Relevance of Pseudogenes to Human Genetic Disease

Abstract Pseudogenes, regarded as ‘plagiarised slip‐ups’ or ‘genomic fossils’, are the dysfunctional genomic copies lingering on in the genome for millions of years. The nonfunctional relics of their functional counterparts are now being recognised as genomic treasures presenting glimpses of the evolutionary chronicles of human genome. The recent flurry of experimental evidences indicating sequence conservation of the pseudogenes constructively regulating their functional homologues are now challenging their ‘junk DNA’ appearance. Pseudogenes are observed to harbour sequence variations that become degenerative disease‐causing mutations when transmitted to their colocalised progenitors through gene conversion event. The issue of pseudogene deregulation in several genetic diseases including cancer is now of the essence in the context of disease progression in humans. Different aspects of the involvement of pseudogenes and their relevance to human genetic disease are recaptured here. Key Concepts: A genetic disorder, which may or may not be heritable, is a disease caused by genomic or chromosomal aberrations. An inherited disease is a genetic disorder that is passed down from the parents' genes to that of the offspring. For example, some forms of cancer may be originated as an inherited genetic condition in some people. Duplicated pseudogenes arise due to unequal crossing over between two homologous chromosomes (during the process of DNA replication) followed by nondeleterious mutations. Processed pseudogenes, often termed as ‘dead on arrival’, are ensued by reverse transcription of mature mRNAs and reinclusion of the cDNAs into the genome. Pseudogenes may interfere with factors regulating the mRNA stability; provide mechanistic linkage between their expression and disease formation and thus anomalous pseudogene expression can be indicative of different physiological conditions, including diseases like diabetes and cancer. Gene conversion involves the unidirectional transfer of genetic material from a ‘donor’ sequence (intact homologous sequence) to a highly homologous ‘acceptor’ containing the ‘double strand break’ and can occur between sister chromatids, homologous chromosomes or homologous sequences on either the same chromatid or different chromosomes. Pseudogenes that are colocalised with their progenitors were seen to be the potential candidates of gene conversion event and thus the sequence variations accumulated in pseudogenes turn out to be disease‐causing mutations when they are transferred to other genes rendering them nonfunctional. Some pseudogenes were observed to post‐transcriptionally regulate their parental genes by three distinct mechanisms: (1) suppression of gene expression by natural antisense RNA; (2) RNA interference by producing siRNAs and (3) act as miRNA decoys. Pseudogenes can partially retain or totally resurrect their original functions. Being the paradigm of neutral evolution pseudogenes provide snapshots of the evolutionary history of the human genome. The neutral characteristic of all pseudogenic regions renders them relevant to determine the nature of neutral sequence evolution among different regions in the genome.