The Human AKNA Gene Expresses Multiple Transcripts and Protein Isoforms as a Result of Alternative Promoter Usage, Splicing, and Polyadenylation

Abstract

We previously showed that the human AKNA gene encodes an AT-hook transcription factor that regulates the expression of costimulatory cell surface molecules on lymphocytes. However, AKNA cDNA probes hybridize with multiple transcripts, suggesting either the existence of other homologous genes or a complex regulation operating on a single gene. Here we report evidence for the latter, as we find that AKNA is encoded by a single gene that spans a 61-kb locus of 24 exons on the fragile FRA9E region of human chromosome 9q32. This gene gives rise to at least nine distinct transcripts, most of which are expressed in a tissue-specific manner in lymphoid organs. Many of the AKNA transcripts originate from alternative splicing; others appear to derive from differential polyadenylation and promoter usage. The alternative AKNA transcripts are predicted to encode overlapping protein isoforms, some of which (p70 and p100) are readily detectable using a polyclonal anti-AKNA antisera that we generated. We also find that AKNA PEST-dependent cleavage into p50 polypeptides is targeted to mature B cells and appears to be required for CD40 upregulation. The unusual capacity of the AKNA gene to generate multiple transcripts and proteins may reflect its functional diversity, and it may also provide a fail-safe mechanism that preserves AKNA expression.

AKNA as Genetic Risk Factor for Cervical Intraepithelial Neoplasia and Cervical Cancer

“…Risk of Cervical cancer associated with Q/Q akna genotype The AT-hook akna motif studied in this work is present in all reported akna isoforms except in the C1 and C2 isoforms (Sims et al, 2005). Therefore, the relevance of our observation is valid for the wide range of isoforms potentially expressed by the akna gene (Sims et al, 2005).…”

“…AKNA is a transcriptional factor that is involved in lymphocyte maturation and in the upregulation of signaling molecules, such as CD40L (Siddiqa et al, 2001;Sims et al, 2005). Even though the precise molecular mechanisms for AKNA function have not been defined, AT-hook transcription factors have emerged as multifaceted regulators that can activate or repress broad A/T-rich gene networks.…”

Coordinate activation of inflammatory gene networks, alveolar destruction and neonatal death in AKNA deficient mice

“…In keeping with this notion, the human AKNA gene is a non-HMG transcription factor with N-and C- terminus AT-hook motifs [9,19], whereas its murine ortholog depicts putative N-terminus AT-hook and C-terminus ALM sequences. Although the structural identity of murine AKNA's ALM remains hypothetical and its physiopathological resemblance to the human protein needs to be established, inferred evidence [9,19] suggests that mouse AKNA may be able to bind pathway-specific genes and regulate their expression. Thus, it is possible that AKNA expression plays a role in mechanisms that, if altered, could result in systemic and potentially fatal disorders.…”

“…Thus, it is possible that AKNA expression plays a role in mechanisms that, if altered, could result in systemic and potentially fatal disorders. In keeping with this hypothesis, human AKNA is encoded by a single gene located within the FRA9E region of chromosome 9q32 [19], a common fragile site (CFS) linked to loss-of-function mutations that often lead to inflammatory and neoplastic diseases [20][21][22]. The recent finding that single nucleotide polymorphisms (SNP) within the human AKNA AT-hook domain increase the risk of cervical cancer [23] lends support to this concept.…”

“…Among these, proteins with AT-hook-like motifs (ALMs) have been found to be capable of binding A/T-rich gene targets and regulating their transcription [16][17][18]. In keeping with this notion, the human AKNA gene is a non-HMG transcription factor with N-and C- terminus AT-hook motifs [9,19], whereas its murine ortholog depicts putative N-terminus AT-hook and C-terminus ALM sequences. Although the structural identity of murine AKNA's ALM remains hypothetical and its physiopathological resemblance to the human protein needs to be established, inferred evidence [9,19] suggests that mouse AKNA may be able to bind pathway-specific genes and regulate their expression.…”