Satellites on the terminal short arm of chromosome 12 (12ps), inherited through several generations in three families: a new variant without phenotypic effect
- a J Hospital, Hills Road, Cambridge CB2 2QQ, UK, b National Centre for Medical Genetics, Our Lady's Hospital, Dublin, Ireland, c Department of Medical Genetics, University College Dublin, Ireland, d Department of Medical Genetics, Addenbrooke's Hospital, Cambridge, UK, e Department of Medical Genetics, The Wellcome Trust Centre for Molecular Mechanisms in Disease, University of Cambridge, UK
- Dr Willatt,
Editor—Five of the human autosomes are acrocentric, chromosomes 13, 14, 15, 21, and 22, and are identified by the presence of satellited short arms. These short arms contain three bands, p11, p12, and p13,1 and are composed of repetitive DNA containing satellite repeats and copies of ribosomal RNA genes. Band p11, the pericentromeric region, is composed of several types of tandemly repeated DNA including satellites I, II, III, and IV, and β satellite DNA.2-4 Band p12, the satellite stalks, contains multiple copies of genes coding for ribosomal RNA5 and is known as the nucleolar organiser region (NOR) as the nucleolus is formed by an aggregation of ribosomal RNA. This can be recognised by staining with silver nitrate (AgNOR staining).6 Band p13 contains β satellite DNA and terminal telomeric sequences.2 7
Loss or gain of the short arm of acrocentric chromosomes occurs without apparent phenotypic effect. For example, Robertsonian translocations occur when two acrocentric chromosomes are joined by centric fusion with the resulting loss of the short arm material and have no associated phenotype in this euchromatically balanced form.8 Chromosomal rearrangements involving the short arms of acrocentric chromosomes are a well known form of chromosomal variation. The most common variation results from rearrangements between the short arms of acrocentric chromosomes. Thus, the satellites of acrocentric chromosomes range in size from no satellites to double or treble satellites as shown by AgNOR staining.8Translocations between the short arm of an acrocentric chromosome and the heterochromatic region of the long arm of the Y chromosome, resulting in acrocentric chromosomes with Y chromosome heterochromatin in place of satellites and satellited Y chromosomes, are also observed.9 10 More rarely, non-acrocentric chromosomes with terminal satellites have been described which arise from a translocation between the short arm of an acrocentric and the terminal region of another chromosome.11-19 Interstitial insertion of NORs from an acrocentric chromosome into another chromosome, giving rise to non-acrocentric chromosomes with interstitial satellites, is a rare form of chromosomal rearrangement without phenotypic effect.19-24 In contrast to these variant chromosomes, satellited non-acrocentric chromosomes resulting from an insertion or translocation between an acrocentric chromosome and another chromosome, in which there is loss of material from the non-acrocentric chromosome, are associated with an abnormal phenotype.25 26 When a satellited non-acrocentric chromosome is observed it is essential to distinguish between these two possibilities.
We describe three pedigrees in which multiple family members have NORs at the telomeric region of the short arm of one chromosome 12 (12ps).
In family 1, parental chromosome studies were undertaken following a stillbirth at 38 weeks. These studies showed a chromosome 12 with positively NOR staining satellite stalks at the end of the short arm (12ps) in the mother, who had an otherwise normal female chromosome complement. The paternal karyotype was normal. Family studies showed that this satellited chromosome had been inherited from her mother. Each of these women was well with no associated phenotype. There was no other significant family history (fig 1).
In family 2, chromosome analysis of an amniotic fluid sample received for maternal age showed a male karyotype with positively NOR staining satellite stalks at the end of the short arm of one chromosome 12 (12ps). Parental chromosome analysis showed that the 12ps chromosome was paternal in origin with no associated phenotype. The family history was unremarkable. The pregnancy proceeded to term and the child at the age of 3 years has no developmental problems (fig 1).
In family 3, a chromosome 22 (tuple 1) microdeletion was identified in a 3 month old child with tetralogy of Fallot and dysmorphic facies, consistent with DiGeorge/VCF syndrome. Parental chromosome studies showed that the microdeletion was de novo. However, the father was found to have one chromosome 12 with positively NOR staining satellite stalks at the end of the short arm (12ps) with no associated phenotype. The 12ps chromosome was not present in the index case (fig 1).
In a fourth family, a 13 year old girl was referred with short stature. Blood chromosome analysis showed the presence of positively NOR staining satellite stalks at the end of the short arm of one chromosome 12 (12ps). The remainder of the karyotype was normal and there was no evidence of mosaicism for 45,X in 30 cells examined. Parental karyotyping indicated that the chromosome 12 with positively NOR staining satellite stalks was maternally derived. The mother was of normal height and there was no family history of short stature. This family was subsequently identified as part of family 3 (fig1).
Chromosome studies including GTL banding, silver staining (AgNOR), and fluorescence in situ hybridisation (FISH) studies were performed by standard techniques.6
GTL banding showed no detectable loss of material from the satellited chromosome 12 in any of the families, with band p13 still present (fig 2, left). Silver (AgNOR) staining confirmed the presence of NOR positive material (fig 2, middle). The NOR bearing satellited chromosome took part in NOR associations with acrocentric chromosomes. The alpha satellite (centromeric) probes for the acrocentric chromosomes 13/21 (D13Z1), 14/22, and 15 did not hybridise to the satellited chromosome 12, thus confirming it was monocentric. FISH studies with the chromosome 12 short arm subtelomeric probe VIJ2 27 showed no detectable loss of material from the 12ps chromosome (fig 2, right).
