Telomere shortening and telomere position effect in mild ring 17 syndrome

Ring chromosomes are unusual circularized chromosomes, the vast majority of which arise sporadically [1]. Two types of ring chromosome can be related to different clinical presentations: the non-supernumerary ring, which replaces one of the normal homologs, with the karyotype 46,(r) and the supernumerary ring, which is often very small, with the karyotype 47,+(r). The non-supernumerary ring chromosomes are usually associated with abnormal phenotypes because of loss of genomic material at one or both ends. In some cases, no deletion has been detected and the phenotypic features have been attributed to mitotic ring instability [2].

The classic mode of ring chromosome formation consists of two terminal breaks in both chromosome arms, followed by a reconnection of the broken ends, leading to a loss of genetic material. Alternatively, they can be formed by fusion of subtelomeric sequences or telomere-telomere fusion with no deletion, resulting in complete ring chromosomes.

Telomeres, the nucleoprotein complexes that cap eukaryotic chromosomes, play two major roles in cell physiology. First, they are critical factors in determining the lifespan of mammalian cells. Second, they are essential for genome integrity maintenance. In addition to their role in protecting the ends of chromosomes, telomeres can influence the expression of nearby genes, an event called the telomere position effect. Previous reports demonstrated that the telomere position effect is an epigenetic process that involves changes in chromatin conformation and depends on both the distance from the telomere and the telomere’s length [3]. The telomere position effect also occurs in mammalian cells, and may play a role in human genetic disease, as a result of the repositioning of active genes near telomeres or subtelomeric sequences following chromosome rearrangements [4–8], such as those seen in individuals with ring chromosomes [9, 10].

Rings of chromosome 17 are relatively rare and only 17 cases have been reported in the literature to date. Among these 17 cases, 11 showed no deletions of the Miller-Dieker critical region with mild phenotypic features [11] consisting of growth delay, intellectual disability, seizures, café au lait skin spots and minor facial dysmorphism.

The ring chromosomes 17 of the majority of these patients were described at the banding level, and only two cases pointed at the molecular level [11–13]. Cytomolecular investigation of the telomeric and subtelomeric regions of ring 17 is essential for the disclosure of candidate genes or gene expression regulating regions, which are potentially responsible for the phenotypic characteristics.

We report here the results of new investigations of a previously described patient with mild ring 17 syndrome and studies of an unusual case of ring chromosome 17 syndrome with a mosaic familial transmission.

Ring chromosomes are rare cytogenetic abnormalities that usually occur as a consequence of breaks and deletions at both ends of the chromosome, with subsequent fusion of the remaining portions. The breakpoints may be located within euchromatic regions, with partial monosomy of the distal ends of the short and long arms of the chromosome, but sometimes they may involve only one or both the telomere repeats. Though uncommon, the last mechanism has been described in the literature as a telomere-telomere fusion with no detectable loss of euchromatin [14, 15].

In a previous report [11], we have described the case of a ring 17 chromosome in a young boy, providing an accurate characterization of the defect, which resulted in complete loss of the telomeric ends with the conservation of the whole euchromatic regions. The clinical picture was reported as a mild ring 17 syndrome with moderate intellectual disability, seizures, dysmorphic traits, and café au lait and hypochromic skin spots. We compared this to the most severe lissencephaly syndrome, which involves functional loss of the LIS1 gene.

Several hypotheses have been proposed to explain the relevant phenotype of the mild ring chromosome 17 syndrome, such as ‘dynamic mosaicism’ or the ‘ring syndrome’. However, they have never been supported by clear experimental evidence. It has been suggested that the epigenetic silencing of gene expression through a telomere position effect is one of the alternative mechanisms that occur in ring chromosome syndrome [9]. This silencing effect contributes to an alteration of the function of genes located near the ring fusion point by modulating (mainly downregulating) their expression in a telomere-dependent fashion [10, 16–18].

Here, we note that, in the absence of detectable telomere repeats in the first patient [11], a group of non-contiguous genes that were within an interval of 1.5 Mb from the telomeres of p and q arms were significantly downregulated compared with the controls. Although the biological functions of these genes are manifold, most of the latter play a key role in cellular and tissue functions (that is, RPH3AL, FN3K, and NARF), cell cycle (that is, ARHGDIA), DNA processing and regulation (that is, FOXK2 and GEMIN4) and cell growth and early development (that is, NXN and FAM57A). Our data suggest that the concomitant downregulation of these genes may lead to an imbalance of cell activities and hamper proper physiological interactions, ultimately producing the phenotypic aspects related to ring 17 syndrome. Moreover, the upmodulation of NF1 and the downregulation of LIS1, located outside the studied 1.5 Mb region, reveal that the observed effects are not linearly correlated to the distance from the telomeres.

