Monogenic lupus with homozygous C4A deficiency presenting as bronchiectasis and immune-mediated thrombocytopenia.

Monogenic lupus is a subset of lupus caused by single-gene disorders, integrating the paradoxical combination of autoimmunity and immunodeficiency. Pulmonary manifestations with recurrent pneumonia and bronchiectasis have rarely been described as the predominant presentation of juvenile lupus and may suggest an alternate differential like primary immunodeficiency, especially in early childhood. We describe a case of 10-year girl who presented with a history of recurrent pneumonia, arthritis, alopecia, and poor weight gain for the past 2 years. On examination, she had respiratory distress, bilateral diffuse crackles and arthritis of the small joints of hands. Lab investigations showed pancytopenia, low complement levels and high titers of ANA and anti-dsDNA antibodies. The patient was diagnosed with juvenile lupus. Imaging studies revealed evidence of multiple lobar collapse and consolidation with bronchiectasis. She was started on steroids, HCQ and supportive measures for bronchiectasis. The child reported relief in initial symptoms of lupus on follow-up but developed recurrent thrombocytopenia requiring IVIG and escalating the doses of oral steroids. The young age and atypical presentation prompted a screening for monogenic lupus, and clinical exome sequencing revealed a novel homozygous missense variation in exon 20 of the C4Agene with clinically reduced C4 levels, consistent with the diagnosis of C4A deficiency.