MED12 Loss-of-Function Variants as a Cause of Congenital Diaphragmatic Hernia in Females With Hardikar Syndrome and Nonspecific Intellectual Disability.

Mediator complex subunit 12 (MED12) is required for the assembly of the kinase module of Mediator, a regulatory complex that controls the formation of the RNA polymerase II-mediated preinitiation complex. MED12-related disorders display unique gender-specific genotype-phenotype associations and include X-linked recessive Opitz-Kaveggia syndrome, Lujan-Fryns syndrome, Ohdo syndrome, and nonspecific intellectual disability in males predominantly carrying missense variants, and X-linked dominant Hardikar syndrome and nonspecific intellectual disability in females known to predominantly carry de novo nonsense/frameshift and nonsense/missense variants, respectively. MED12 was previously identified as a low-penetrance candidate gene for non-isolated congenital diaphragmatic hernia (CDH+). At the time, however, there was insufficient evidence to confirm this association. In a clinical database search, we identified 18 individuals who were molecularly diagnosed with MED12-related disorders by exome or genome sequencing, including eight missense, four frameshift, two nonsense, and one splice variant. Nine of these variants have not been previously reported. Two females with nonspecific intellectual disability were found to carry a de novo frameshift variant, indicating that potentially truncating variants causing nonspecific intellectual disability are not limited to nonsense variants. Notably, CDH was reported in three out of seven females with Hardikar syndrome or nonspecific intellectual disability but was not reported in males with MED12-related disorders. These results suggest that pathogenic MED12 variants are a cause of CDH+ in females with Hardikar syndrome and nonspecific intellectual disability.

Expanded clinical phenotype and untargeted metabolomics analysis in RARS2-related mitochondrial disorder: a case report.

BACKGROUND: RARS2-related mitochondrial disorder is an autosomal recessive mitochondrial encephalopathy caused by biallelic pathogenic variants in the gene encoding the mitochondrial arginyl-transfer RNA synthetase 2 (RARS2, MIM *611524, NM_020320.5). RARS2 catalyzes the transfer of L-arginine to its cognate tRNA during the translation of mitochondrially-encoded proteins. The classical presentation of RARS2-related mitochondrial disorder includes pontocerebellar hypoplasia (PCH), progressive microcephaly, profound developmental delay, feeding difficulties, and hypotonia. Most patients also develop severe epilepsy by three months of age, which consists of focal or generalized seizures that frequently become pharmacoresistant and lead to developmental and epileptic encephalopathy (DEE). CASE PRESENTATION: Here, we describe a six-year-old boy with developmental delay, hypotonia, and failure to thrive who developed an early-onset DEE consistent with Lennox-Gastaut Syndrome (LGS), which has not previously been observed in this disorder. He had dysmorphic features including bilateral macrotia, overriding second toes, a depressed nasal bridge, retrognathia, and downslanting palpebral fissures, and he did not demonstrate progressive microcephaly. Whole genome sequencing identified two variants in RARS2, c.36 + 1G > T, a previously unpublished variant that is predicted to affect splicing and is, therefore, likely pathogenic and c.419 T > G (p.Phe140Cys), a known pathogenic variant. He exhibited significant, progressive generalized brain atrophy and ex vacuo dilation of the supratentorial ventricular system on brain MRI and did not demonstrate PCH. Treatment with a ketogenic diet (KD) reduced seizure frequency and enabled him to make developmental progress. Plasma untargeted metabolomics analysis showed increased levels of lysophospholipid and sphingomyelin-related metabolites. CONCLUSIONS: Our work expands the clinical spectrum of RARS2-related mitochondrial disorder, demonstrating that patients can present with dysmorphic features and an absence of progressive microcephaly, which can help guide the diagnosis of this condition. Our case highlights the importance of appropriate seizure phenotyping in this condition and indicates that patients can develop LGS, for which a KD may be a viable therapeutic option. Our work further suggests that analytes of phospholipid metabolism may serve as biomarkers of mitochondrial dysfunction.