Expansion of CD22lo B cells in the spleen of autoimmune-prone flaky skin mice

Multiple intestinal atresia with combined immune deficiency

PURPOSE OF REVIEW

To update on the molecular and cellular basis of multiple intestinal atresia (MIA).

RECENT FINDINGS

Mutations of the tetratricopeptide repeat domain 7A gene have been identified in patients with MIA and other related disorders, including MIA associated with combined immunodeficiency and very early onset inflammatory bowel disease with apoptotic enterocolitis. Pathological findings in patients with MIA and MIA associated with combined immunodeficiency include abnormalities of enterocyte apicobasal polarity, increased apoptosis of intestinal cells, decreased proliferation of intestinal crypts, and defects of thymic architecture associated with lymphoid depletion. Dysregulated RhoA signaling and defective expression of phosphatidylinositol 4-kinase IIIα represent biochemical cellular hallmarks of the disease.

SUMMARY

The study of patients with MIA and related disorders has established that tetratricopeptide repeat domain 7A plays a critical role in intestinal and immune homeostasis. Identification of biochemical defects may pave the way to novel pharmacological interventions for this group of severe congenital disorders.

Whole-exome sequencing identifies tetratricopeptide repeat domain 7A (TTC7A) mutations for combined immunodeficiency with intestinal atresias

BACKGROUND

Combined immunodeficiency with multiple intestinal atresias (CID-MIA) is a rare hereditary disease characterized by intestinal obstructions and profound immune defects.

OBJECTIVE

We sought to determine the underlying genetic causes of CID-MIA by analyzing the exomic sequences of 5 patients and their healthy direct relatives from 5 unrelated families.

METHODS

We performed whole-exome sequencing on 5 patients with CID-MIA and 10 healthy direct family members belonging to 5 unrelated families with CID-MIA. We also performed targeted Sanger sequencing for the candidate gene tetratricopeptide repeat domain 7A (TTC7A) on 3 additional patients with CID-MIA.

RESULTS

Through analysis and comparison of the exomic sequence of the subjects from these 5 families, we identified biallelic damaging mutations in the TTC7A gene, for a total of 7 distinct mutations. Targeted TTC7A gene sequencing in 3 additional unrelated patients with CID-MIA revealed biallelic deleterious mutations in 2 of them, as well as an aberrant splice product in the third patient. Staining of normal thymus showed that the TTC7A protein is expressed in thymic epithelial cells, as well as in thymocytes. Moreover, severe lymphoid depletion was observed in the thymus and peripheral lymphoid tissues from 2 patients with CID-MIA.

CONCLUSIONS

We identified deleterious mutations of the TTC7A gene in 8 unrelated patients with CID-MIA and demonstrated that the TTC7A protein is expressed in the thymus. Our results strongly suggest that TTC7A gene defects cause CID-MIA.

CD22: an inhibitory enigma

CD22 is an inhibitory coreceptor of the B-cell receptor (BCR), and plays a critical role in establishing signalling thresholds for B-cell activation. Like other coreceptors, the ability of CD22 to modulate B-cell signalling is critically dependent upon its proximity to the BCR, and this in turn is governed by the binding of its extracellular domain to alpha2,6-linked sialic acid ligands. Manipulation of CD22 ligand binding in various experimental settings has profound effects on B-cell signalling, but as yet there is no complete model for how ligand binding in vivo controls normal CD22 function. Several elegant studies have recently shed light on this issue, although the results appear to suggest two mutually exclusive models for the role of ligand binding; in either promoting or inhibiting, CD22 function. We shall therefore discuss these results in detail, and suggest possible approaches by which these conflicting experimental findings might be reconciled. We shall also consider a second important issue in CD22 biology, which relates to the role that defects in this receptor might play in mediating autoimmune disease. We review the current evidence for this, and discuss the importance of genetic background in modifying CD22 function and predisposition to autoimmunity.