The second case we studied regards a young girl with a cytogenetic finding of mosaic ring 17 chromosome. The clinical phenotype presents, as in the previous case, the classical features of mild ring 17 syndrome, with the key symptoms being seizures, intellectual disability, and café au lait skin spots. Nevertheless, chromosome and molecular cytogenetic investigations demonstrated that the telomere ends were preserved. Familial studies revealed that the mother was the carrier of a mosaic ring 17 chromosome with the sole phenotypic feature of café au lait skin spots. Ring chromosomes usually follow the conventional method of transmission resulting, when the rearrangement is passed on, in a constitutional ring syndrome with all, or the majority, of cells containing the abnormal chromosome. The familial transmission of a mosaic ring is extremely rare and represents a challenge still to be clarified [1]. The proband’s maternal grandparents, sister, and aunt exhibited a normal karyotype in accordance with the data of the segregation analysis that clearly showed that the ring chromosome derived from the maternal grandmother and was transmitted only to the affected subjects. We also excluded uniparental disomy, since all of the informative markers were found on both the normal and the rearranged chromosomes. Moreover, we tested a set of microsatellites on normal skin and a café au lait skin spot biopsy of the patient, and this showed the presence of a biallelic pattern of transmission related to each parent.

Analysis of telomere lengths in the patient’s lymphocytes revealed that telomeres of chromosome 17 were the shortest compared with the other chromosomes of the cell in both normal and ring metaphases (Figure 2A,B). This evidence raises the hypothesis that chromosome 17 could be prone to reaching critical telomere lengths, leading to an increased risk of ring formation.

When we compared the telomere lengths of the ring chromosome of the patient and her mother with the mean telomere lengths of chromosome 17 in age-matched controls, we found that telomeres of the ring chromosome of the patient were at least three times shorter (48 T/C% vs. 155 T/C%) (Figure 2D,E), whereas no remarkable differences were found for the patient’s mother (76 T/C% vs. 80 T/C%). Therefore, assuming that the ring chromosome is generated during prenatal life and that ring chromosome telomeres do not shorten upon cell divisions (as telomeres usually do), we can hypothesize that the generation of the ring chromosome is the result of an extensive loss of telomere repeats at an early stage of development.

The mosaicism rate is another important aspect to consider in determining the phenotypic features of the syndrome. Values less than 10% do not usually present clinical manifestation, whereas higher rates are associated strictly with different degrees of disease expression. Conversely, this rate tends to decline through one’s lifespan. In fact, the rearranged chromosome is preferentially lost during cell divisions, leading to apoptosis and premature cell death. In our case, the patient and her mother showed respectively 72% and 44% of mosaic cells in the peripheral blood. Nevertheless, only the patient presented relevant symptoms. The telomere shortening observed in the patient with respect to the mother appears as an additional factor that might have influenced the generation of the ring chromosome and the occurrence of this peculiar phenotype. These hypotheses are also confirmed by the first case that we investigated, which completely lacked the telomeres, as well as previous reports describing ring chromosome 17 with subtelomeric deletions, but conserving the LIS1 gene.

In summary, our studies clearly outline that telomere length reduction plays a determining role in at least two important aspects of the ring 17 syndrome.

First, the cases we presented show a strong genotype-phenotype correlation regarding clinical symptoms and telomeric shortening. Patient 1, with the most severe phenotype, lost the telomeric ends completely. Patient 2, with an intermediate picture, displayed a high shortening of the telomeric repeats, while her mother, affected by a lower reduction of telomere length, presented with only skin spots without any other symptom.

Second, telomere shortening acts as a novel recognized mechanism responsible for ring formation and mosaicism assessment. In fact, we demonstrated that the ring chromosome was not selectively lost during the cell cycle, being the autologous maternal linear chromosome present on all cells in the skin biopsy. Moreover, the ring chromosome of the patient’s mother, being unable to change its structural characteristic during gametogenesis, has telomeres longer than that of the patient. These observations led us to hypothesize that the chromosome transmitted is the normal autologous one and that, as a consequence of the telomere shortening during embryogenesis, it underwent a circularization process, ultimately leading to the formation of a ring chromosome. The stage in which this occurrence takes place is responsible for the different mosaicism rate.

The influence of telomere shortening and the telomere position effect should also be investigated in other patients carrying the ring chromosome 17; however, since the ring chromosome 17 syndrome is very rare, a big effort by all the scientific community will be necessary to recruit a larger number of patients.