Combined methylmalonic acidemia and homocystinuria (cblC) is the most common inborn error of intracellular cobalamin metabolism and due to mutations in Methylmalonic Aciduria type C and Homocystinuria (MMACHC). Recently, mutations in the transcriptional regulators HCFC1 and RONIN (THAP11) were shown to result in cellular phenocopies of cblC. Since HCFC1/RONIN jointly regulate MMACHC, patients with mutations in these factors suffer from reduced MMACHC expression and exhibit a cblC-like disease. However, additional de-regulated genes and the resulting pathophysiology is unknown. Therefore, we have generated mouse models of this disease. In addition to exhibiting loss of Mmachc, metabolic perturbations, and developmental defects previously observed in cblC, we uncovered reduced expression of target genes that encode ribosome protein subunits. We also identified specific phenotypes that we ascribe to deregulation of ribosome biogenesis impacting normal translation during development. These findings identify HCFC1/RONIN as transcriptional regulators of ribosome biogenesis during development and their mutation results in complex syndromes exhibiting aspects of both cblC and ribosomopathies.

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Combined methylmalonic acidemia and homocystinuria (cblC type), an inherited disorder of cobalamin (vitamin B12) metabolism, is a rare metabolic and multi‐systemic disease caused by mutations in MMACHC. Patients with cblC can have severe neurodevelopmental defects including microcephaly, hydrocephaly, and seizures as well as renal, cardiac and hematological defects. Recently, two new variants of cblC were discovered and termed cblX and cblX‐like. Rather than being due to mutations in MMACHC, cblX and cblX‐like result from homozygous mutations in the transcription cofactor HCFC1 and its transcription factor partner RONIN (THAP11), respectively. Patients with either HCFC1 or RONIN mutations were shown to have a dramatic reduction in MMACHC transcription. We have known that HCFC1 is an obligatory partner for RONIN and we have previously shown that the HCFC1/RONIN transcriptional complex directly regulates mouse Mmachc expression. These findings suggest that cblX and cblX‐like disorders comprise a novel family of rare and severe cblC‐like disorders that are transcriptional in nature. To better understand the cellular and molecular mechanisms underlying the pathophysiology of these devastating neurodevelopmental diseases, we have generated Hcfc1A115V and RoninF80L mice, which have the same point mutations observed in cblX and cblX‐like patients. Both Hcfc1Y/A115V hemizygous and RoninF80L/F80L homozygous mice exhibit defects in cobalamin metabolism consistent with an inherited vitamin B12 disorder, as well as defects that recapitulate those observed in the human cblX and cblX‐like patients, including severe brain developmental defects, cardiac malformations, and anemia. Additionally, these mice also exhibit craniofacial deformities, a developmental phenotype that has not been previously linked to vitamin B12. Our data identifies, for the first time, a role for RONIN/HCFC1 in craniofacial development. Together, the phenotypic and molecular data confirm that the RoninF80L and Hcfc1A115V mouse models will serve as powerful tools to further uncover the pathophysiology of this complex family of diseases, as well as to achieve a better understanding of the complexities of craniofacial development. Support or Funding Information This work is supported by R01 DE028298.

Combined methylmalonic acidemia and homocystinuria (cblC type), a form of inherited intracellular vitamin B12 deficiency, is a rare metabolic and multi‐systemic disorder caused by mutations in MMACHC. Patients with cblC can have severe neurodevelopmental defects including microcephaly, hydrocephaly and seizures as well as renal, cardiac and hematological defects. Mutations in MMACHC have been the sole known causes of cblC, until recently when an X‐linked variant of the disease was described, termed cblX. This variant was found to be due to mutations in the X‐linked gene coding for the transcription cofactor HCFC1, which is known to be an obligatory partner for the transcription factor RONIN (THAP11). Intriguingly, a single patient exhibiting cblX‐like findings has been found to carry a mutation in RONIN. Patients, with both HCFC1 and RONIN mutations, were shown to have a dramatic reduction in MMACHC transcription. We have previously shown that the Hcfc1/Ronin transcriptional complex directly regulates mouse Mmachc expression. These findings suggest that cblX and the new Ronin (THAP11) disorder comprise a novel family of rare and severe cblC‐like disorders that are transcriptional in nature. As a result, we have generated a mouse model that carries the human mutation in Ronin (RoninF80L), in an effort to better understand the cellular mechanisms underlying the pathophysiology of these devastating neurodevelopmental diseases. Here we report the generation of the first mouse model, RoninF80L along with its phenotypic and molecular characterization. RoninF80L homozygous mice die soon after birth and exhibit severe brain developmental defects that recapitulate those observed in the human cblX and Ronin‐deficient patients. Moreover, consistent with a vitamin B12 deficiency, cells from RoninF80L homozygous embryos exhibit defects in cobalamin metabolism. Surprisingly however, RNA‐seq and ChIP‐seq analyses also revealed a role for ribosome biogenesis in the pathophysiology of the disease. Furthermore, we were able to show that there is also a functional deficit in ribosome biogenesis and hence protein translation. This identifies for the first time, a role for Ronin in ribosome biogenesis. Together the phenotypic and molecular data confirm that the RoninF80L mouse model will serve as a powerful tool to further uncover the pathophysiology of this complex family of diseases.Support or Funding InformationBaylor College of Medicine, Department of Molecular Physiology and Biophysics (Seed)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.