Diagnosis and treatment of MN1 C-terminal truncation syndrome

MN1 C-terminal truncation (MCTT) is a rare syndrome; only 27 cases have been reported. We report the first case of an 8-year-old girl with MCTT syndrome complicated with moderate obstructive sleep apnea (OSA).

MCTT syndrome was diagnosed by whole-exome sequencing (WES) and validated by Sanger sequencing. The patient received 2 years of treatment with continuous positive airway pressure (CPAP) to relieve sleep apnea and hypoxia, and a reverse sector fan-shaped expander for maxillary expansion.

WES revealed a de novo MN1 variant, c.3760C>T (p.[Q1254*]). An arachnoid cyst was found in the right occipital brain. The patient presented mild symptoms of classic MCTT syndrome. The patient did not experience hearing loss and only mild intellectual disability. Radiological examinations showed cleft secondary palate, narrow upper arch, narrow upper airway, and mandibular skeletal retrusion. Polysomnography indicated moderate OSA, with an apnea/hypopnea index of 6.8, which decreased to 1 after CPAP during the night. Two-year maxillary expansion widened the upper arch, and the cleft secondary palate became visible. The mandible moved forward spontaneously, resulting in the improvement of profile and upper airway widening. General physical conditions, such as motor delay, muscle weakness, and developmental delay, were significantly improved two years later.

In conclusion, we discovered a MN1 variant [NM_002430.2: c.3760C>T, p.Q1254*] that causes mild MCTT symptoms compared to other MN1 variants. For patients with MCTT complicated with OSA, multidisciplinary combination therapy can improve maxillofacial development, widen the upper airway and relieve sleep apnea, improving the general physical condition.