According to the orientation of the duplicated segment, duplications may be classified either as tandem or inverted, being the last usually associated with deletion of the distal region of the duplicated chromosome 1. The best studied cases of inverted duplications (inv dup) are the inv dup(8p) 23 and bisatellited inv dup(15) 4, which are usually non-mosaic. In contrast, mosaic inverted duplications are derived from different post-zygotic mechanisms for which various possible origins have been proposed 567. There is also a particular subset of inv dup in which the duplication ends terminally on the chromosome and which are named terminal inv dup 89.

Tetrasomy of distal 3q segments is associated with adverse phenotypic manifestations, ranging from mild developmental delay to deep facial dysmorphisms, resembling patients with the dup(3q) and Brachmann-de Lange syndromes. Accordingly, some of the patients with 3q tetrasomy show hirsutism, synophrys, broad nasal root, anteverted nares, thin upper lip with downturned mouth corners, craniosynostosis, urinary tract anomalies, micrognathia, cleft palate and malformed ears, characteristics also seen in patients with the dup(3q) syndrome 10. Brachmann-de Lange syndrome (BDLS) has overlapping features with dup(3q) syndrome, but with apparently normal chromosomes 1112.

In this study we characterize by molecular cytogenetics a case of an autistic child previously reported by our group, with a mosaic partial tetrasomy of a distal chromosome 3q segment translocated to the short arm of the chromosome 12 13. To our knowledge, this is the first report of a mosaic terminal inv dup(3q) captured in an intact 12p subtelomere. Using fluorescence in situ hybridization (FISH) and array-based comparative genomic hybridization (aCGH) we have better characterized the extra material of chromosome 3 as qter → q26.31::q26.31 → qter. Furthermore, the mechanism of formation for this rearrangement is discussed.

The male child was the first born of non-consanguineous healthy parents and was delivered at term after an uneventful pregnancy. His birth weight was 2850 g (<5th centile) and there were no major neonatal problems, except for the club foot surgically corrected. However, a general developmental delay was noted soon after birth and fever convulsions were observed between the age of 9 and 24 months old.

As a consequence of suspicions of a pervasive developmental disorder of autistic type, at the age of 3, the proband was referred to a child psychiatrist, who directed him to an autism unit. At 11 years old, his height was between the 25th and 50th centile, weight was on the 90th centile and head circumference was on the 50th centile. Some minor dismorphic features were also observed, such as low set and slightly enlarged ears, high and arched palate and round face. No skin pigmentation disorders were observed. He was submitted to a multidisciplinary neurodevelopmental assessment and showed an adaptive behavior at the 30 month age level and a nonverbal IQ of 61, corresponding to a mild delay, a diagnosis of autism was based on Autism diagnostic interview-revised and the Statistical Mental Disorder IV edition criteria 131415. At adult age the proband maintain a clinic of autism and has an adaptative behaviour evaluated with Vineland Adaptative behaviour scales – survey form with a standard score of 30 for communication domain, 59 for daily living skills domain and 63 for socialization domain 16.

In this work we have characterized by molecular cytogenetics a tetrasomy of a 3qter fragment. The carrier presents minor facial dysmorphisms and general developmental delay associated with an autistic disorder. His karyotype was initially established from peripheral blood lymphocytes as mos 46,XY,add(12)(p13.3) [56]/46,XY [44] 13. The use of MCB and aCGH techniques allowed the characterization of the extra material as being derived from chromosome 3, involving an inv dup(3)(qter → q26.3::q26.3 → qter) and with a length of 24.92 Mb.

