Rare case of Killian-Pallister syndrome associated with idiopathic short stature detected with fluorescent in situ hybridization on buccal smear
© Sukarova-Angelovska et al. 2016
Received: 21 February 2016
Accepted: 1 April 2016
Published: 3 May 2016
Killian-Pallister syndrome (KPS) is a rare form of chromosomal mosaicism and is defined by the existence of an extra chromosome 12 in some cell lines in one individual. The degree of mosaicism varies among tissues and dictates the clinical presentation of the syndrome. The clinical features of Killian-Pallister syndrome include mental retardation, typical facial dysmorphism and pigmentation defects.
We present a rare case of Killian-Pallister syndrome with severe form of the disease associated with isolated growth hormone deficiency and low-rate mosaicism on buccal smear. The absence of a marker chromosome 12p in lymphocyte cultures and the low degree of mosaicism lead to frequent misdiagnosis of this condition.
The selection of tissue sampling is crucial in establishing the diagnosis of Killian-Pallister syndrome. Fluorescent in situ hybridisation on buccal smear remains the golden standard as a screening method if a suspicion of the syndrome exists.
Killian Pallister syndrome (KPS) is a rare chromosomal disorder caused by mosaic tetrasomy of 12p. The syndrome has been described independently by Pallister in 1977 , afterwards confirmed by Killian in 1983 . The incidence is estimated to be 1/25,000 [http://www.orpha.net], however these data are still uncertain due to the diagnostic difficulties and unrecognized cases.
In most cases of KPS, the 12p chromosome is present in four copies ; 12p trisomy or other complex chromosomal rearrangements involving the short arm of chromosome 12 have rarely been described . The mosaic appearance of isochromosome 12p is tissue-limited and often absent in blood culture. Therefore the diagnosis is frequently delayed until the appearance of clinical features later in life.
The extra chromosome is present in 30–100 % of all analyzed skin fibroblasts and buccal cells, in 10–100 % of bone marrow cells and amniocytes, whereas in lymphocytes the mosaic cell line can be found in 0–2 % [5–7]. Identifying the condition prenatally often fails, since the amniotic cell cultures reveal no chromosomal abnormalities .
The clinical presentation is variable, including mild to severe mental retardation, facial dysmorphism, alteration in skin pigmentation (both hypo- and hyperpigmentation) and several nonspecific congenital anomalies. The most significant features that infer the diagnosis are coarse face, full cheeks, sparse hair on the lateral sides of the head, diaphragmal hernias and supernumerary nipples . The presence, severity and combination of the above mentioned features are highly variable, which makes the clinical recognition difficult.
Additionally, certain infrequent and subtle features have been described, such as ear pits, palm vs. finger length, typical upper lip, offering a full dysmorphological spectrum of the syndrome [10, 11]. Epileptic seizures, usually described as generalized or myoclonic, are frequent . Other types of seizures, such as hypsarrhytmia, are rarely described . Alike other syndromic epilepsies, the seizures are hard to control, even with combinations of several antiepileptic drugs.
The growth characteristics in KPS include high birth weight and high growth velocity during the first months, followed by growth retardation and delayed bone age later in childhood. Reynolds  suggested that the initial overgrowth typical of KPS needs further elucidation since most of the aneuploidies involving other chromosomes are associated with fetal growth retardation. It is likely that the presence of 3 or 4 copies of certain genes on chromosome 12 provoke fetal overgrowth. Lower birth measurements combined with microcephaly are rarely present  and are usually associated with additional anomalies - cardial and intestinal defects, renal cysts, etc.
Some authors suspect that the clinical presentation depends on the degree of mosaicism, while others consider the size of the supernumerary chromosome i.e., the number of present gene copies, more important. However, there are reports of similar degrees of mosaicism but inconsistent severity of clinical features . Some authors suggested that existence of the supernumerary chromosome in particular tissues is responsible for this clinical range rather than degree of mosaicism .
