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Partial deletion of 4p and 4q in a fetus with ring chromosome 4: phenotype and molecular mapping of the breakpoints

Molecular breakpoint mapping in phenotypically well defined patients with structurally rearranged chromosomes is an established strategy to narrow down the physical interval for genes that are critical in the formation of the phenotype. In patients with balanced translocations, the breakpoints themselves are likely to contain the disease associated gene, whereas in patients with segmental aneuploidy the deleted or duplicated chromosome fragments are the candidate regions for one or more dosage regulated genes. The phenotypes in patients with segmental aneuploidy are syndromes which often vary in their clinical manifestation depending on the size of the chromosomal region involved. Two relatively frequent syndromes involve chromosome 4 and may coincide in the same person in the form of a ring chromosome 4. Formation of a ring chromosome 4, r(4), often involves loss of 4p and 4q telomeres and of more proximal regions on either or both chromosome arms. Several r(4) chromosomes have been reported in patients who had a combination of features typical of the deletion 4p− and deletion 4q− syndromes.1–3

Deletions of the distal short arm of one chromosome 4 involving parts of 4p16 cause the Wolf-Hirschhorn (4p−) syndrome (WHS, OMIM 194190), a complex malformation syndrome characterised by low birth weight, severe psychomotor delay, microcephaly, typical facial features (“Greek warrior helmet” appearance), midline fusion defects, heart defects, and other abnormalities. Most likely, WHS is a contiguous gene syndrome caused by the deletion of more than one gene. Several attempts have been made to identify the genes involved by genotype-phenotype mapping.4–6 The minimal critical region for some of the WHS features (typical facial appearance, mild mental and growth retardation, and congenital hypotonia) has been mapped to a 165 kb region in 4p16.3 (WHSCR).7, 8 Other features of WHS patients, like midline fusion defects and heart defects, clearly map …