The rare X-linked female-restricted Hardikar syndrome (HDKR, OMIM # 301068) is characterized by multiple congenital anomalies including orofacial clefts, gastrointestinal, genitourinary, and cardiac anomalies, but cognitive and neurobehavioral development is rarely impaired. HDKR is caused by heterozygous frameshift, splice or nonsense variants in the MED12 gene. Besides HDKR, MED12 pathogenic variants cause a broad spectrum of developmental disorders, collectively referred to as MED12-related disorders, including Opitz-Kaveggia syndrome or FG syndrome type 1 (OKS, OMIM #305450), Lujan-Fryns syndrome (MRXSLF, OMIM #309520), X-linked Ohdo syndrome (OHDOX, OMIM #300895) and isolated intellectual disability. Here we report four individuals with HDKR, including the first of maternally inherited HDKR, and we review molecular and clinical data from 33 individuals with HDKR and 215 individuals with other MED12-related disorders retrieved through a literature and public database search. We highlight sella turcica cysts as a new Hardikar syndrome-related feature, and we introduce clinical guidelines for the diagnosis and management of individuals with HDKR.

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.

We describe the phenotype of 22 male patients (20 probands) carrying a hemizygous missense variant in MED12. The phenotypic spectrum is very broad ranging from nonspecific intellectual disability (ID) to the three well-known syndromes: Opitz-Kaveggia syndrome, Lujan-Fryns syndrome, or Ohdo syndrome. The identified variants were randomly distributed throughout the gene (p = 0.993, χ2 test), but mostly outside the functional domains (p = 0.004; χ2 test). Statistical analyses did not show a correlation between the MED12-related phenotypes and the locations of the variants (p = 0.295; Pearson correlation), nor the protein domain involved (p = 0.422; Pearson correlation). In conclusion, establishing a genotype-phenotype correlation in MED12-related diseases remains challenging. Therefore, we think that patients with a causative MED12 variant are currently underdiagnosed due to the broad patients' clinical presentations.

MED12-related disorders are a rare group of intellectual disability syndromes with a broad range of phenotypic characteristics. The phenotypic spectrum of MED12-related disorders currently includes X-Linked Ohdo Syndrome, Lujan-Fryns Syndrome (LS), and FG syndrome type 1 (FG), also known as Opitz-Kaveggia Syndrome. The MED12 gene encodes the largest component of the mediator complex of RNA polymerase II, which is critical for recruiting activators and repressors to regulate the transcription of genes critical to growth, development, and differentiation. We performed a systematic literature review of previously published cases to highlight the key ocular features in individuals with MED12-related disorders. In addition, we present a new case of a female patient with a de novo pathogenic c. 3866A>G, p.Q1289R variant. Ocular manifestations are not uncommon in MED12-related disorders, but have not been characterized in literature reports. Commonly reoccurring reported eye and ocular adnexa features within the spectrum include ptosis, downslanting palpebral fissures, and hypertelorism. Other less common findings include strabismus, astigmatism, and optic nerve hypoplasia. Our patient presented with developmental delay, mild hypotonia and dysmorphic features including frontal bossing, high arched palate, and syndactyly of the 2nd and 3rd toes bilaterally. Ocular manifestations identified in this patient included intermittent esotropia, hyperopic astigmatism, epicanthal folds and ptosis bilaterally.

Pathogenic variants in the MED12 gene located on the X-chromosome have primarily been reported in males with Lujan-Fryns syndrome, Ohdo syndrome and the Opits-Kaveggia syndrome. However, earlier reports of female patients and female mice suggest that MED12 deficiency causes severe malformations. We report a novel example of a MED12 de novo nonsense variant in a female fetus with severe malformations identified by trio-exome sequencing. This finding further expands the clinical spectrum of MED12-related disorders, which is vital for prenatal diagnosis and genetic counselling of couples.