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Editor—Hypomelanosis of Ito (HI) (MIM 146150) is an aetiologically heterogeneous physical finding characterised by a swirling pattern of hypopigmentation of the skin, typically distributed along the lines of Blaschko, reflecting pigmentary mosaicism. Chromosomal abnormalities are common and very heterogeneous. They include X;autosomal translocations and mosaicism of aneuploidy, segmental trisomies, or monosomies.1-5 As HI is not a specific disorder, it has also been referred to as pigmentary mosaicism.6 This report adds a further chromosomal anomaly which has not been described in pigmentary mosaicism previously.
The proband was the first son born to a 29 year old mother and a 30 year old father. There was no family history of recurrent abortions, consanguinity, or mental retardation. He was born by caesarian section at term after an uncomplicated pregnancy with a weight of 3560 g (50th centile), a length of 50 cm (50th centile), and an occipitofrontal circumference (OFC) of 32 cm (<3rd centile). Apgar scores were 8, 9, and 10. No complications were reported in the perinatal period. Despite delayed psychomotor milestones (sitting at 9 months, walking at 2 years, first words at 3 years), the first evaluation was only done at the age of 5 years. At the age of 10 years he was referred to our endocrinological outpatient clinic because of cryptorchidism and glandular hypospadias. At this time he was found to be a shy and uncommunicative boy. He showed moderate mental retardation and attended a school for handicapped children. He had a height of 1.3 m (3rd centile), a weight of 26 kg (3rd centile), and an OFC of 50 cm (<3rd centile). On physical examination, mild facial asymmetry, epicanthic folds, a broad nose with a broad nasal tip, retrognathia, dysplastic ears, and a low set posterior hair line were noted (fig 1). Areas of linear hypopigmentation were distributed along the lines of Blaschko and were more evident on the legs. He also showed hemihypertrophy of the right leg, bilateral pes equinovarus, and pectus excavatum. There were no organic defects.
Chromosome analysis of peripheral lymphocytes showed the karyotype 46,XY/46,XY,add(12q). The GTG banding pattern was suggestive of an interstitial duplication involving chromosomal segment q13-q21 or q13-q22 (fig 2). Parental karyotypes were normal. Fibroblast cultures were obtained from a normal and a hypopigmented skin area on the left arm. Fifty cells from each first subculture were analysed and identified 70% abnormal cells in the normal skin and 15% abnormal cells in the hypopigmented skin fibroblast culture. In addition, 2/50 cells from the normally pigmented skin showed the karyotype 46,XY,del(7)(q31).
Multiplex-FISH (M-FISH) allows the identification of chromosomes with distinct colours and was performed as described elsewhere.7 The M-FISH analysis showed that the der(12) consisted of chromosome 12 material only, excluding an interchromosomal rearrangement. No other abnormalities were noted. In order to characterise the der(12) in more detail a chromosome 12 specific multicolour bar code was constructed with YAC clones from the CEPH library, as previously described8 (table 1). On the der(12), two signals (blue) were observed for the 12q13 specific YAC 926h3 (fig 2). These signals were located close to each other indicating a duplication of the respective region. The 12q21-22 specific YAC 923a3 displayed a normal signal (red) distal and a weak signal proximal to the 926h3 double spot, indicating that this particular YAC is partially duplicated and spans a breakpoint of the duplication. The multicolour banding pattern clearly showed an inverted duplication involving the segment 12q13-q21/22 and allowed the differentiation between the two possibilities of inverted duplications (pter-q13::q22-q13::q22-qter versus pter-q21/22::q22-q13::q22-qter). Using the GTG banding pattern in addition to the FISH results, the structure of the der(12) could be designated as dup(12)(pter-q13::q22-q13::q22-qter) (fig 2). The patient was thus mosaic for a pure trisomy of the segment 12q13-q21.
