Sun exposure causes somatic second-hit mutations and angiofibroma development in tuberous sclerosis complex

Abstract

Tuberous sclerosis complex (TSC) is characterized by the formation of tumors in multiple organs and is caused by germline mutation in one of two tumor suppressor genes, TSC1 and TSC2. As for other tumor suppressor gene syndromes, the mechanism of somatic second-hit events in TSC tumors is unknown. We grew fibroblast-like cells from 29 TSC skin tumors from 22 TSC subjects and identified germline and second-hit mutations in TSC1/TSC2 using next-generation sequencing. Eighteen of 22 (82%) subjects had a mutation identified, and 8 of the 18 (44%) subjects were mosaic with mutant allele frequencies of 0 to 19% in normal tissue DNA. Multiple tumors were available from four patients, and in each case, second-hit mutations in TSC2 were distinct indicating they arose independently. Most remarkably, 7 (50%) of the 14 somatic point mutations were CC>TT ultraviolet 'signature' mutations, never seen as a TSC germline mutation. These occurred exclusively in facial angiofibroma tumors from sun-exposed sites. These results implicate UV-induced DNA damage as a cause of second-hit mutations and development of TSC facial angiofibromas and suggest that measures to limit UV exposure in TSC children and adults should reduce the frequency and severity of these lesions.

Tuberous sclerosis complex inactivation disrupts melanogenesis via mTORC1 activation

“…Skin biopsies of hypomelanotic macules and nearby normal skin were obtained for cell culture. Fibroblasts were isolated as described previously (21). The melanocytes were isolated as described previously (90).…”

“…We have recently shown that facial angiofibroma and other TSC cutaneous tumors are due to 2-hit mutational inactivation of both alleles of either TSC1 or TSC2 and that lesions on the face often sustain UV-induced CC>TT mutations in the second allele (21). Hypopigmented macules typically appear similar to an ash leaf in configuration, with relative hypopigmentation in comparison with surrounding skin, and are most commonly seen on the limbs and trunk (22,23).…”

Estudo mutacional em pacientes com o complexo da esclerose tuberosa

“…Outro estudo recente, utilizando técnicas similares para sequenciamento de todos os éxons e a grande maioria intrônica de ambos os genes, não identificou mutações em 18% dos pacientes (Tyburczy et al, 2013 Knudson, 1971). …”

“…Perda de heterozigose foi demonstrada mais frequentemente para hamartomas do que hamártias de TSC (Au et al, 2004;Crino et al, 2010;Campos et al, 2013). Outros estudos relatam mutações de ponto como a mutação somática em hamartomas de pele desses pacientes (Tyburczy et al, 2013), o que não seria observado por métodos indiretos como perda de heterozigose. A limitação de demonstração da mutação somática em hamártia cerebral de pacientes com TSC, seja por método direto ou indireto, sugere haploinsuficiência da mutação germinativa (Niida et al, 2001;Wei et al, 2010).…”

“…Em outro cenário, pacientes sem mutação identificada em leucócitos poderiam apresentar mosaicismo somático do primeiro evento mutacional, podendo este ser detectável em tecido afetado por hamartoma. Entretanto, análises recentes com sequenciamento massivo, em paralelo de DNA de angiofibromas de 4 pacientes com TSC não detectou mutações em 18% deles (Tyburczy et al, 2013). Ainda, a combinação de polimorfismos de DNA em genes codificadores para proteínas da via do mTOR poderia, em outra hipótese, causar TSC em pacientes NMI.…”