As defined by the World Health Organization (WHO), an osteochondroma is a cartilage-capped osseous outgrowth that is broad-based or stemmed and is made up of cortex and a marrow cavity, which are both continuous with the host bone. The Online Mendelian Inheritance in Man (OMIM) categorizes it under number 133700 and terms it hereditary multiple exostoses (HME), a synonym of the WHO term multiple osteochondroma (MO). MO is a monogenic autosomal-dominant disorder, caused by loss-of-function mutations in either exostosin-1 (EXT1) (8q23-q24)1 or exostosin-2 (EXT2) (11p11-p12)2,3. EXT1 and EXT2 mutations are found in 90% of all cases4. Most often, one side is predominantly affected within a person and family, and sidedness rarely shifts5. Possible modulators are sex, age, and gene environment.
We report on a pair of monozygotic twins with MO. We found phenotypic differences, and there was a tendency toward mirror-image involvement. We reviewed the literature for explanations, searching for genotype-phenotype relationships and the presence of mirror image or variable disease expression within monozygotic twins. Based on the difference in phenotype in these twins, it seems that mere genetic factors insufficiently explain differences in phenotype. The patients and their parents were informed that data concerning their cases would be submitted for publication, and they provided consent.
Ten-year-old monozygotic twin girls presented to our outpatient clinic. Family history was positive for MO in the father, who was known to carry an EXT1 mutation. Molecular analysis by polymorphic DNA markers proved the twins to be monozygotic, and sequence analysis showed a c.572delT (p.Leu191X) mutation in exon 1 of the EXT1 gene. The twins had never undergone surgery before presentation, nor did they have a history of trauma. No specific comorbidity was present in either of the girls.
Patient A's height was measured at 147.5 cm, and patient B's height was measured at 145 cm. Weight was 44 kg and 40 kg, respectively. In comparison with their peers, height was average, but weight was one standard deviation above average.
Upper Extremities
We found mirror-image affliction of unilateral ventral scapular osteochondromas: the right side was affected in patient A, and the left side was affected in patient B. Both patients presented with osteochondromas on the proximal part of the humerus of both arms; however, size, aspect, and extension were asymmetrical. The osteochondroma on the right side was more prominent in patient A, and the osteochondroma on the left side was more prominent in patient B. Shoulder motion was normal in both patients.
Mirror-image involvement was evident for the typical Masada type-I deformity of the forearm on the left side in patient A and the right side in patient B (Fig. 1)6. In both of these forearms, active pronation was possible to 45°, whereas supination was normal. Clear differences were seen between the twins concerning the contralateral forearms. Patient A had a Masada type-III deformity with an ulnar plus variance, as well as osteochondromas on the distal part of the right radius, most notably on the radioulnar side, but with absence of ulnar involvement. Patient B had osteochondromas on the radiovolar side of the distal part of the left radius and on the distal part of the right ulna, demonstrating an ulnar minus variance. There was more pronounced bowing of the distal part of the radius in patient B and of the distal part of the ulna in patient A. Both patients had a horizontal course and normal aspect of the distal ulnar growth plate, in contrast to the opposite (Masada type-I deformity) side. Rotations were unimpaired in these two forearms. Dorsal extension, palmar flexion, and ulnar deviation were within normal range of motion in all four wrist joints, whereas radial deviation was not possible in both Masada type-I deformities and in patient B's left wrist.
Mirrored shortening of the fourth and fifth metacarpals was evident in the right hand of patient A and the left hand of patient B. In patient B, the left first metacarpal bone and the proximal phalanx of the fourth digit were shortened as well. We also noticed mirror-image affliction, with osteochondromas being more present in the right hand of patient A and the left hand of patient B. Generally, the hands of patient B were slightly more affected.
Lower Extremities
Developmental dysplasia of the hip was present in both patients. The femoral neck showed mirror-image broadening by osteochondromas on patient A's left side and patient B's right side.
The distal part of the femur showed mirror-image disorders, not only in the configuration of the metaphyses, but also in the varus joint orientation of the distal femoral end (Fig. 2). A higher than normal anatomic lateral distal femoral angle was found. Valgus deformity was seen in the proximal part of the tibiae, with varying degrees on both sides, leading to an oblique orientation of the knee joints, especially on the left side of patient A and the right side of patient B. Leg-length discrepancy was noticed in both patients, with a shorter lower leg evident on the left side of patient A and the right side of patient B. Patient A partially compensated for this discrepancy with a longer left femur. Patient B did not compensate, yielding an increased leg-length discrepancy by comparison.
