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Localized cutaneous amyloid at an insulin injection site

We report a 40-year-old man with insulin-dependent diabetes, who developed localized cutaneous amyloid deposition at the site of insulin injection (Humulin ; Eli Lilly & Co. Ltd, Basingtoke, Hampshire, UK). A lump, clinically suggestive of a lipoma, was excised from the upper arm. On histological examination, calcified hyaline deposits were seen, staining positively with Congo red and displaying apple-green birefringence under polarized light. Under electron microscopy (EM), fibrils 12 nm in size, consistent with amyloid, were seen (Fig. 1), and immunohistochemical staining for amyloid P (1 : 200; product code A0302; Dako, Glostrup, Denmark) further supported this. The amyloid deposit also stained specifically with a mouse monoclonal antihuman insulin antibody (1 : 20; product code: NCLINSULIN; Novocastra, Newcastle upon Tyne, UK). Amyloid is not a single chemically distinct substance, as it may develop from several precursor proteins. Amyloidosis may be primary or secondary, localized or systemic. It is characterized histologically by green birefringence in polarized light when stained with Congo red and by a fibrillary structure under EM, due to the formation of b-pleated sheets. Insulin is known to be able to convert into a fibrillary form in vitro when subjected to certain physical or chemical stimuli such as heat or acidity. Although cutaneous amyloid is well recognized as a localized secondary phenomenon within some skin tumours, there are few reports of localized amyloid deposition secondary to insulin injection. The first report of localized cutaneous amyloid secondary to insulin injection was published in 1983. Amyloid was induced iatrogenically using subcutaneous porcine insulin in six rats and one human subject. In a subsequent clinical report, an area of presumed lipohypertrophy at a site of repeated insulin injection was excised. Congo red and EM studies confirmed amyloid. Immunohistochemistry revealed binding to several anti-insulin antibodies but amino acid sequencing confirmed porcine insulin as the precursor protein. In another report, a patient using porcine insulin developed a firm mass adjacent to an area of lipohypertrophy. On histological examination of the lesion, localized cutaneous amyloid was seen, which bound to antimammalian insulin antibody, confirming porcine insulin as the amyloid precursor protein. One further case is reported in the literature, but the precursor protein was not elucidated. Localized secondary cutaneous amyloid deposition is a well-recognized phenomenon and yet there is a paucity of literature concerning localized cutaneous amyloid at insulin injection sites. It is possible that this is because it is occurs rarely, but given the high incidence of diabetes in