Aggressive cutaneous malignancies following cardiothoracic transplantation

The Australian experience

Abstract

BACKGROUND

The development of malignancies in recipients of a cardiothoracic transplant (CTT)—that is, heart, lung, or heart and lung recipients—is of concern. Cutaneous and lymphoproliferative malignancies comprise the two major groups of malignancies encountered. A small subgroup of patients will develop potentially life-threatening aggressive cutaneous malignancies (ACM); these are poorly defined and documented in the literature. The authors report the results for 619 CTT recipients from a single institution.

METHODS

Between 1984 and 1995, 619 recipients received a CTT. With a minimum follow-up of 2 years, 66 patients (10.7%) were diagnosed with a major malignancy, and 27 of these 66 patients developed ACM. ACM were defined as having one or more of the following characteristics: local invasion and/or regional metastases at diagnosis, poor differentiation, and locoregional and/or systemic relapse following therapy. All malignant melanomas were considered ACM. Data on malignancy occurrence were documented in the clinical notes of the heart and lung transplant unit. A retrospective analysis was undertaken from these notes.

RESULTS

Tumor histology was predominantly poorly differentiated squamous cell carcinoma (55%) (SCC) and malignant melanoma (30%) (MM). No patient developed Kaposi sarcoma (KS). The median time from transplant to diagnosis of ACM was 52 months (range, 8–127 months). Thirteen of 27 patients have died; 10 of them died of metastatic disease. The mean time to death was 20 months (range, 8–54 months). Of 14 patients alive, 5 have disease. All but one of the 19 patients diagnosed with nonmelanoma ACM received radiotherapy, either as part of initial treatment or on relapse. Eight patients have subsequently suffered an infield relapse.

CONCLUSIONS

The development of ACM in CTT recipients resulted in substantial morbidity and mortality. Poor results were obtained with standard surgery and radiotherapy. Treatment modalities for and the underlying pathobiology of ACM in organ transplant recipients require detailed research if improved outcomes are to be achieved. Cancer 1999;85:1758–64. © 1999 American Cancer Society.

The relation between immunosuppression associated with solid organ transplantation and the development of malignancies is well documented.1, 2 This is of particular concern in the setting of cardiothoracic transplantation (CTT) as the number of transplant recipients increases and life expectancy improves. Malignancies developing in CTT recipients are predominantly posttransplantation lymphoproliferative disorders (PTLD) and cutaneous malignancies (mainly superficial and in situ squamous cell carcinoma [SCC] and basal cell carcinoma [BCC]) plus a group of miscellaneous malignancies. Malignancy has accounted for 13% of all deaths posttransplantation in our study population.3

The majority of cutaneous malignancies are adequately treated locally without significant morbidity. We identified a subgroup of patients who developed cutaneous malignancies that exhibited aggressive clinical behavior often leading to considerable morbidity and mortality. This group has previously been referred to in the transplantation literature as patients with aggressive SCC,4 but they still remain poorly defined and documented. We have used the broader term “aggressive cutaneous malignancies” (ACM) to include all primary cutaneous malignancies that have exhibited an aggressive natural history. The aim of this study was to assess the incidence, treatment, and clinical behavior of ACM in a group of Australian CTT recipients. As far as we know, this study represents the largest single-institution series documenting the development and behavior of ACM in CTT recipients.

MATERIALS AND METHODS

Between February 1984 and December 1995, 619 CTT were performed: 437 orthotopic heart, 3 heterotopic heart, 55 bilateral sequential lung, 54 heart and lung, and 70 single lung. All 619 patients received their transplants at St. Vincent's Hospital, Sydney, Australia. The mean age at transplantation was 44 years, and the majority of patients were male (81%). Sixty-six patients (10.7%) have developed a major malignancy with a minimum follow-up of at least 2 years (range, 2–11 years). Patients developing a major malignancy or relapsing after treatment for a major malignancy after December 1997 were not included in this study. A major malignancy was defined as any solid or hematologic malignancy with invasive or metastatic potential. Excluded from this definition was any nonhematologic malignancy diagnosed within the first 12 months following transplantation (9 patients fitted this criterion). One ACM patient was diagnosed at 8 months posttransplantation with malignant melanoma, and for completeness was included in the study. Patients diagnosed with superficial cutaneous malignancies (SCM) were not considered to truly fulfill the criteria of a major malignancy. The definition of SCM included all in situ SCC and SCC with only minimal deep invasion that did not invade local structures and did not exhibit features fitting the definition of ACM. One patient was found to have a primary small cell carcinoma in the explanted lung; she died of metastatic disease at 10 months posttransplantation and was not included. Also excluded were 3 patients diagnosed with pituitary, renal, and parathyroid adenomas, respectively; these did not meet the strict criteria of an invasive malignancy.

