RESUMEN
INTRODUCTION: Complement deficiencies are rare primary immunodeficiency disorders, the diagnosis of which is often underestimated. Only a small number of molecular studies have been carried out for the characterization of the underlying genetic defects in these cases. PURPOSE: Reporting the first family from the Arabian Gulf region with multiple members affected by meningococcemia and abscent serum complement 5 (C5). We tried to correlate clinical, biochemical and molecular genetics features of this family. METHODS: Determination of the serum level of all complement proteins including the terminal cascade (C5-9), followed by mutation analysis on DNA extracted from fresh blood samples of each alive family member. RESULTS: Molecular studies showed a homozygous nonsense mutation in exon 1, with the change of cytosine to thymine at position 55 (55C > T) leading to change of the glutamine amino acid at position 19 to a stop codon (Q19X), and serologically absence of C5 in the serum. A similar but compound heterozygous mutation has been reported in one African-American family. previously. CONCLUSION: Characterization of the underlying mutations in C5 deficient families is important, to understand this uncommon complement deficiency, and try to elucidate structure-function relationships in the C5 gene. This report also highlights the importance of complement screening in cases of sporadic meningococcal Infections, especially in communities with high prevalence of consanguineous marriages, which will ensure timely and adequate clinical interventions.