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In the March 1999 issue of the journal, Portnoï et al1 reported on a patient with a marker chromosome with neocentromere which originated from distal 3q. Here we describe a further example of this type of marker chromosome from distal 3q, which is substantially smaller.
The marker was ascertained during cytogenetic diagnosis of a 5 year old girl with marked developmental delay and mild facial dysmorphism. Body measurements were birth length 49 cm, birth weight 2965 g, and OFC 34 cm. At the age of 8.5 years, height was 131 cm, and weight was 37 kg (>97th centile) as a result of hyperphagia which developed two years ago. Dysmorphic signs included hypognathia, broad and flat nasal root, and abnormally shaped alae nasi (fig 1). The median upper lids showed atypical epicanthus. Furthermore, the girl showed slight hirsutism and a bilateral ichthyosiform hyperkeratosis of the palms and soles. On clinical investigation at the age of 5 years our proband presented with slight muscular hypotonia and hyporeflexia. Perceptive skills and visuomotor coordination were retarded corresponding to a developmetal age of 3-3.5 years. At the age of 8.5 years the girl attends a special school for mentally handicapped children. She cannot speak properly, searching for words, can recognise only between 10 and 15 letters, and is still not able to write.
A very small supernumerary de novo marker chromosome was ascertained during cytogenetic diagnosis of a 5 year old girl with developmental delay and mild facial dysmorphism. The marker was C band negative but appeared mitotically stable in cultured lymphocytes.
FISH with an alpha satellite probe detecting all human centromeres gave hybridisation signals on all of the proband’s chromosomes except the marker chromosome. The chromosomal origin of the marker was clarified by microdissection and reverse painting; DNA collected from the marker hybridised exclusively on the marker and on distal 3q.
The extent of the marker chromosome was characterised by FISH using mapped YAC or BAC clones. Each positive clone gave a double signal on the marker. Thus the marker belongs to the expanding category of analphoid marker chromosomes originating from duplication of a distal chromosome arm, which must have gained centromere function by neocentromere formation.
According to the ENSEMBL database, the most proximal clone present on the marker, RP11-634l24, maps to chromosome band 3q28, and the extent of the duplicated segment is approximately 9 Mb.
Standard cytogenetic investigation showed a very small, nearly metacentric supernumerary de novo marker chromosome (fig 2), which was C band negative (not shown) but appeared mitotically stable in cultured lymphocytes, since it was present in all metaphases investigated. FISH with an alpha satellite probe detecting all human centromeres (fig 3) gave hybridisation signals on all chromosomes of the proband, except the marker chromosome. This finding implies that it is an analphoid marker. The chromosomal origin of the marker was clarified by reverse painting. Twenty copies of the marker chromosome were collected by microdissection, subsequently amplified, and labelled by DOP-PCR,2,3 and the resulting FISH probe was hybridised to metaphase spreads of a male control (results not shown) and of the proband, respectively. DNA from this microdissection library hybridised exclusively on the marker and on distal 3q (fig 4). FISH with the Vysis subtelomeric probe 3QTEL05 of chromosome 3q showed two terminal signals on the marker chromosome (fig 5).
In order to characterise the extent of the marker chromosome along distal 3q, FISH with mapped YAC or BAC clones was performed (table 1). All clones tested gave either no FISH signal on the marker chromosome or a double signal, a finding which suggests a (presumably inverted) duplication of the terminal chromosome 3q segment (with no monosomic segment in between). Therefore, one can assume that this chromosome belongs to the expanding category of analphoid marker chromosomes with a neocentromere, the majority of which have derived from inversely duplicated terminal chromosome arms.4 According to the ENSEMBL database the most proximal clone RP11-634l24 maps to chromosome band 3q28. The G banding pattern of the marker chromosome is in agreement with an inverse duplication of the distal G bands 3q28 and 3q29 (fig 2).
To our knowledge there are only two reports on marker chromosomes representing a duplication of distal 3q. Portnoï et al1 described a marker chromosome with inverse duplication of 3q27→qter, which is substantially larger than the marker reported here. Cockwell et al5 reported on an analphoid supernumerary marker chromosome derived from terminal 3q in a fetus with multiple malformations. The breakpoint of this marker was assigned to band 3q26.2 according to its banding pattern but was not precisely determined by molecular genetic methods. Kroisel et al6 reported on partial tetrasomy 3q resulting from triplication within a single chromosome. A comparison of the extent of the resulting tetrasomic segment of our proband with those of the reported cases with more precise determination of the breakpoint is shown in table 1.
The marker chromosome reported here is smaller than the one described by Portnoï et al1 and the resulting tetrasomic segment is also smaller than on the derivative chromosome 3 described by Kroisel et al.6 Even standard banding analysis shows that the marker reported by Cockwell et al5 is also substantially larger, since it exceeds the size of a G chromosome. According to the FISH analysis, the marker described here contains at least 9 Mb from the distal chromosome 3q28-29.
In the female patient described by Portnoï et al,1 the clinical picture is restricted to pigmentary anomalies of the skin following the lines of Blaschko.7 Mental development is not impaired and she has no dysmorphic features. In this girl the mild clinical manifestation and the skin pigmentary pattern might both be explained by the finding that she represents a mosaic with a euploid cell line. The patient reported by Kroisel et al6 displays a very similar pattern of skin hyperpigmentation to the patient of Portnoï et al1 and he also represents mosaic tetrasomy 3q. However, this patient is developmentally retarded. Contrary to these cases, in the patients described here and by Cockwell et al5 no mosaicism was detected. Though the patient reported by Kroisel et al6 is a mosaic, he can be compared with our proband. Both patients are mentally retarded and both share some typical non-specific symptoms commonly observed in patients with unbalanced karyotypes, namely coarse facial features and a prominent supraorbital region. However, a broad nasal root, long philtrum, and large, prominent ears were only reported in the patient of Kroisel et al6 and not in the girl reported here. Particularly, the skin anomalies observed in our patient are distinct from the pigmentary anomalies observed in the patients described by Portnoï et al1 and Kroisel et al.6 In the latter patient, hyperpigmentation of the skin was already manifest at birth, whereas in the patient of Portnoï et al1 this phenotype developed only after the age of 10-12 years.
The marker chromosomes of Portnoï et al,1 Cockwell et al,5 and the one described in this report most probably represent inverse duplications with a neocentromere acquired to stabilise a de novo acentric fragment. The different breakpoints in these three examples suggest that such “acentric fragments stabilised by neocentromere formation” can originate at many, if not any, chromosomal sites. This is in accordance with the observation of variable breakpoints within the growing collection of this type of marker chromosome representing the different chromosome arms4 and even within the same chromosome arm.8