Tetrasomy 18p syndrome is an extremely rare chromosomal disorder that is often the result of an additional isochromosome for the short arm of chromosome 18. Most cases result from de novo mutations, although familial cases have also been reported. In this article, we report for the first time a case of a 17-year-old female patient with mosaic tetrasomy 18p. This study aimed to investigate the genetic and clinical characteristics of patients with tetrasomy 18p and to enhance the understanding of tetrasomy 18p. A female patient presenting with oligomenorrhea, distinctive physical features, and intellectual disability underwent chromosomal karyotype analysis via G-, C-, and N-banding techniques. Upon the detection of abnormal results, high-throughput genomic copy number variation sequencing (CNV-seq) was performed to establish a definitive diagnosis. Additionally, chromosomal karyotype analysis was conducted on both parents and the patient's three younger sisters. The G-banding karyotype results indicated 47,XX,+mar[85]/46,XX[15], suggesting a mosaic condition with one additional marker chromosome. C-banding revealed one centromere on the small marker chromosome, whereas N-banding revealed no satellite. CNV-seq analysis revealed a 14.86 Mb mosaic duplication spanning 18p11.32-p11.21, with an estimated copy number of 3.5. On the basis of these findings, the final chromosomal karyotype was designated 47,XX,+i(18)(p10)[85]/46,XX[15]. No chromosomal abnormalities were identified in the karyotypes of the patient's parents or three sisters, indicating that this is a de novo mutation. The patient's oligomenorrhea, distinctive features, and intellectual disability may be associated with a partial duplication of the short arm of chromosome 18. Previous reports on patients with tetrasomy 18p have focused primarily on preschool or prepubescent patients. Here, we present the first reported case of a 17-year-old female with mosaic tetrasomy 18p. The absence of fingerprints on the palms and oligomenorrhea may represent novel characteristics of this condition. For such rare chromosomal abnormalities, the combined use of G-banding, C-banding karyotype analysis, CNV-seq technology, and comprehensive clinical evaluation can facilitate early diagnosis, genetic counseling, and personalized treatment.

Tetrasomy 18p syndrome is a rare chromosomal disorder that is caused by the presence of isochromosome 18p. Most tetrasomy 18p cases are de novo cases and maternal origin of trisomy 18p is a rare condition. At present, only four cases of maternal origin have been reported in worldwide.This is the fifth case of tetrasomy 18p originating from maternal trisomy 18p. The mother of the fetus studied had no apparent disease phenotype. The current case report is to describe a fetus with confirmed 18p tetrasomy as detected by karyotyping and Single Nucleotide Polymorphism array (SNP array) analysis. However, the fetus showed normal phenotypic features that were observed using ultrasound scans. The mother and maternal grandfather were phenotypically normal and healthy; however, they were diagnosed with trisomy 18p, which was confirmed by conventional karyotyping and SNP array. We report a case of 18p tetrasomy in a fetus whose mother and grandfather had 18p trisomy. The mother and grandfather were phenotypically normal. Our case report findings provide an important reference for the genetic counseling of trisomy 18p in the future.

Tetrasomy 18p is a rare genetic condition characterized by a supernumerary 18p isochromosome with two copies of the p arm of chromosome 18 causing patients to have an extra chromosome. Most cases are de novo; however, a few maternally inherited cases have been reported. The most commonly reported manifestations of this condition are developmental delay, cognitive impairments, muscle tone abnormalities, and dysmorphic facial features. This case details a new diagnosis of tetrasomy 18p in a 42-year-old adult who was initially diagnosed with cerebral palsy as a child. We compare the phenotypic traits of our patient with the ones reported in the literature.

Tetrasomy 18p syndrome (Online Mendelian Inheritance in Man 614290) is a rare chromosomal disorder that is seen in approximately 1 in every 180,000 live births. It is caused by the presence of isochromosome 18p, which is a supernumerary marker composed of two copies of the short arms of chromosome 18. Isochromosome 18p is one of the most commonly observed isochromosomes. We report tetrasomy 18p syndrome diagnosed prenatally after noninvasive prenatal testing (NIPT) was positive for trisomy 18. Tetrasomy 18p was finally diagnosed by G-banding and fluorescence in situ hybridization of chromosome 18p, before invasive confirmatory testing the karyotype findings by NIPT showed an increase in the DNA fragments from chromosome 18p, indicating duplication of chromosome 18p. NIPT can detect not only trisomy 13, 18, and 21, but also structural chromosomal anomalies, such as deletions and duplications. An NIPT report "positive for trisomy 18" indicates the possibility of tetrasomy 18p, and detailed analysis of NIPT data can reveal subchromosomal copy number variations, to a certain extent, before definitive diagnostic testing.

Small Supernumerary Marker Chromosomes (sSMC) are rare chromosomal abnormalities, which have abnormal banding arrangement and take many shapes. Several disorders have been correlated with sSMC presence. The aim of this study is to characterize the sSMC derived from chromosome 18 by Fluorescence in situ hybridization (FISH) and Array Comparative Genomic Hybridization (aCGH). Nine children with dysmorphic features have been investigated. They have these features in common: a triangular face, low-set ears, a large mouth with a thin upper lip, and a horizontal palpebral fissure. Epicanthus and strabismus were present in two patients. In addition, we have noticed microcephaly and mental and/or developmental delay with low birth weight. However, two patients had standard birth weight; one patient had hypospadias; two had skin problems; and three showed different congenital heart defects. One patient had corpus callosum hypoplasia. Systematic karyotype analysis revealed a de novo supernumerary chromosome. Array CGH showed a gain in copy number on the short arm of chromosome 18 in the nine cases. In one case, the sSMC seemed to be in mosaic. The breakpoints of the marker were identified using aCGH and FISH. Thus, the sSMC led to 18p tetrasomy with approximately 14 Mb lengths, between 364344 and 14763575 based on the human genome version 18. These results have been completed by FISH in order to ascertain the shape of the sSMC. Our results confirm the uniqueness and particularity of the iso18p syndrome on the phenotypic as well as on the genetic level.