Mosaic Intrachromosomal Triplication of (12)(p11.2p13) in a Patient with Pallister-Killian Syndrome.

Pallister-Killian syndrome (PKS) is a rare genetic disorder usually characterized by mosaic tetrasomy of isochromosome 12p detected in cultured fibroblast cells. We describe here a patient with PKS and intrachromosomal triplication of the short arm of chromosome 12. Her karyotype was mos 46,XX,inv trp(12)(p11.2p13)[34]/ 46,XX[16]de novo by conventional cytogenetics and fluorescent in situ hybridization (FISH) analysis. However, this chromosomal abnormality was not detected from the patient's cultured blood lymphocytes. We report here the third patient with intrachromosomal triplication on the short arm of chromosome 12, presenting a PKS phenotype.

INTRODUCTION

Pallister-Killian syndrome (PKS, OMIM #601803) is a rare sporadic disorder caused by mosaicism for tetrasomy of the short arm of chromosome 12 resulting from an supernumerary isochromosome 12p [1,2]. The karyotype in the cultured blood lymphocytes is normal in most cases, but one supernumerary isochromosome 12p is present in high percentage in cultured skin fibroblasts and bone marrow cells of the patients [3]. Supernumerary analphoid inverted duplicated chromosome 12p and a supernumerary ring chromosome consisting of two copies of chromosome 12p have also been reported in the rare cases with PKS [4,5]. Clinical features of this syndrome include; mental retardation, pigmentary skin abnormalities, seizures, prominent forehead with temporal balding, hypertelorism, short nose, short neck, flat nasal bridge, flat occiput and macrosomia [6,7].

Intrachromosomal triplications leading to partial tetrasomies of the certain chromosome 12 regions have been reported in two patients [8,9]. In this report, we present the third patient who has mosaic intrachromosomal triplication on the short arm of chromosome 12.

CASE REPORT

A 9-year-old girl was referred to us because of developmental delay and mental retardation. She was the third child of non consanguineous parents. Both parents were 43 years old at the time of the patient’s birth. The family history was remarkable because the child of the patient’s paternal uncle had severe mental retardation and a supernumerary inverted duplicated marker chromosome derived from chromosome 15. The patient’s siblings were clinically normal. She was born at term by Cesarian section (birth weight was 6200 gr).

On examination, she could not speak or walk, her weight was 20.5 kg (3–10 percentile) and height was 110 cm (below 3 percentile). She had a prominent forehead, high frontal hair line, low-set ears, sparse eyebrows, hypertelorism, full cheeks, long philtrum, high arched palate, macroglossia, gingival hypertrophy, mandibular prognathism and muscular hypotonia (Figure 1). She also had hypermobile joints and pes equinovarus of the left foot. Cranial computed tomography showed bilateral frontal sub-dural hygroma (Table 1). Her other system examinations were normal.

Figure 1

Frontal view of the patient’s head at 9 months of age.

Table 1

Comparison of the patient’s clinical findings with a previously reported patient with an intrachromosomal triplication.

Clinical Findingsa	This Study	[9]
Triplicated segment	trp(12)(p11.2p13)	trp(12)(p11.2p13)
Age at examination	9 years old	at birth
Sex	female	male
Coarse face	[+]	[+]
Mandibular prognathism	[+]	[−]
Thin upper lip	[−]	[+]
Sparse eyebrows	[+]	[−]
Hyperterlorism	[+]	[−]
High broad forehead	[+]	[+]
Abnormal ears	[+]	[−]
Epicanthal folds	[−]	[−]
Broad nasal bridge	[−]	[+]
Long philtrum	[+]	[−]
Bitemporal alopecia	[−]	[+]
Macroglosia	[+]	[−]
High arched palate	[+]	[+]
Microcephaly	[−]	[−]
Pigmentation anomalies	[−]	[−]
Terminal hypoplasia of fingers	[−]	[−]
Muscular hypotonia	[+]	[+]
Abnormal fat accumulation	[−]	[−]
Visceral anomalies	[−]	[−]
Anal anomalies	[−]	[−]
Abnormal genitalia	[−]	[−]
Severe mental retardation	[+]	[+]
Hearing loss	[−]	[−]
Macrosomia at birth	[−]	[−]
	gingival hypertrophy; frontal
Other findings	subdural	single palmar crease
	hygroma; pes equinovarus

Table is adapted from Huang et al.[19].

