A new variant of chromosome 16

Directly transmitted unbalanced chromosome abnormalities and euchromatic variants

In total, 200 families were reviewed with directly transmitted, cytogenetically visible unbalanced chromosome abnormalities (UBCAs) or euchromatic variants (EVs). Both the 130 UBCA and 70 EV families were divided into three groups depending on the presence or absence of an abnormal phenotype in parents and offspring. No detectable phenotypic effect was evident in 23/130 (18%) UBCA families ascertained mostly through prenatal diagnosis (group 1). In 30/130 (23%) families, the affected proband had the same UBCA as other phenotypically normal family members (group 2). In the remaining 77/130 (59%) families, UBCAs had consistently mild consequences (group 3). In the 70 families with established EVs of 8p23.1, 9p12, 9q12, 15q11.2, and 16p11.2, no phenotypic effect was apparent in 38/70 (54%). The same EV was found in affected probands and phenotypically normal family members in 30/70 families (43%) (group 2), and an EV co-segregated with mild phenotypic anomalies in only 2/70 (3%) families (group 3). Recent evidence indicates that EVs involve copy number variation of common paralogous gene and pseudogene sequences that are polymorphic in the normal population and only become visible at the cytogenetic level when copy number is high. The average size of the deletions and duplications in all three groups of UBCAs was close to 10 Mb, and these UBCAs and EVs form the "Chromosome Anomaly Collection" at http://www.ngrl.org.uk/Wessex/collection. The continuum of severity associated with UBCAs and the variability of the genome at the sub-cytogenetic level make further close collaboration between medical and laboratory staff essential to distinguish clinically silent variation from pathogenic rearrangement.

Euchromatic variant 16p+. Implications in prenatal diagnosis

BACKGROUND

Euchromatic imbalances at the cytogenetic level are usually associated with phenotypic consequences. Among the exceptions are euchromatic variants of chromosome 16 (16p+) with normal phenotype. There is a growing list of euchromatic duplications and deletions involving both G-positive and G-negative bands that seem to be phenotypically neutral, but these euchromatic variants are rare.

OBJECTIVE

The aim of this report is to describe a new familial case of euchromatic variant 16p+ and to emphasise the misinterpretation of these rare euchromatic variants particularly when ascertained at prenatal diagnosis.

METHODS AND RESULTS

Fluorescence in situ hybridisation with clone RP11-261A7 showed an amplified signal in the larger chromosome 16. This clone contains FLJ43855 gene, similar to sodium- and chloride-dependent creatine transporter.

CONCLUSION

So, this 16p+ variant that involves amplification of pseudogenetic sequences is considered a polymorphism in normal individuals.

The euchromatic 9p+ polymorphism is a locus-specific amplification caused by repeated copies of a small DNA segment mapping within 9p12

A large duplication involving the proximal euchromatic region of chromosome 9p was detected by conventional cytogenetics in a healthy 33-year-old woman and in two unrelated foetuses; both of them received the rearrangement from their healthy father. The duplicated segment was R(RBG) and C(CBG)-negative and G(GTG)-positive and was also positive for a 9-specific painting probe. It was preliminarily interpreted as a pathological quantitative change of the genome in the foetuses. FISH analyses allowed us to characterise the chromosome boundaries of this polymorphism, being identified by the RP11-15E1 BAC clone, proximally, and by the RP11-402N8 clone, distally, both probes falling within the 9p12 region. The contiguous, distally, RP11-916H19 probe was not included in the amplification, and may represent the discriminating genetic locus between chromosome polymorphism and chromosome mutation. The 9p12 amplification was approximately 12, 7 and 8 Mb in the three different families and was stable through generations. Our observations confirm the already provided evidence that proximal 9p duplications represent a benign euchromatic polymorphism. However, we demonstrated that these variants are not a simple duplication of the region 9p11.2-p13.1, as already suggested, but that they result from a many-fold amplification of a segment mapping within 9p12. These results provide important insights both in the genetic counselling and in the prenatal diagnosis of rare euchromatic chromosome variants and in understanding the architecture of the human genome.

