CD55-deficiency in Jews of Bukharan descent is caused by the Cromer blood type Dr(a-) variant

Abstract

The complement system regulator CD55 was initially found to carry the Cromer blood group system antigens, and its complete loss of function was subsequently revealed to cause a severe monogenic gastrointestinal syndrome characterized by protein-losing enteropathy and susceptibility to venous thrombosis. Here we present homozygosity to the CD55 c.596C>T; p.Ser199Leu variant, which was previously described as the Cromer Dr(a-) genotype, in two Bukharan Jewish CD55-deficiency patients with variable disease severity. We confirm that this missense variant causes aberrant splicing and deletion of 44 bp in exon 5, leading to premature termination and low expression of the CD55 protein. Furthermore, Patient 1 exhibited a mildly abnormal B cell immunophenotyping profile. By population screening we established that this variant is highly prevalent in the Bukharan Jewish population, with a carrier frequency of 1:17, suggesting that many similar patients are un- or mis-diagnosed. The phenotypic variability, ranging from abdominal pain when eating a high-fat diet to the full CD55-deficiency phenotype, is likely related to modifiers affecting the proportion of the variant that is able to escape aberrant splicing. Establishing the diagnosis of CD55-deficiency in a timely manner, even in patients with milder symptoms, may have a critical effect on their management and quality-of-life since treatment with the complement inhibitor eculizumab is highly effective in ameliorating disease manifestations. Awareness of founder mutations within certain populations can further guide genetic testing and prevent a diagnostic odyssey, by placing this CD55 variant high on the differential diagnosis.

Fig. 1

Abnormal intestinal resections in a CD55-deficiency patient. Gastrointestinal resections from Patient 1 revealed a structural abnormalities in intramural veins; b intimal lesion involving mesenteric veins consistent with recanalized vessels; and c inflammatory polyposis

Fig. 2

Loss of CD55 expression in c.596C>T homozygosity. Staining whole blood for surface CD55 revealed that patient cells had lower CD55 expression (red) compared to a healthy control (blue). Isotype control is shown in gray. The panels are showing CD55 staining while gating on different cell populations based on SCC and FSC size (lymphocytes in upper panel, monocytes in middle panel and neutrophils in lower panel). Numbers show mean fluorescent intensity (MFI) levels

Fig. 3

The CD55 c.596C>T variant causes abnormal splicing of exon 5. a The c.596C>T; p.Ser199Leu variant affects weakly conserved nucleotide and amino acid; and b the c.596C>T substitution abolishes an SF2/ASF splicing motif sequence in exon 5; c RNA analysis reveals the hypomorphic nature of c.596C>T. Despite homozygosity for the variant, Patient 1 (P1) cDNA comprises of two mRNA isoforms—one in the predicted size (as in the control samples), which contains the missense change, and another 44 bp shorter, caused by abnormal splicing of exon 5; d densitometry analysis of the cDNA PCR product for Patient 1 reveals an abundance of 88% of the misspliced mRNA isoform (44 bp deletion—c.580_623del); e qRT-PCR analysis using isoform-specific primers confirms the densitometry findings with the misspliced isoform showing 9-times higher expression than the correctly spliced isoform; f a schematic representation of the splicing aberration caused by c.596C>T, which leads to deletion of 44 bp in the 5’ end of exon 5 (depicted in stripes) and causes a frameshift with a premature stop codon (c.580_623del; p.Tyr194Glnfs*7)

Fig. 4

Abnormal B-cell immunophenotyping. B-cell immunophenotyping shows a skewed maturation pattern with a low percent of CD27⁺ memory B cells, and b increased percent of transitional CD38^highCD24^high and naïve memory CD38^dimCD24^low B-cells, and low percent of CD38^lowCD24^high memory cells. Despite that, c CD27⁺ B cells showed the normal percent of IgG⁺ and IgA⁺ class-switched B cells