Inverted duplications are a kind of genetic lesions that can appear either as mosaic, or non-mosaic, depending on the time that they are formed 18. There are different well established mechanisms proposed for the origin of terminal inv dup. The present case, however, does not adjust entirely with the usual mechanisms proposed for terminal inv dup that would imply a concomitant deletion or a distal extra marker chromosome stabilized by neocentromerization. We propose three alternative mechanisms (Fig. 4A–C), two of them could have in common a first step in gametogenesis with a double-strand break event at 3q, repaired by fusion of the two sister chromatids (U-type exchange) and giving rise to an acentric inv dup (Fig. 4A–B). One possible mechanism for the subsequent step would be the healing by telomere capture from a non-homologous chromosome 12. A non-disjunction of the 12 homologous at meiosis I, led to the formation of a gamete that after fertilization originated a trisomic zygote, which included the der(12). In the first mitotic division, in an attempt of cell rescue, two cell lines would have been generated, by loosing either the der(12) or the normal 12 chromosome. A consequence of the heterodisomy generated by the segregation error at meiosis I could be a chromosome 12 uniparental disomy 19. The other alternative mechanism (Fig. 4C) would be the generation of a terminal acentric inv dup either as a result of a post-zygotic mitotic division, which would necessarily lead to mosaicism, or at meiosis. In this case, the post-zygotic stabilization delay of the acentric inv dup, associated with the telomere capture from a non-homologous chromosome 12, could explain the mosaicism 72021.

Vanneste and colleagues have described that post-zygotic chromosome instability is highly frequent in cleavage-stage embryos, leading to segmental chromosomal imbalances and mosaicism, probably a common cause of constitutional chromosomal disorders. In this study, fifty-five percent of embryos carried terminal segmental imbalances, that were the result of DNA double-stranded breaks possibly followed by non-disjunction of the acentric fragment 22. This study reinforces the theory that mosaic inv dup formation is a post-zygotic event.

There are only twelve reported cases in the literature of inv dup associated with tetrasomy for distal chromosome 3q 102324252627282930313233. Of all documented cases, our proband is the only one in which the tetrasomy is not an intrachromosomal triplication 3233 or a supernumerary marker chromosome 10232425262728293031.

The chromosome region of the present rearrangement has been reported to be involved in both the BDLS (q26.3-q27) and the dup(3q) syndromes (with q26.3 being the critical region) 34353637. Nonetheless, besides the mental retardation, the low set ears, the arched palate, our patient does not have any of the other 23 physical features compiled for those syndromes by Faas and colleagues 34.

Taking into account the twelve cases previously reported and the present one, it becomes evident that the phenotypes associated with tetrasomy of distal 3q segments are heterogeneous 102324252627282930313233. As a consequence, we failed to establish any genotype-phenotype correlation once neither the region involved nor the degree of the mosaicism could be correlated with a consistent pattern. Nevertheless, the presence of skin pigmentary disorders is a particular feature that connects the majority of the cases reported 2325272829303132. Indeed, hyperpigmentation is present in 8 of the reported patients, with a pattern concordant with lines of Blaschko in 5 patients. Correlating these cases, Gimelli and colleagues proposed the presence of a gene involved in skin pigmentation defects located at 3q27.1-qter region 30. However, hypopigmentation and hyperpigmentation following the Blaschko's lines are relatively common in individuals with chromosomal mosaicism. In the present case, although involving the 3q27.1-qter region, there are no skin pigmentary alterations.

The autistic behavior observed in our patient, and not reported in any other case with distal 3q tetrasomy, is an interesting feature. According to the results of a genome-wide screen performed by Auranen and colleagues, there is evidence for a major susceptibility locus on chromosome 3q25-q27 for the autism-spectrum disorders 38. Accordingly, a study of the same group revealed the existence of an allelic association on chromosome 3q25-q27 in families with autism spectrum disorders originating from a sub-isolate of Finland 3839. Since no other reported patient with 3qter tetrasomy demonstrated autistic behavior, we could be facing a random occurrence. However, it would be interesting to evaluate children with autistic behavior for micro-rearrangements in this region of chromosome 3.

To the best of our knowledge, this is the first study describing a mosaic interchromosomal inverted duplication of a 3qter segment captured in a non-homologous intact subtelomere (12pter). Also new is the fact that the proband presents an autistic behavior, not observed in any other patients with the same genomic imbalance. The concomitant employment of aCGH and multicolor FISH techniques contributed to the understanding of this unusual rearrangement.

The authors declare that they have no competing interests.

IMC, drafted the manuscript and coordinated the study.

JBM, performed the array-CGH analysis and has been involved in the drafting of the manuscript.

CR, provided valuable support.

AW and NK, carried out the application of multicolor banding.

LB and JRV, were involved in array-CGH analysis.

GO, provided the clinical data and biological samples.

EM, performed the cytogenetic studies as well as FISH experiments and has been involved in the drafting of the manuscript.

All authors read and approved the final manuscript.