease relationship. This classification was approved by the ClinGen Motile Ciliopathy GCEP on December 8, 2022 (SOP Version 9).

ClinGen Antibody Deficiencies GCEP Addendum:

Early and late stages of B cell development in the bone marrow is complex and sequential and involves immunoglobulin gene rearrangement as a key event. There are several transcription factors (TFs) that regulate B cell fate decisions, and these TFs target specific enhancers, which interact with cognate gene promoters to regulate specific gene expression programs. The BROMO and WD40 domains of BRWD1 are essential for preparing the Ig kappa locus, assembly of RAG genes and subsequent Igkappa recombination. The expression of BRWD1 has been shown to be lineage- and stage-specific. It has also been shown that BRWD1 regulates a genome-wide reordering of enhancer accessibility resulting in opening of those critical for late B cell lymphopoiesis to TF binding (Mandal et al., 2018, PMID: 30250168).

BRWD1 pathogenic variants have been identified in 4/50 patients with hypogammaglobulinemia. These patients also had either undetectable or very low peripheral B cell counts. Mandal et al. (2018) postulate that BRWD1 pathogenic variants are common in patients with idiopathic hypogammaglobulinemia. Also, BRWD1-mutated patient cell-derived data reveal similar transcriptional and epigenetic profiles as those observed in the Brwd1-/- (null) mice. All 4 patients had pan-hypogamma (pediatric and adult patients) with Giardia infections, sinusitis. One patient each had Norovirus and Campylobacter infections. All patients also had low class-switched memory B cells (PMID: 30250168).

Two of the 4 patients in Mandal et al. (2018) had a variant in BRWD1 predicted to affect splicing 3’ of exon 22. Their mother had the same variant but no phenotype suggesting incomplete penetrance. One patient had an indel in exon 21 leading to a frameshift and premature stop codon. The 4th patient had a missense variant in exon 17, encoding the WD40 domain. The clinical and immunological phenotype of these patients supports a humoral deficiency association with heterozygous BRWD1 variants (autosomal dominant) (PMID: 30250168).

BRWD1 is currently not included in Table 3 ('Predominantly Antibody Defects') of the 2022 IUIS classification (Tangye et al., 2022, PMID: 35748970).