Prenatal diagnosis of a karyotypically normal pregnancy in a mother with a supernumerary neocentric 13q21 -->13q22 chromosome and balancing reciprocal deletion

An adult female patient with a history of miscarriages was found to be carrying a stable supernumerary chromosome. The patient also carried a reciprocal paracentric deletion in chromosome 13q21/22. Microdissection and reverse fluorescence in situ hybridization FISH revealed that this supernumerary chromosome was derived from region 13q21 --> 13q22. The presence of a neocentromere on this supernumerary chromosome was confirmed by the absence of detectable alpha satellite DNA using FISH and the presence of centromere proteins CENP-C and CENP-A using immunofluorescence. The absence of telomere sequences suggests that the marker is a ring chromosome (r(13)). FISH using ordered BACs from the chromosome region 13q21 --> 13q31 permitted the precise positioning of the r(13) chromosome and the corresponding deletion to chromosome bands 13q21.32 --> 13q22.2. BAC 280J7 from within the r(13) was used as a FISH probe for the prenatal analysis of amniocytes at 16 weeks of gestation, which revealed a normal karyotype for the fetus. This r(13) chromosome represents the first description of chromosome 13 of the rarer class of neocentric chromosomes that are derived from interstitial deletions. It represents the first example of prenatal diagnosis in a phenotypically normal female that was ascertained to carry a neocentric marker. The presence of such a neocentric marker/deletion karyotype in a parent presents unique possible karyotypic outcomes for conceptions and unusual challenges for genetic counseling.