ACM were defined by one or more of the following features: local infiltration and/or regional metastases at diagnosis, poor differentiation, and locoregional and/or systemic relapse following therapy. Malignant melanomas were considered to be ACM even if there was superficial spreading (Clark's level 1); 2 patients fit into this category. Patients were treated initially with either surgery alone or surgery and adjuvant radiotherapy. On relapse, patients were treated with surgery, radiotherapy, or chemotherapy, often in combination. The ACM were reviewed for histology, treatment, and outcome. The literature was reviewed to determine the incidence and management of ACM at other organ transplant units.

Data on malignancy occurrence were not routinely compiled until 1992, but all major malignancies were documented in the clinical notes of the Heart and Lung Transplant Unit. Retrospective analysis of these clinical notes and correspondence with referring units allowed compilation of an extensive data set on patients diagnosed with major malignancies.

RESULTS

Patients and Outcomes

Sixty-six (10.7%) of the 619 CTT recipients have developed major malignancies. This group contains 52 males and 14 females. The mean age at diagnosis of major malignancy was 52 years (range, 36–67 years). ACM accounted for 27 (41%) of the 66 major malignancies. ACM was diagnosed in 25 cardiac and 2 lung transplant recipients. The only female diagnosed with ACM developed MM. Three patients were treated for two major malignancies each; all but one of these second malignancies was an ACM. The median time from transplantation to ACM diagnosis was 52 months (range, 8–127 months). At the close of the study, 13 of 27 patients had died. The mean time to death, from ACM diagnosis, was 20 months (range, 8–54 months). Ten of these 13 patients had died of metastatic disease, 2 of uncontrolled local disease, and 1 of an unrelated cause.

Histology and Immunosuppression

The histologic diagnosis was SCC in 18, MM in 8 patients, and both Merkel cell carcinoma (MCC) and SCC together (though separated in time) in 1 patient. No patients developed KS. No patient developed a BCC fitting the criteria of ACM. Of 20 individual SCC (in 19 patients), 15 were poorly differentiated, 3 were moderately differentiated, and 2 were diagnosed at other institutions and not ascribed a degree of differentiation. All patients had more than one poor prognostic feature previously documented as fitting the definition of ACM. All patients were prescribed triple immunosuppression consisting of cyclosporine, azathioprine, and prednisolone maintenance therapy. Prior to May 1994, polyclonal antithymocyte globulin (ATG) was routinely used part of an induction immunosuppressive regime.

Outcomes Based on Initial Treatment of NonMelanoma Patients (Patients 1–19)

The treatment and outcomes of these patients are described in Table 1. Initial treatment involved wide local excision and/or lymph node dissection plus, in some cases, adjuvant radiotherapy. Six patients (Patients 8, 9, 11, 12, 14, and 16) underwent superficial parotidectomies; all had metastatic SCC to peri/intraparotid lymph nodes. Most did not have an obvious cutaneous primary present at the time of operation. Four also had simultaneous ipsilateral neck dissections; two of these patients had pathologically involved lymph nodes. In total, nine patients underwent neck dissections as either initial treatment or at the time of relapse, and all but two had pathologically involved lymph nodes. No patient was treated with radiotherapy alone. Nineteen patients were diagnosed with 22 major malignancies, 21 of these being ACM. This occurred because 3 patients (Patients 1, 9, and 11) developed a second major malignancy. One was a synchronous sigmoid adenocarcinoma (Patient 9) treated simultaneously with the ACM, whereas the other two developed a second ACM at 16 (Patient 1) and 32 (Patient 11) months, respectively, following initial treatment.

