3MC syndrome is a rare autosomal recessive disorder with characteristic craniofacial dysmorphism and multiple anomalies. It is caused by biallelic mutations in one of three genes, MASP1, COLEC11 and COLEC10, all encoding factors of the lectin complement pathway. In MASP1, either truncating mutations or missense variants in exon 12 encoding the C-terminal serine protease domain specific for isoform MASP-3 are causative. By trio exome sequencing we now identified a novel, homozygous 2kb deletion, partially affecting exon 12 in an adult female with the typical facial gestalt of 3MC syndrome and hearing loss, but without the main feature cleft lip/palate, and without intellectual disability, or short stature. We therefore expand the MASP1 associated mutational and clinical spectrum and describe the development of her clinical presentation over a period of 21 years. As the homozygous deletion in our patient was only found by thorough and visual evaluation of the whole exome sequencing data, such deletions might escape detection in some routine diagnostic workflows and might explain a few of the so far molecularly unconfirmed cases of 3MC syndrome.

The 3MC syndromes are a group of rare autosomal recessive disorders where the main clinical features are cleft lip and palate, hypertelorism, highly arched eyebrows, caudal appendage, postnatal growth deficiency, and genitourinary tract anomalies. Ophthalmological abnormalities, most notably anterior chamber defects may also be seen. We describe the clinical and molecular findings in 13 individuals with suspected 3MC syndrome from 12 previously unreported families. The exclusion of the MASP1 and COLEC11 Loci in two individuals from different consanguineous families and the absence of mutations in four further individuals sequenced for both genes raises the possibility that that there is further genetic heterogeneity of 3MC syndrome.