Multiple sequences from downstream of the Jκ cluster can combine to recruit somatic hypermutation to a heterologous, upstream mutation domain

Abstract

Recruitment of somatic hypermutation to the Igκ locus has previously been shown to depend on the enhancer elements, Ei/MAR and E3 ′ . Here we show that these elements are not sufficient to confer mutability. However, hypermutation is effectively targeted to a chimeric β‐globin/Igκ transgene whose 5 ′ end is composed of the human β‐globin gene (promoter and first two exons) and whose 3 ′ end consists of selected sequences derived from downstream of the J_κ cluster (Ei/MAR, C_κ + flank and E3 ′ ). Thus, multiple downstream Igκ sequences (all derived from 3 ′ of the J_κ cluster) can combine to recruit mutation to a heterologous mutation domain. The location of this hypermutation domain is defined by the position of the transcription start site and this applies even if the Igκ Ei/MAR is positioned upstream of the promoter. Hotspots within the mutation domain are, however, defined by local DNA sequence as evidenced by a new hotspot being created within the β‐globin domain by a mutation within the transgene. We propose that multiple, moveable Igκ sequences (that are normally located downstream of the transcription start site) cooperate to bring a hypermutation priming factor to the transcription initiation complex; a mutation domain is thereby created downstream of the promoter but the local sequence defines the detailed pattern of mutation within that domain.