Deletions of the short arm of chromosome 6 are relatively rare, the main features being developmental delay, craniofacial malformations, hypotonia, and defects of the heart and kidney, with hydrocephalus and eye abnormalities occurring in some instances. We present the molecular cytogenetic investigation of six cases with 6p deletions and two cases with unbalanced translocations resulting in monosomy of the distal part of 6p. The breakpoints of the deletions have been determined accurately by using 55 well-mapped probes and fluorescence in situ hybridization (FISH). The cases can be grouped into two distinct categories: interstitial deletions within the 6p22-p24 segment and terminal deletions within the 6p24-pter segment. Characteristics correlating with specific regions are: short neck, clinodactyly or syndactyly, brain, heart and kidney defects with deletions within 6p23-p24; and corneal opacities/iris coloboma/Rieger anomaly, hypertelorism and deafness with deletions of 6p25. The two cases with unbalanced translocations presented with a Larsen-like syndrome including some characteristics of the 6p deletion syndrome, which can be explained by the deletion of 6p25. Such investigation of cytogenetic abnormalities of 6p using FISH techniques and a defined set of probes will allow a direct comparison of reported cases and enable more accurate diagnosis as well as prognosis in patients with 6p deletions.

Orofacial clefting is genetically complex, no single gene being responsible for all forms. It can, however, result from a single gene defect either as part of a syndrome (e.g. van der Woude syndrome, Treacher-Collins syndrome, velo-cardio-facial syndrome) or as an isolated phenotypic effect (e.g. X-linked cleft palate; non-syndromic, autosomal dominant orofacial clefting). Several studies have suggested that chromosome 6p is a candidate region for a locus involved in orofacial clefting. We have used YAC clones from contigs in 6p25-p23 to investigate three unrelated cases of cleft lip and palate coincident with chromosome 6p abnormalities. Case 1 has bilateral cleft lip and palate and a balanced translocation reported as 46,XY,t(6,7)(p23;q36.1). Case 2 has multiple abnormalities including cleft lip and palate and was reported as 46,XX,del(6)(p23;pter). Case 3 has bilateral cleft lip and palate and carries a balanced translocation reported as 46,XX,t(6;9)(p23;q22.3). We have identified two YAC clones, both of which cross the breakpoint in cases 1 and 3 and are deleted in case 2. These clones map to 6p24.3 and therefore suggest the presence of a locus for orofacial clefting in this region. The HGP22 and AP2 genes, potentially involved in face formation, have been found to flank this region, while F13A maps further telomeric in 6p24.3/25.