[Comprehensive diagnosis and genetic analysis of two children with ring chromosome 18]

To clarify the genetic diagnosis of two children with ring chromosome 18 and explore their mechanisms and clinical phenotypes.

Two patients treated at the Children's Hospital of Henan Province respectively in June 2022 and March 2023 were selected as the study subjects. Genetic testing and diagnosis were carried out through copy number variation sequencing (CNV-seq), G-banded chromosomal karyotyping, and whole exome sequencing (WES).

Child 1 had mainly manifested developmental delay, white matter hypoplasia, type 1 diabetes mellitus, and micropenis. He was found to have a chromosomal karyotype of 46,XY,r(18)(p11.21q22.1)[40]/46,XY[7], and CNV-seq results showed that he has a 14.86 Mb deletion at 18p11.21p11.32 and a 14.02 Mb deletion at 18q22.1q23. Child 2 had peculiar facial features, delayed white matter myelination, developmental delay, atrial septal defect, severe sensorineural deafness, and congenital laryngeal stridor. He was found to have a chromosomal karyotype of 46,XY,r(18)(p11.2q23). CNV-seq result proved that he had a 14.86 Mb deletion at 18p11-21p11.32 and a 20.74 Mb deletion at 18q21.32q23. WES has failed to detect single nucleotide variants (SNVs) in either child, but revealed a large segmental deletion at chromosome 18 in both of them.

Both children were diagnosed with ring chromosome 18 syndrome. The different size of the deletional fragments in the 18q region and mosaicism of ring chromosome 18 in child 1 may underlay the variation in their clinical phenotypes. The type 1 diabetes mellitus and micropenis noted in both children are novel features for ring chromosome 18 syndrome.