The molecular basis of familial hemolytic uremic syndrome: Mutation analysis of factor H gene reveals a hot spot in short consensus repeat 20

The aim of the present study was to clarify whether factor H mutations were involved in genetic predisposition to hemolytic uremic syndrome, by perfor ming linkage and mutation studies in a large number of patients from those referred to the Italian Registry for Recurrent and Familial HUS/TTP. PCR an d Western blot analyses were conducted to characterize the biochemical cons equences of the mutations. Five mutations in the factor H gene were identif ied. Three, identified in two families and in a sporadic case, are heterozy gous point mutations within the most C-terminal short consensus repeat 20 ( SCR20) of factor H, resulting in single amino acid substitutions. The other two mutations introduce premature stop codons that interrupt the translati on of factor H. A heterozygous nonsense mutation was identified in SCR8 in one family, and a homozygous 24-bp deletion within SCR20 was identified in a Bedouin family with a recessive mode of inheritance. Reverse transcriptio n-PCR analysis of cDNA from peripheral blood leukocytes from the Bedouin fa mily showed that the deletion lowered factor H mRNA levels. Although hetero zygous mutations were associated with normal factor H levels and incomplete penetrance of the disease, the homozygous mutation in the Bedouin family r esulted in severe reduction of factor H levels accompanied by very early di sease onset. These data provide compelling molecular evidence that genetica lly determined deficiencies in factor H are involved in both autosomal-domi nant and autosomal-recessive hemolytic uremic syndrome and identify SCR20 a s a hot spot for mutations in the disease. The mutations identified here gi ve an important hint to the relevance of the C-terminus of factor H in the control of the alternative complement activation pathway.