A short-term phytohemagglutinin-stimulated peripheral blood lymphocyte culture was performed according to a standard procedure [10]. Analysis of 100 of the patient’s lymphocyte metaphases by G-bands by trypsin using Giemsa banding showed the karyotype to be 46,XX (Figure 2,3). Skin fibroblasts from the patient were cultured in two different flasks including 2 mL of AmnioGrow (Cytogen GmbH, Bienenweg, Germany) and Chang Medium-D (Irvine Scientific, Santa Ana, CA, USA) at 37°C in a CO2 incubator and harvested by standard methods [10]. Intrachromosomal triplication of the p11.2p13 region on one chromosome 12 was found in 68% of the metaphases of the skin fibroblasts. C-band by barium hydroxide using Giemsa analysis showed the triplicated chromosome to be monocentric. The parents’ lymphocytes showed a normal karyotype.

Figure 2

a) The patient’s GTG-banded karyotype, b) The patient’s CBG- banded metaphase chromosomes.

Figure 3

The patient’s partial karyotype and an ideogram showing chromosome 12 pair.

Fluorescent in situ hybridization (FISH) analysis [11] was performed on metaphase plates from the patient using To TelVysion subtelomeric probe set (Vysis Inc., Downers Grove, IL, USA) according to the manufacturer’s instructions. Images were recorded using a Zeiss Axioplan epifluorescence microscope equipped with a CCD camera (Photometrics Sensys, Tucson, AS, USA) and analyzed using MacProbe v4.3 software. The FISH results on 15 metaphases showed two signals for the 12p subtelomeric region on the abnormal chromosome 12 and one on the normal chromosome 12. This analysis confirmed that the tetrasomy 12p arose by an inverted duplication mechanism. The patient’s karyotype was designated as mos 46,XX,inv trp(12)(p11.2p13)[34]/46,XX[16] according to International System for Human Cytogenetic Nomenclature (ISCN 2009) [12].

DISCUSSION

Intrachromosomal triplications are rare events and the middle segment usually being inverted in orientation, as in our case [8]. In the majority of cases, triplications were interstitial, whereas there is only one published report of triplication of a whole chromosome arm [9]. In most cases, triplications had originated from maternal chromosomes [13]. Our case is the second patient with intrachromosomal triplication of a whole chromosome arm.

To the best of our knowledge, only two patients with intrachromosomal triplication of the short arm of chromosome 12 have been presented in the literature. In the article by Eckel et al. [8], the 12p11.22-p12.3 region was triplicated and this region does not cover the critical PKS region, which defined the chromosomal region 12pter-p12.3. Therefore, the patient’s clinical phenotype was similar to trisomy 12p syndrome rather than the PKS. Unexpectedly, the intrachromosomal triplication was found in all peripheral blood lymphocytes. Also, this intrachromosomal triplication was found in 12% of the mother’s peripheral blood lymphocytes. The clinical findings were compatible with the PKS that were reported by Powis et al. [9]. During conventional cytogenetic analyses, triplicated chromosome 12 was observed in 30% of the cultured fibroblasts but not in peripheral blood lymphocytes. However, array-based comparative genomic hybridization analysis showed a triplication of the 12p in the peripheral blood lymphocytes in a low level mosaicism. A parental transmission of the triplication could not be excluded since parental blood samples were not available [9]. However, this has been excluded in our patient because her parents’ karyotypes were normal.

Mechanisms for the formation of the intrachromosomal triplication include; i) fusion of the inverted duplicated supernumerary marker chromosome with the normal homologue; ii) unequal crossover or interhomologue translocation followed by the inverted insertion at the former breakpoint junction; iii) two U-type exchanges among three chromatids [14-16]. Powis et al. [9] speculated that triplication of chromosome 12p could have arisen from telomere to telomere fusion of supernumerary analphoid isochromosome 12p with a normal chromosome 12. Indeed, there are three reports about PKS patients with analphoid inverted duplicated supernumerary marker chromosomes consisting of chromosome 12p [4,17-18].

The clinical findings of the patient reported by Powis et al. [9] included hypotonia, brachycephaly with upslanting palpebral fissures, thin upper lip, low nasal bridge, high arched palate, a single transverse palmar crease on each hand and abnormal hair pattern. The clinical differences between this case and our case could result from differences in the degree of mosaicism and in the distribution of abnormal cells in different tissues.

As a result, we report here the third patient with intrachromosomal triplication of the short arm of chromosome 12. We conclude that intrachromosomal triplication might be a new mechanism of formation for PKS.