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Polycystic lipomembranous osteodysplasia with sclerosing leucoencephalopathy (PLOSL; MIM 221770), also known as Nasu-Hakola disease, is a recessively inherited disorder characterised by systemic bone cysts and progressive presenile dementia associated with sclerosing leucoencephalopathy.1 The onset usually occurs in the third decade of life with pathological fractures; later on, symptoms of frontal lobe dysfunction appear, with upper motor neurone involvement and epileptic seizures. Some patients, however, do not have clinically manifest osseous problems despite the radiological demonstration of cystic bone lesions. The disease leads to death before the age of 50.1
The disease is characterised by genetic heterogeneity: mutations in two genes (TYROBP and TREM2) encoding different subunits of a membrane receptor complex in natural killer and myeloid cells have been associated with the disease.2,3
This rare disorder was initially described in Finland and Japan but is now recognised to have a worldwide distribution.1 In particular, sporadic cases have been described in Italy,4,5 and a homozygous mutation in the splice donor consensus site at intron 3 of TREM2 has been identified in two affected siblings.3
We report here the clinical and genetic analysis of an Italian family in which two siblings are affected by PLOSL.
After giving their informed consent, all the family members were submitted to neurological examination, psychological interview, bone radiographs, and brain computed tomography (CT) or magnetic resonance imaging (MRI). Genomic DNA was extracted from whole blood by standard methods. The entire coding sequences and the intron–exon boundaries of TYROBP and TREM2 genes were amplified from the DNA of each patient. After purification with a QIAquick PCR purification kit (Quiagen, Milan, Italy), polymerase chain reaction (PCR) products were directly sequenced on both strands using the Big Dye terminator kit (Applied Biosystems, Milan, Italy) and a model 310 automated sequencer (Applied Biosystems).
Linkage analysis was undertaken using the microsatellite markers D19S608, D19S610, and D19S876. The order on chromosome 19 is as follows: centromere – D19S610 – TYROBP – D19S876 – D19S608 – telomere. Briefly, primers specific for each locus were used to amplify the repeat sequences in template DNA by PCR. The forward primers were labelled by 6-carboxyfluorescein, and PCR products were analysed by a model 310 automated sequencer (Applied Biosystems).
The family pedigree is shown in fig 1. The family originated from a restricted area of northern Italy (Piacenza) and pedigree analysis seems to exclude consanguinity in the last five generations.
The proposita (II,1) is a 46 year old woman. She was of normal psychomotor development. She had been in good health until aged 23 years, when pathological fractures of both extremities started to occur, with radiological evidence of multiple cystic lesions in the distal bones. At the age of 30 she began to have insidious personality changes, depression of mood with suicidal ideas, and loss of social inhibition and judgment. Aged 40, psychological assessment suggested frontal dysfunction, and neurological examination showed the presence of primitive reflexes, mild apraxia, dyscalculia, and spatial and temporal disorientation. An EEG showed theta and delta activity dominating in the frontal areas, and brain CT showed a marked and diffuse cerebral atrophy with calcification in the basal ganglia. The disease progressed, with marked worsening of cognitive and motor functions, cerebral ictal events and epileptic seizures, leading finally to a vegetative state.
The affected sister (II,2) is 35 years old. At the age of 30 she began showing progressive loss of judgment, depressed mood, changes of personality, and uninhibited attitudes. No pathological fractures occurred, but x ray imaging showed cystic bone lesions in the metatarsal bones. Neuropsychological assessment revealed deterioration of intellectual function with frontal signs, dyscalculia, and dysgraphia. Cerebral MRI showed severe diffuse cerebral atrophy with basal ganglia calcification.
Neither cystic bone alterations nor pathological cerebral signs were found in the relatives.
Sequencing analyses did not detect any mutation in the five exons and in the intron–exon boundaries of TYROBP gene. Microsatellite analysis was undertaken with molecular markers spanning 120 kb of the genomic region containing the TYROBP gene. Although only marker D19S610 was fully informative, the linkage analysis excluded any association between the presence of the disease in our family and the PLOSL locus on chromosome 19.
In the two affected sisters, sequencing analysis identified a homozygous C to T mutation at position 191 (191 C→T) in exon 2 of the TREM2 gene. The mutation changes glutamine 33 to a stop codon (Q33X). To screen the family members for the identified mutation, we investigated a possible change in enzymatic restriction sites introduced by the mutation. The mutation abolished a Pst I site. This allowed us to propose a simple test to screen the family members: the parents (I,1; I,2), the proposita’s daughter (III,1), and the brother (II,4) were found to be heterozygous carriers of the mutated allele, while the other sister (II,3) was homozygous for the wild type allele (fig 1).
The clinical features of our cases are typical of PLOSL, but this family presents a novel homozygous mutation in exon 2 of TREM2. This mutation generates a premature stop codon and it is unlikely to be a polymorphism. Our findings confirm that PLOSL is characterised by a remarkable genetic heterogeneity, showing that mutations in different components of a single signalling pathway may lead to the same clinical condition.
In conclusion, in Italy PLOSL is explained by two different mutations in TREM2 gene.3 Its prevalence is undetermined because the disease is likely to go unrecognised. We believe that if physicians were more aware of this disease and were able to identify more cases, this would lead to a better clinical and genetic understanding of the condition.
We are grateful to the family who participated in this study and to the “Associazione Laura Fossati ONLUS”. A special thank you to Dr Ileana Ranzini for her secretarial support.
Competing interests: none declared