Interstitial deletion of chromosome 13 and associated congenital anomalies

Prenatal diagnosis of de novo mosaic deletion 13q associated with multiple abnormalities

A case of prenatal diagnosis of de novo mosaic deletion of the long arm of chromosome 13 (del(13)(q13.3)) is presented. Routine scanning in a 27-year-old primigravida at 25 weeks' gestation showed fetal bilateral hydronephrosis. Detailed anomaly scanning in our tertiary referral center further demonstrated posterior meningoencephalocele, sloping forehead, microcephaly, syndactyly and hypoplastic thumbs. Both genetic amniocentesis and cordocentesis revealed a mosaic karyotype, 46,XY/46,XY,del(13)(q13.3). Sonographic findings were confirmed by postmortem autopsy and additional abnormalities such as agenesis of corpus callosum, hypoplastic cerebellum and macroglossia were diagnosed. Detailed sonography in this case thus revealed multiple malformations that prompted fetal karyotyping at 25 weeks' gestation.

Deletion of band 13q21 is compatible with normal phenotype

A deletion of band 13q21, of maternal origin, was found in a male whose wife had had two miscarriages. The proband and his mother were both phenotypically normal. Repeated studies by high resolution banding techniques failed to demonstrate a translocation of the deleted band in the two subjects. The absence of pathological consequences of the deletion is explained by the fact that this band is one of the latest replicating in the human karyotype, which may indicate, by analogy with heterochromatin, that it carries no transcriptionally active genetic material.

Dicentric chromosome 13 and centromere inactivation

The karyotype of a child with dysmorphic findings suggestive of both trisomy 13 and the 13q--syndrome was found to have cells with one of two different dicentric chromosomes: one bearing a duplication of chromosome 13q [46,XX,-13, + psu dic (13)t(13;13)(pter leads to cen leads to q34::q34 leads to pter)] and the other a deletion of 13q [46,XX,-13, + psu dic (13)t(13;13)(pter leads to cen leads to q22::q11 leads to pter]. Longitudinal cytogenetic studies in leukocytes demonstrated a loss of those cells possessing the small dicentric [psu dic(13)(q22;q11)], whereas fibroblasts from two separate skin biopsies contained only this marker. Q-band polymorphisms indicated that both dicentrics were of paternal origin, with the smaller dicentric derived from the larger via the bridge-breakage-fusion cycle. The presence of two active centromeres could not be confirmed in either dicentric.