In many kinds of de novo chromosomal aberrations, it is impossible to determine the chromosomal origin of the extra material by banding patterns alone and characterisation of the rearrangement often needs FISH analysis with single copy probes. In the present case, a partial trisomy 12q was suspected from the G banding pattern and was confirmed by M-FISH within a single hybridisation experiment. Chromosome specific bar codes obtained from well characterised YAC clones have been shown to be useful for characterising chromosomal rearrangements.8 By performing multicolour FISH using a set of chromosome 12 specific YAC clones, the type of duplication in the proband was identified as inverted. Moreover, we could also differentiate between the two possibilities of inverted duplication occurring upwards or downwards on the chromosome arm. Using YAC mapping data and GTG banding pattern analysis, the rearrangement in the proband was defined as an inverted duplication in an upward direction involving the segment 12q13-q22. This case illustrates the potential of multicolour chromosome specific bar codes for the characterisation of intrachromosomal rearrangements by a single hybridisation experiment. Other methods, such as CGH and reverse painting, in general allow the accurate mapping of over-represented regions. However, CGH analysis would have failed in this case because of the mosaicism with a high percentage of normal cells. At best, reverse FISH could only allow the differentiation between direct and the two types of inverted duplication. In contrast, the fortuitous determination of a breakpoint YAC from the bar code enables mapping of the duplication at a molecular level, which would be impossible by CGH or reverse painting.
The trisomy 12q13-q22 was detected in 10% of peripheral lymphocytes, in 70% of fibroblasts obtained from the normally pigmented skin, and in 15% of fibroblasts from the hypopigmented skin. Thus, the proportion of abnormal cells in the two different skin areas did not correlate with the skin pigmentation in the patient. This is not surprising as fibroblasts were investigated and it is expected that melanocytes, as the carriers of the biochemical defect, have a different distribution, because the two cell types have different embryological derivations. As a consequence, the selection of the skin biopsy area should not be guided by the localisation of skin pigmentation abnormality, and biopsy of more than one site may be required to identify the chromosomal anomaly in fibroblasts. A deletion 7q was found in two fibroblast cells of the normally pigmented skin, but not in other cells. We do not know if deletion 7q is the result of a cultural artefact or a third cell line in skin fibroblasts. The low proportion of abnormal lymphocytes shows the importance of analysis of a large number of cells. Moreover, a normal karyotype in lymphocytes of HI patients does not rule out a chromosomal defect, as has been shown for many HI associated chromosomal anomalies, which were detected only in fibroblasts but not in blood cultures.5 Like the proband described here, more than 70% of HI patients have one or more abnormalities of the central nervous system, the musculoskeletal system, facies, epidermal structures, and inner organs. The variability of the features associated with HI can be interpreted as a consequence of different chromosomal imbalances. Our proband also showed asymmetrical clinical features, which can also be considered as a non-specific sign of chromosomal mosaicism and as an indication for repeated chromosomal analysis of different tissues, especially if it is associated with mental retardation or dysmorphic features or both.9
The proband's phenotype is different from other patients with trisomy of a more distal part of 12q. Trisomy for the segment 12q13-q22 has not been described previously and a characteristic phenotypic pattern cannot be determined. To our knowledge, only three other patients have been reported with an interstitial duplication 12q.10 11All of them showed mosaicism suggesting that non-mosaic interstitial trisomy 12q may be lethal.
Cytogenetic diagnosis of structural mosaicism enables accurate genetic counselling of families. Given that the parents have normal karyotypes, as in the present family, the recurrence risk is very small. Mosaicism of structural abnormalities with a 46 chromosome complement is rare, and a significant proportion of such cases are direct or inverted duplications.12 If chimerism were excluded, a postzygotic origin of the rearrangement in a chromosomally normal conceptus can be postulated for intrachromosomal duplications, which would rule out the small risk of parental gonadal mosaicism.
In summary, the present case stresses the importance of careful chromosomal analysis of different tissues in patients with pigmentary anomalies or asymmetrical clinical findings or both, and has shown the usefulness of multicolour FISH with single copy probes resulting in chromosome specific bar codes to characterise intrachromosomal rearrangements.
This study was supported by the Deutsche Forschungsgemeinschaft (Sp 460/3-1) and the Friedrich-Baur-Stiftung.
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