Isolation of chromosome-21-specific DNA probes and their use in the analysis of nondisjunction in Down syndrome.

Thirteen single-copy, chromosome-21-specific DNA probes were isolated from a recombinant library made from flow-sorted chromosome 21 DNA and regionally mapped using a panel of somatic cell hybrids. Five probes mapped in the 21q21-q22.1 region, six to the 21q22.1-qter region, and one to each of the regions 21q22.1-q22.2 and 21q22.3. Two of these probes, one of which maps in the critical region for Down syndrome, have recently been shown to be expressed at high levels in Down syndrome brain tissue (Stefani et al. 1988). Following preliminary screening for restriction fragment length polymorphisms (RFLPs), five polymorphisms were discovered with four of the chromosome 21 DNA probes. A frequent MspI polymorphism detected by one of the probes was used in conjunction with four previously described polymorphic chromosome 21 probes to analyse the origin of nondisjunction in 33 families with a child or fetus with trisomy 21. The parental origin of the additional chromosome 21 was determined in 12 cases: in 9 (75%) of these it was derived from the mother and in the other 3 cases (25%) it was of paternal origin. Cytogenetic analysis of Q-banding heteromorphisms was informative in three of five families tested, and in each case the RFLP results were confirmed. The meiotic stage of nondisjunction was defined with confidence in five families, the results being obtained with pericentromeric RFLP or cytogenetic markers. Recombination between two nondisjoined chromosomes was demonstrated in one family and is consistent with the view that a lack of recombination between chromosome 21 homologues or failure of their conjunction is not the invariable cause of trisomy 21.