We have described three families in which a satellited chromosome 12 (12ps) is inherited. There was no evidence for loss or gain of genetically significant material. The clinical indication for chromosome analysis of the proband was different in each case and both maternal and paternal transmission of the satellited chromosome was evident. In view of the lack of a consistent phenotype in the probands and the normal phenotype in the heterozygous parents, this satellited chromosome 12 appears to be a previously undescribed heritable variant chromosome, without phenotypic effect. Our detection of this rare variant in these three families makes it probable that they are related, especially as they all share a travelling, semi-nomadic lifestyle.
It is notable that the majority of reported cases of familial satellited non-acrocentric autosomes have been identified at prenatal diagnosis for maternal age, as was the case for family 2 in the present study.11-19 When a satellited non-acrocentric chromosome is identified at prenatal diagnosis, it is essential to distinguish between significant loss or gain of material and variant chromosomes. In some cases, satellited non-acrocentric chromosomes identified at prenatal diagnosis have been shown to be unbalanced products of a parental balanced translocation25 26 and the satellited chromosome in the proband has been associated with an abnormal phenotype. Gene density is high in the subtelomeric regions of chromosomes so particular care has to be taken before deciding that the integrity of this region has been maintained. In particular, cases of Wolf- Hirschhorn and cri du chat syndromes involving a satellited short arm of chromosomes 4 or 5 respectively have been frequently reported.27-29 In the family with a satellited 4 (qs) variant, described by Babu et al,30 the index case had an interstitial deletion of chromosome 4 in the region adjacent to the satellites (q35), thus explaining his developmental problems. Careful conventional banding analysis, exclusion of loss of subtelomeric regions by using commercially available subtelomeric FISH probes, and the demonstration of inheritance of the identical rearrangement from a phenotypically normal parent should provide reassurance. In the family described by Mihelick et al,31 the proband had craniorachischisis whereas other family members who carried the satellited chromosome 4 (qs) had a normal phenotype. This family was studied before the advent of FISH and it is probable that the craniorachischisis was a coincidental finding. The satellited chromosome 10 (qs) described by Storto et al 18 was unusual in being present in mosaic form in the father. This chromosome 10 with satellites must presumably have occurred for the first time in the father. Interestingly, as far as we are aware, there are no other published reports of de novo satellited non-acrocentric variant chromosomes. The lack of a clinical phenotype, and the normal hybridisation analysis with the chromosome 12p subtelomeric probe would indicate that there has been no loss of the subtelomeric region of 12p in these families. However, it is impossible to exclude the possibility that this rearrangement is an interstitial insertion of a NOR into the most subtelomeric part of 12p, rather than a terminal attachment to 12p. The satellited chromosome 12 reported in the present study is different from the familial dicentric satellited chromosome 12 variant described by Watt et al.20 In their family, satellite material and a centromere from an acrocentric chromosome had been interstitially inserted into the proximal short arm of chromosome 12.
The case described by Miller et al 14 was unusual in that the satellited chromosome 4 (qs) did not stain with AgNOR staining in the proband, but was NOR positive in other family members including the father of the fetus. As judged by silver (AgNOR) staining, not all NORs are active in every cell. Of the 10 NORs present on the acrocentric chromosomes, most people have between four and seven that are active per cell.32 The satellited 4 (qs) in the proband appeared morphologically the same as in the other family members. It is possible that examination of more cells in the proband would have shown AgNOR staining in some cells. The possibility that the satellited chromosome 4 (qs) in the proband had lost its NOR activity cannot be excluded.
When a chromosome anomaly appears to be an apparently harmless variant, then there may be a dilemma regarding further family follow up. Taking blood samples for chromosome analysis may raise anxiety in the family needlessly and might also be a time consuming exercise. This has to be balanced against the problems associated with the unexpected finding of a satellited chromosome at prenatal diagnosis.
The satellites on acrocentric chromosomes show a considerable variation in size in the population. This is considered to arise from unequal recombination between acrocentric short arms at meiosis occurring in these regions of sequence homology. The close proximity of the repetitive sequences of the acrocentric chromosomes in a common nucleolus is also likely to favour recombination, even in mitosis.33 It is likely, for example, that the case of mosaic satellited chromosome 10 (qs) described by Stortoet al 18 arose mitotically. Rearrangements resulting in non-acrocentric chromosomes may also arise because of sequence homology at the breakpoints and scattered within the genome are repeat sequences with homology to the acrocentric repetitive DNA.34 Furthermore, there is strong homology between the satellite sequences of acrocentric chromosomes and the subtelomeric repetitive sequences on some autosomes.35It is possible that these variants arise from abnormal pairing and subsequent crossing over at meiosis between the satellites of acrocentric chromosomes and complementary subtelomeric sequences. Sequences similar to the D4Z4 complex repeat associated with FSHD, which maps to 4q35, are found on the short arm of acrocentric chromosomes36 and it is of interest that chromosome 4 (4qs) is the most frequently described satellited variant chromosome.
Description and proper evaluation of these rare variants is vital when counselling subjects and families in whom a satellited chromosome is found, particularly when detected prenatally.
We describe three apparently unrelated families in whom a translocation of a nucleolar organiser region (NOR) onto the short arm of chromosome 12 has been inherited through several generations.
Conventional banding and FISH studies showed no detectable loss of material from this satellited chromosome 12 (12ps).
There was no evidence of reproductive problems or phenotypic effects in the carriers of this satellited chromosome 12, indicating that it is a previously unreported variant chromosome.