Pathogenetic mechanisms of muscle-specific ribosomes in dilated cardiomyopathy (AC16 RNA-seq)

The heart employs a specialized ribosome in its muscle cells to translate genetic information into proteins, a fundamental adaptation with an elusive physiological role. Its significance is underscored by the discovery of neonatal patients suffering from often fatal heart failure caused by rare biallelic variants in RPL3L, a muscle-specific ribosomal protein that replaces the ubiquitous RPL3 in cardiac ribosomes. RPL3L-linked heart failure represents the only known human disease arising from mutations in tissue-specific ribosomes, yet the underlying pathogenetic mechanisms remain poorly understood despite an increasing number of reported cases. While the autosomal recessive inheritance pattern suggests a loss-of-function mechanism, Rpl3l-knockout mice display only mild phenotypes, attributed to up-regulation of the ubiquitous Rpl3. Interestingly, living human knockouts of RPL3L have been identified. Here, we report two new cases of RPL3L-linked severe neonatal heart failure and uncover an unusual pathogenetic mechanism through integrated analyses of population genetic data, patient cardiac tissue, and isogenic cells expressing RPL3L variants. Our findings demonstrate that patient hearts lack sufficient RPL3 compensation. Moreover, contrary to a simple loss-of-function mechanism often associated with autosomal recessive diseases, RPL3L-linked disease is driven by a combination of gain-of-toxicity and loss-of-function. Most patients harbor a recurrent toxic missense variant alongside a non-recurrent variant. The non-recurrent variant is often loss-of-function and enables partial compensation through RPL3, similar to Rpl3l-knockout mice. However, the recurrent missense variant exhibits increased affinity for the RPL3/RPL3L chaperone GRWD1 and 60S biogenesis factors, retains 28S rRNA in the nucleus, disrupts ribosome biogenesis, and induces severe cellular toxicity that extends beyond the loss of ribosomes. These findings elucidate the pathogenetic mechanisms underlying muscle-specific ribosome dysfunction in neonatal heart failure, providing critical insights for genetic screening and therapeutic development. Our findings also suggest that gain-of-toxicity mechanisms may be more widespread in autosomal recessive diseases, especially for those involving genes with paralogs.