Novel STAG2 variant expands Mullegama-Klein-Martinez Syndrome phenotype.

We present a novel case of a female patient with a de novo heterozygous splice site pathogenic variant STAG2 (NM_001042749.3):c.1196 + 4_1196+7del. The STAG2 gene encodes "Stromal Antigen 2" (STAG2), a fundamental subunit of the cohesin complex, which regulates the separation of sister chromatids during cell division. Pathogenic STAG2 gene variants are associated with Mullegama-Klein-Martinez Syndrome (MKMS), a rare X-linked cohesinopathy. This patient exhibits symptoms not previously associated with MKMS, thereby expanding the known clinical phenotype of this rare disease. The patient has severe intellectual disability, microcephaly, short stature, and a single front tooth. She also has an ectopic posterior pituitary gland, resulting in vasopressin deficiency (formerly known as central diabetes insipidus), which was associated with adipsia, causing profound hypernatraemic dehydration and acute renal failure necessitating peritoneal dialysis. STAG2 pathogenic variants should prompt in-depth imaging of the pituitary fossa for pituitary malformations. Clinical and biochemical screening and surveillance should be performed to identify pituitary dysfunction.