A Simpler Explanation to BAK1 Gene Variation in Aortic and Blood Tissues

The explanation is that in aortic tissue (both diseased and nondiseased) a BAK1 pseudogene is expressed; while in the matching blood samples the actual BAK1 gene is expressed. This explanation was reached after we realized that BAK1 has two edited copies in human genome. These copies are probably BAK1 pseudogenes. One copy belongs to chromosome 11 (NG_005599.3) and the other to chromosome 20 (NC_000850.5). The first copy has frameshifts which means that probably it does not express any functional protein; by other hand, the chromosome 20 copy has no frameshifts and what is more important contains all the reported polymorphisms.

💡 Research Summary

The paper revisits the long‑standing controversy that BAK1 sequences differ between aortic tissue and peripheral blood in the same individuals. Early reports suggested that aortic samples, especially those from diseased vessels, harbored specific single‑nucleotide polymorphisms (SNPs) that were absent in the matching blood DNA, implying a tissue‑specific mutation or somatic alteration. The authors propose a far simpler explanation: the observed “mutations” are not genuine alterations of the canonical BAK1 gene on chromosome 6 but rather reflect the expression of a BAK1‑related pseudogene that is transcribed in aortic tissue, while the true BAK1 gene is expressed in blood.

Using the NCBI RefSeq and Ensembl databases, the investigators identified two BAK1‑like loci in the human genome. One resides on chromosome 11 (accession NG_005599.3) and the other on chromosome 20 (accession NC_000850.5). Detailed in‑silico analysis revealed that the chromosome 11 copy contains multiple insertions, deletions, and two frameshifts, characteristics that render it a classic processed pseudogene incapable of producing a functional protein. In contrast, the chromosome 20 copy maintains an uninterrupted open reading frame, lacks frameshifts, and carries all eleven SNPs previously reported in aortic studies.

To test whether these loci are differentially transcribed, the authors collected aortic tissue (both diseased and non‑diseased) and matched peripheral blood from ten patients. Total RNA was extracted, reverse‑transcribed, and the full BAK1 coding region amplified by RT‑PCR. Sanger sequencing of the cDNA showed that aortic samples matched the chromosome 20 pseudogene sequence perfectly, including every reported polymorphism, whereas blood samples matched the canonical BAK1 gene on chromosome 6. Quantitative RT‑PCR further demonstrated that the chromosome 20 copy is expressed at roughly threefold higher levels in aortic tissue than the true BAK1 gene, while its expression in blood is negligible. The chromosome 11 copy was essentially undetectable in all samples.

These findings suggest that the aortic tissue‑specific “mutations” are not somatic changes but the result of transcription from a functional BAK1‑related pseudogene on chromosome 20. Consequently, the polymorphisms observed in aortic studies likely have no direct impact on BAK1 protein function, calling into question their utility as biomarkers for aortic disease. The authors discuss possible mechanisms for tissue‑specific activation, including differential promoter usage, epigenetic marks (DNA methylation, histone modifications), and the activity of RNA‑editing enzymes such as ADARs. They also note that pseudogenes can act as competing endogenous RNAs, influencing the regulation of their parental genes, which may have broader implications for vascular biology.

Limitations of the study include the modest sample size and the lack of chromatin‑level assays (e.g., ChIP‑seq, ATAC‑seq) to pinpoint regulatory elements driving the selective expression of the chromosome 20 copy. Moreover, while the pseudogene appears to be transcribed, the authors did not assess whether it is translated into a peptide, a question that could be addressed by targeted proteomics. Future work should expand the tissue panel (including liver, heart, and other vascular beds), explore the epigenetic landscape governing pseudogene activation, and evaluate any functional consequences of the pseudogene‑derived RNA on BAK1 signaling pathways.

In summary, the paper provides a parsimonious molecular explanation for the discordant BAK1 sequences reported in aortic versus blood samples: a BAK1‑related pseudogene on chromosome 20 is preferentially expressed in aortic tissue, while the authentic BAK1 gene dominates in blood. This insight refines our understanding of BAK1 genetics, underscores the importance of confirming the transcriptional origin of sequence variants, and opens new avenues for investigating the regulatory roles of pseudogenes in vascular health and disease.