Standard chromosome analysis of blood lymphocytes was performed, with 3-day cultivation using phytohaemagglutinin and standard harvesting, followed by G-banding . A total of 50 metaphases were analysed using Microscope Olympus BX51 and standard Metasystems karyotyper. The results revealed a normal karyotype in all analysed cells.
Establishing the diagnosis of Killian-Pallister syndrome (KPS) is often difficult and demanding, requiring thorough clinical observation and different cytogenetic and molecular methods. The underlying mechanism for this rare chromosomal error is prezygotic , occurring in maternal meiosis 2 followed by mitotic loss of the marker chromosome in some cells. The predominance of the meiosis 2 error in the formation of i(12p), unlike other chromosomopathies, is still unresolved. Dufke  clarified the smallest critical region of the chromosome (12pter-12p12.3) using different molecular-cytogenetic techniques. The phenotypic characteristics of KPS were confirmed to be a result of the dosage effect of LDH-B gene in some cases , but others indicate that a fitting phenotype is possible without the presence of this gene . In all cases, however, heterozygosity is reduced to homozigosity in the derivative chromosome 12. Kaur  gives a comprehensive analysis chromosome 12 genes that could produce a KPS phenotype, narrowing the critical region on band 12p13.31. He identified several genes that, when misexpressed (either up or down regulated), yield clinical features of the syndrome.
The growth curve in KPS patients is biphasic - prenatal overgrowth followed by postnatal growth retardation. Although there are several genes on chromosome 12 that promote growth in various tissues (ADIPOR2, FGF6, NTF3, KRAS1, etc.), there are no specific studies about the reason for fetal overgrowth in these patients. Unlike other children with KPS, in our patient growth retardation was present both prenatally (the baby was born small for gestational age) and postnatally (constantly 3SDS below the mean). Growth hormone deficiency is an uncommon finding in KPS, described only once in the literature  and should be further evaluated. Overexpression of insulin-like growth factor 2 (IGFBP2) is a probable reason for postnatal growth retardation . The reason for prenatal growth retardation in our case is unclear.
The cytogenetic finding of i(12p) is rarely detected in blood leukocytes. Some speculate that this is due to the distortion of abnormal cells during the culturing procedure, during which i(12p) becomes dramatically reduced [3, 22]. This reduction occurs in vivo as well; therefore the aneuploidy can be detected in bone marrow tissue only in the early neonatal period . In all other tissues that divide less frequently (i.e.fibroblasts) the extra chromosome persists for longer periods and can be detected later in life. Therefore skin biopsy is most reliable method in detecting the syndrome.
The use of the FISH method in tissues other than blood leukocytes was first described by Speleman  who detected i(12p) in fibroblasts; the value of the method was afterwards confirmed by other authors [25, 26] who used FISH on buccal smear. The method allows detection of i(12p) on interphase nuclei without cultivation and is considered to be a reliable, rapid and effective method for the detection of the chromosomal abnormality in this syndrome. The presence of low-rate mosaicism of i(12p) broadens the phenotypic spectrum of the syndrome. In most of the studies so far [25, 27], an alpha satellite probe of the centromeric region of chromosome 12 was used, but some authors [26, 27] suggest the usage of a dual colour, locus specific probe in order to avoid cross-hybridisation with other cetromeres and false positive results, as well as to detect the number and length of the supernumerary 12p. Recently, other methods such as array CGH are proposed as more sensitive [14, 22]; however these methods are more expensive, require more sophisticated equipment and could not be used as a screening procedure. Yet essential for diagnosis is not the method used, but the adequate choice of tissue for sampling.
The FISH method on buccal smear remains invaluable for the detection of Killian-Pallister syndrome. In order to avoid diagnostic delay and invasive diagnostic procedures, the buccal smear FISH analysis remains a superior method due to the less invasive nature of sample collection.
Underlying mechanisms of the biphasic growth pattern in Killian Pallister syndrome are not yet elucidated and need further investigations. Rarely, both prenatal and postnatal growth pattern are affected and could be associated with growth hormone deficiency, as shown in our case.
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