Repercusión clínica de las anomalías cromosómicas

The incidence of chromosome anomalies in newborn infants is 0.7-0.8 %. The phenotypic manifestations of chromosomal abnormalities are highly diverse. These anomalies may be present in phenotypically normal individuals in whom they can increase the risk of recurrent miscarriage and birth defects and/or mental retardation. It is important to determine this risk to provide patients with appropriate genetic counseling.

Partial distal 5p trisomy resulting from paternal translocation (5;15)(p15.1;p13) in a boy with no mental retardation

We report a boy with partial distal 5p15.1-->pter trisomy and normal development. We compared the clinical findings in our patient with those previously reported of the same 5p duplicated region. Several cases of autosomal duplications and normal development have been described. The present case is another example of a chromosomal anomaly with little, if any, phenotypic effect without mental retardation.

Duplication of chromosome region (16)(p11.2 --> p12.1) in a mother and daughter with mild mental retardation

We report a 40-year-old female with mild mental retardation and behavior problems and her 6-year-old daughter. Chromosome analysis showed that both patients had a proximal duplication in the short arm of chromosome 16. The aberration was characterized further with band-specific probes, resulting in a 46,XX,dir dup(16)(pter --> p11.2::p12.1 --> qter) karyotype. The clinical and cytogenetical findings are compared to other patients with partial trisomy 16p reported in the literature.

A brief history of human autosomes

Comparative gene mapping and chromosome painting permit the tentative reconstruction of ancestral karyotypes. The modern human karyotype is proposed to differ from that of the most recent common ancestor of catarrhine primates by two major rearrangements. The first was the fission of an ancestral chromosome to produce the homologues of human chromosomes 14 and 15. This fission occurred before the divergence of gibbons from humans and other apes. The second was the fusion of two ancestral chromosomes to form human chromosome 2. This fusion occurred after the divergence of humans and chimpanzees. Moving further back in time, homologues of human chromosomes 3 and 21 were formed by the fission of an ancestral linkage group that combined loci of both human chromosomes, whereas homologues of human chromosomes 12 and 22 were formed by a reciprocal translocation between two ancestral chromosomes. Both events occurred at some time after our most recent common ancestor with lemurs. Less direct evidence suggests that the short and long arms of human chromosomes 8, 16 and 19 were unlinked in this ancestor. Finally, the most recent common ancestor of primates and artiodactyls is proposed to have possessed a chromosome that combined loci from human chromosomes 4 and 8p, a chromosome that combined loci from human chromosomes 16q and 19q, and a chromosome that combined loci from human chromosomes 2p and 20.

Variant euchromatic band within 16q12.1

The enlarged short arm of chromosome 16 resulting in an additional euchromatic band has been regarded as a variant. We present an unreported case with an unusual variant of chromosome 16, where the mother and daughter both have an additional band (q12.1) in the long arm. Its origin is chromosome 16, as revealed by FISH-technique, and its familial nature suggests that it has no clinical significance.

A rare inherited euchromatic heteromorphism on chromosome 1

Extra genetic material that is euchromatic is generally regarded to be associated with phenotypic abnormalities. However, recent studies suggest that this is not always the case. Chromosome analysis was performed on amniotic fluid cells from a 37-year-old phenotypically normal patient referred for advanced maternal age. All the cells analysed showed a karyotype of 46,XY,1p+. The 1p+ chromosome had extra genetic material of uncertain origin in chromosome band region 1p21-->31. Chromosome analysis on the father revealed a normal 46,XY male karyotype. The mother's karyotype showed the same 1p+ chromosome. C and Q banding, as well as silver staining studies, in both the mother and the fetus support the interpretation that the extra chromosomal material was euchromatic in nature. This 1p+ chromosome may be characterized as a euchromatic heteromorphism. Euchromatic heteromorphisms not associated with phenotypic abnormalities have been reported for chromosomes 9 and 16. To the best of our knowledge, this is the first report involving this type of cytogenetic anomaly on chromosome number 1 in a phenotypically normal mother and infant.