Valgus joint orientation and type-II deformity was seen in both ankles of both patients. There was a mirror-image tendency for the two maximally affected ankles. Patient B showed predominant osteochondromas on the distal part of the right lower leg, whereas patient A showed bilateral affliction with slight right-sided preference.
The twins showed partly mirrored presentation with shortened metatarsals. The third metatarsal on the right side was shortened in patient A, whereas the third metatarsal on the left side was shortened in patient B. However, the shortened second metatarsal in patient A and the shortened fourth metatarsal in patient B were singular findings.
When quantifying the pathophysiological role of genetics in a disease, twin studies offer vital clues concerning the impact of environment since twins have the same genetic makeup. To the best of our knowledge, we present the third report on monozygotic twins with MO7,8. Previously reported cases did not document mirror-image tendencies.
Objectifying mirroring in MO is difficult because the essential mutation is omnipresent, and most bones not formed by membranous bone are affected to some extent. However, based on radiographic evaluation and clinical features, we concluded that there were strong mirror-image tendencies in our twin patients. No known discriminators explain phenotypic differences for monozygotic twins with MO. Jäger et al. described a low frequency of the shifting of the predominant side within a family, mostly attributed to a possible modifier-gene variability9. Therefore, mirror imaging in twins with MO does not seem to be a random event.
Benign mirror imaging occurs in 10% to15% of monozygotic twins. Cases of pathological mirroring have been documented in various medical specialties. In the orthopaedic literature, mirror-image coxa vara, congenital longitudinal radial deficiency, bone cysts, first and second branchial arch syndrome, osteoarthritis, slipped capital femoral epiphysis, Legg-Calvé-Perthes disease, and idiopathic scoliosis all have been documented10-20.
Mirror-image twinning occurs in the late blastocyst stage, when the left-right axis is defined21,22. The embryo splits along this axis, yielding two identical phenotypic halves. Having already been defined as either left or right, the damaged residual halves of the embryo generate the new contralateral half23. If the disease is multigenetic or environmentally influenced, the phenotype might differ from the original side. Therefore, discordance or mirror imaging ensues.
In MO, mirror imaging does not seem to be of mere genetic origin. Considering the monogenic autosomal-dominant nature of this condition, explanations for mirror-image tendencies must be sought in epigenetic modifications. The postzygotic events that happen in utero, including allocation of blastomeres, reorientation of the right-left axis, differentiation delays or oppression, variable X-chromosome inactivation, variations in placental circulation/anatomy, and availability of cytoplasmic components, can all contribute to phenotypic differences and limit the usefulness of monozygotic twins as an experimental model24. In order to determine which factors contribute to the phenotypic differences, numerous sets of twins with and without the condition, who additionally need to be sex and age matched since epigenetics is an ever-changing process, would need to be evaluated. Because we believe we are the first to describe mirror-image tendency in this disease entity, prior evidence regarding pathogenesis is absent.
The occurrence of mirror imaging in MO could be coincidental; however, it possibly could be due to the unique function of the EXT proteins. The protein complex is involved in the synthesis of heparan sulfate proteoglycans (HSPGs). These chains are then incorporated into the cell membrane and extracellular space. In this aspect, HSPGs are a keystone in cell signaling, differentiation, and migration. They have been shown to be involved in growth plate as well as embryological and developmental signaling pathways, including hedgehog, Wnt, and bone morphogenetic proteins. The hedgehog proteins are particularly famous for their role in developing the left-right axis.
In an EXT1-deficient mouse model, Mundy et al. recently showed deformity in developing limb buds, disorganization of the growth plate, and resulting pathologically developing joint formation25. Strangely enough, even in the EXT1-deficient scenario, there is residual heparan sulfate26. Disorganization of primary cilia in the cartilage cap of the osteochondroma and microRNA signaling also has recently been shown to play a part in osteochondroma development27. Mouse models showed typical development of osteochondromas and joint deformation. Even though the mice were clones, no mirror imaging was noted, although this was not a focus point of the authors25,28.
To the best of our knowledge, our report is the first to describe monozygotic twins with MO with mirror-image tendencies, based on alignment discrepancies, deformities, and osteochondroma formation. Our findings could be a result of the physiological developmental role of the MO genes, as well as epigenetic pathophysiological modification.
Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. None of the authors, or their institution(s), have had any financial relationship, in the thirty-six months prior to submission of this work, with any entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Also, no author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.