Congenital and childhood sensorineural hearing loss (SNHL) is a multifactorial disease that severely impacts quality of life. The single most important etiology of congenital SNHL is prenatal human cytomegalovirus (HCMV) infection, which accounts for 20-30% of all deafness in infants and children. Approximately 1 in 150 children is born with congenital HCMV, and 1 in 5 of these children will be born with or will develop permanent neural disabilities, the most common of which is SNHL. SNHL can be either bilateral or unilateral, and the severity of the hearing loss and its progression varies widely. Although the association of congenital HCMV infection and SNHL has been recognized for 50 years, how infection induces the hearing loss is unknown. Since the target of HCMV is cells of the neural lineage, a major goal of our research is to understand at high-resolution the effects of HCMV infection on neural lineage specification and maturation of stem and progenitor cells. Elucidation of the genes and cellular processes that are affected will serve as a basis for therapeutic strategies to ameliorate the effects of HCMV infection in newborns. The significance of our studies also extends to the problem of HCMV infection in immunocompromised individuals, with recipients of allogeneic transplants having a high risk of severe disease and allograft rejection. The proposed use of stem cell transplantation in treating neuronal injury and neurodegenerative diseases, as well as transplantation of other organ-specific precursors, makes it imperative to understand how disseminated HCMV infection in immunosuppressed recipients will affect the function and differentiation of the cells.
This past year, we made significant progress in accomplishing the goals of this project. To study in greater depth the molecular basis of the interaction of HCMV with cells of the neural lineage, we continued high-throughput genomics approaches to analyze HCMV microRNAs, alterations in cellular microRNA and gene expression profiles, and global defects in host alternative splicing in infected and uninfected pNSC-derived NPCs. We also analyzed our infected NPC RNA-seq database for differences in host mRNA polyadenylation patterns and found that over a hundred transcripts were significantly altered in terms of their 3′ end cleavage site preference, with the majority of these events resulting in shortened 3′ UTRs.
One of our most striking findings was that there was strongly enhanced expression of three miRNAs that play a critical role in auditory development. Other genes involved in cochlear development were also dysregulated by infection. These results implicate a novel molecular mechanism of damage to the developing inner ear of congenitally infected children, based on altered developmental gene regulation. These findings coupled with our success in inducing differentiation of human ES into otic progenitors that can be further differentiated to hair cell-like cells and immature auditory neurons have provided a strong foundation to pursue in depth HCMV infection of auditory progenitor cells, with the goal of determining how infection compromises the gene expression and function of human ES-derived inner ear cells and to use this information to develop new strategies for the treatment and prevention of hearing loss in congenitally infected children.
Regrettably, we are not able to continue these highly important studies, as this CIRM grant ended and a new proposal to CIRM was not funded. This was very disappointing because there is a gap in the portfolio of grants awarded by CIRM, which is a lack of attention to important problems relating to Child Health. One of the most important areas in stem cells that have been neglected is congenital sensorineural hearing loss. Only 3 grants relating to hearing loss have ever been awarded by CIRM. Yet the incidence of just congenital hearing loss (not including hearing loss from other causes or associated with aging) is 400 in 100,000 newborns. Moreover, the disability will last for life, causing a significant burden to the patient and family and a high economic cost to society. There is no drug or vaccine to prevent congenital hearing loss due to CMV, and thus it is essential develop new therapeutic strategies, including stem cells, gene targeting, and drug discovery. Unfortunately, there is no accepted mechanism by which HCMV infection leads to hearing loss, and a lack of public awareness regarding the serious medical problems resulting from congenital HCMV Infection has made it difficult to garner support for studies to identify an etiology.
We appreciate the support CIRM has provided and hope that in the future there will be sufficient public awareness of the devastating effects of congenital HCMV infection to bring pressure upon funding agencies to recognize and support this important research.