An unusual variant of chromosome 16 in three generations

An unusual variant of chromosome 16, 16p+, in three generations is described. This is the first clinical report of 16p+ in Japan.

The importance of further cytogenetic and molecular investigation of acrocentric variants: justification by presentation of a case [t(8;14)(q24;p11)]

On routine chromosome analysis a moderately retarded 18-year-old man was found to have an unusual short arm on one chromosome 14. With GTL-banding this chromosome showed an enlarged short arm with no evident secondary constriction. Negative CBG-banding of the short arm suggested the possibility of a translocation involving euchromatin. Interpretation of the abnormality as an unbalanced translocation relied on chromosome analysis using GTL-, CBG-, and Ag-NOR-banding of the proband's phenotypically normal mother, who was found to be carrying a balanced translocation involving chromosomes 8 and 14. In situ hybridization of sequences known to map to the short arm of chromosome 14 confirmed the interpretation and established that the breakpoint was within p11. The patient, whose karyotype is 46,XY, -14, +der(14)t(8;14)(q24.1;p11), is trisomic for the terminal end of the long arm of chromosome 8. The patient's clinical features are described and compared with those reported in patients trisomic for this region. This study demonstrates the importance of using a number of different banding techniques in conjunction with in situ hybridization for the investigation of morphologically unusual acrocentric short arm variants seen at routine diagnosis.

A familial case of chromosome 16p variant

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Euchromatic 16p+ heteromorphism: first report in North America

A heteromorphism of the short arm of 16 (16p+) was discovered in 2 unrelated infants. By G banding, the euchromatic variant appears as a light and a medium dark band just distal to the centromere. This results in an increase of the short arm by about 1/3. The same variant was present in the normal father and the normal paternal grandmother in one family and mildly retarded mother in the 2nd family. The anomalies of the 2 infants are not similar and are apparently unrelated to the 16p+ variant. Though the discovery of such euchromatic variants is highly significant for clinical diagnosis, their genetic significance and mode of origin remain to be elucidated.

Chromosome imbalance, normal phenotype, and imprinting

A duplication of the sub-bands 1q42.11 and 1q42.12 was found in a boy and his mother. The proband has short stature (around the 10th centile) but a normal phenotype and psychomotor development. His mother is also asymptomatic. We found 30 published cases of normal subjects with an imbalance of autosomal euchromatic material. In these cases the imbalance involved either only one G positive band or a G positive and a G negative band. Thus the absence of a phenotypic effect cannot always be ascribed to the deficiency in the G positive bands of coding DNA. Moreover, in some cases, the method of transmission of the chromosome abnormality was such that an imprinting effect could be postulated.

Duplication of euchromatin without phenotypic effects: a variant of chromosome 16

A family with an unusual variant of chromosome 16 is presented. The mother and son both with additional material present in the short arm of chromosome 16 adjacent to the centromere are phenotypically normal. The extra C-band negative region has been shown not to be composed of alpha satellite DNA. The literature regarding other familial cases of what appears to be the same variant of chromosome 16 is reviewed.

Replication studies in the 16p+ variant

We report a further case of the 16p+ chromosome studied by replication banding. The extra euchromatic material was shown to be uniformly light staining, indicating that it is late replicating.

Non C-banding variants in some normal families might be homogeneously staining regions

Three families are reported showing transmission of a previously described variant, which is not associated with any clinical abnormality. The variant involves additional material at the band 9p12, which shows homogeneous staining of intermediate density with GTL- and RBG-banding, and negative staining with CBG-banding. The region stains positively with Feulgen stain. In situ hybridization with total genomic human DNA, cloned alpha satellite, satellite III, and ribosomal DNA all show no hybridization to the 9p12 variant. Two members of one of the families show the largest 9p12 variant yet reported; two other carriers in this family have inherited a variant of decreased size. It is suggested that the 9p12 variants are homogeneously staining regions. Using the ISCN three-letter convention, this variant could be described as hsr(9)(p12).

An unusual variant of chromosome 16. Two new cases

Two new cases of an unusual chromosome 16 variant, 16p+, in non-related normal carriers are reported.