Combined immunodeficiency due to TBX1 deficiency

Combined immunodeficiency due to TBX1 deficiency is an extremely rare primary immunodeficiency disorder caused by biallelic (homozygous or compound heterozygous) loss-of-function mutations in the TBX1 gene, which encodes a T-box transcription factor critical for thymic development and immune system function. TBX1 is well known for its role in 22q11.2 deletion syndrome (DiGeorge syndrome) where haploinsufficiency causes variable immunodeficiency, but complete TBX1 deficiency due to biallelic mutations results in a more severe combined immunodeficiency phenotype. The condition primarily affects the immune system, with profound T-cell lymphopenia due to thymic hypoplasia or aplasia, leading to severely impaired cellular and humoral immunity. Patients typically present in infancy with recurrent, severe, and opportunistic infections. Additional features may include congenital heart defects, hypoparathyroidism with hypocalcemia, palatal abnormalities, and facial dysmorphism, reflecting the broader role of TBX1 in pharyngeal arch development. The immunological phenotype can resemble severe combined immunodeficiency (SCID) or complete DiGeorge syndrome. Management depends on the severity of the immune defect. Patients with severe T-cell deficiency may require thymus transplantation or hematopoietic stem cell transplantation to reconstitute immune function. Supportive care includes antimicrobial prophylaxis, immunoglobulin replacement therapy, and management of associated congenital anomalies such as cardiac defects and hypocalcemia. Early diagnosis through newborn screening for T-cell receptor excision circles (TRECs) may facilitate timely intervention. Given the rarity of this condition, management is best guided by specialized immunology centers with experience in primary immunodeficiencies.

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