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Selective IgG2 deﬁciency due to a point mutation causing abnormal splicing of the C c 2 gene

The mechanism underlying selective IgG subclass deﬁciency is largely unknown in humans. We have previously reported the acquisition of a complete IgG2 deﬁciency in a leukemia patient after bone marrow transplantation. Southern blot analysis showed a deletion including the C c 2 and C c 4 genes on one chromosome in the donor, suggesting that the remaining C c 2 gene allele was silent. In the patient and his two IgG2 deﬁcient brothers, the silent C c 2 gene showed both germ-line transcription and switch recombination and no structural defects were found in the intronic promoter or the switch region of the gene. However, an A ! G transition in the fourth nucleotide in the 5 9 portion of intron 1 was identiﬁed. Transfection of artiﬁcial constructs into the human B cell lines demonstrated that this A ! G transition inactivated the normal splice site, and instead, a cryptic splice site in the CH1 exon was used in RNA post-transcriptional processing, leading to a 16 bp deletion of the c 2 CH1 exon. This aberrantly spliced RNA that is mostly derived from germ-line transcription in vivo was also detected in both homozygous and heterozygous individuals carrying this mutation. These ﬁndings suggest a novel genetic mechanism as the cause of IgG subclass deﬁciency in selected patients.