Table 1. Summary of Patients with Nonmelanomas

Patient no.	Age (yrs)/gender	Site/histology	Time to diagnosis from transplant	Initial treatment	Result	Time to relapse from previous treatment	Type of retreatment	Outcome and time to death or length of follow-up (from diagnosis)	Site of metastases or residual disease
SCC: squamous cell carcinoma; MMC: Merkel cell carcinoma; LN: lymph nodes; LC: local control; LR: local relapse; RR: regional relapse; SR: systemic relapse; DF: disease free; DP: disease present; S: surgery; RT: radiotherapy; CT: chemotherapy; WLE: wide local excision; IND: ipsilateral neck dissection; BND: bilateral neck dissection. a These two patients were lung transplant recipients. b These results followed retreatment. c These patients had two major malignancies diagnosed. d All patients underwent a superficial parotidectomy for pathologically positive periparotid/intraparotid lymph node SCC metastases, all had previously treated superficial skin cancers of the head and neck, and most did not have an obvious primary present at the time of operation.
1	57 Mc	1. Left scalp MCC	1.36 mos	1. WLE/RT (50.4 Gy)	1. LR infield	21 mos	1. CT/RT (20Gy)	Dead (54 mos)	Left scalp, liver
	2. Right scalp SCC	2. 52 mos	2. WLE/RT (50.4 Gy)	2. LC
2	50 M	External Ear SCC	64 mos	WLE	LR	1 mos	IND/RT (50Gy)
	LR infieldb	25 mos	WLE/RT (46Gy)	Alive DP (34 mos)	Lung, liver
3	56 M	Scalp SCC	70 mos	WLE	LR	3 mos	WLE/RT (55Gy)
	LR infieldb	8 mos	No treatment	Dead (19 mos)	Bone marrow
4	46 M	External ear SCC	48 mos	WLE/RT (53 Gy)	LR infield	12 mos	WLE	Alive DF (23 mos)
5	55 M	Forehead SCC	31 mos	WLE	LR	3 mos	WLE/RT (60Gy)
	RRb	8 mos	BND/RT (50.4Gy)	Dead (18 mos)	Thorax/bone
6	48 M	Lip SCC	37 mos	WLE	RR	7 mos	IND/RT (50Gy)	Dead (25 mos)	Lung/pancreas
7	49 M	Neck SCC	89 mos	WLE/RT (60 Gy)	Uncontrolled locally	<1 mos	Palliative RT/CT	Dead (15 mos)	Neck
8	45 M	Parotid LN SCCd	80 mos	WLE/RT (50 Gy)	LC/SR	7 mos	No treatment	Dead (8 mos)	Brain/lung
9	67 Mc	1. Parotid LN SCCd	56 mos	1. WLE/IND	1. LC
	2. Sigmoid colon (adenocarcinoma)	56 mos	2. S/CT/RT (50.4 Gy)	2. LC/SR	17 mos	No treatment	Dead (18 mos)	Metastatic adenocarcinoma
10	52 M	Lower neck SCC	29 mos	WLE/RT (50 Gy)	LC/SR	12 mos	Lung lesion excised	Alive DP (41 mos)	Anterior mediastinum
11	45 Mc	1. Lower lip SCC	1. 45 mos	1. WLE/RT (50.4 Gy)	1. LC
	2. Parotid LN SCCd	2. 77 mos	2. WLE/IND/RT (50.4 Gy)	2. LR infield	14 mos	Palliative RT (40Gy)	Alive DP (55 mos)	Left neck/face
12	55 M	Parotid LN SCCd	127 mos	WLE/IND/RT (50 Gy)	LC	Alive DF (3 mos)
13	42 M	Upper chest SCC	89 mos	WLE/RT (50 Gy)	LR infield/RR	3 mos	Palliative RT (30Gy)	Dead (8 mos)	Chest wall Lymph nodes
14	63 M	Parotid LN SCCd	44 mos	WLE/RT (54 Gy)	LC	Alive DP (51 mos)	Skeletal mets
15	48 M	Right ear SCC	40 mos	WLE	LR	12 mos	WLE/RT (55Gy)	Alive DF (20 mos)
16	49 M	Parotid LN SCCd	78 mos	WLE/IND RT (60 Gy)	LR infield	22 mos	WLE	Alive DF (25 mos)
17	55 Ma	Left axilla SCC	54 mos	WLE/RT (50 Gy)	LR infield	2 mos	No treatment	Alive DP (5 mos)	Left axilla/arm
18	55 Ma	Left upper neck SCC	47 mos	WLE	LC	Alive DF (5 mos)
19	49 M	Forehead SCC	101 mos	WLE/RT (45 Gy)	LC	Alive DF (14 mos)

Of 7 patients who received surgery as sole initial treatment (Patients 2, 3, 5, 6, 9, 15, and 18), all but 1 (Patient 18) have relapsed. Four have relapsed locally, 1 relapsed in regional lymph nodes, and 1 relapsed systemically. The mean time to relapse was 5.2 months (range, 1–12 months). Of the remaining 12 patients treated initially with surgery and adjuvant radiotherapy, 6 have experienced infield relapse (Patients 1, 4, 11, 13, 16, and 17). The mean dose of radiotherapy delivered as initial treatment was 52gray (Gy) (range, 45–60 Gy). The mean time to infield relapse was 12 months (range, 2–22 months). Patients 2 and 3 also experienced an infield relapse, but only after retreatment with surgery and radiotherapy following initial relapse after surgery alone. In total, 8 of 15 patients (53%) experienced an infield relapse following high dose (50 Gy) radiotherapy.

Retreatment and Outcomes of Nonmelanoma Patients (Patients 1–19)

Eight patients diagnosed with nonmelanomas have died; of the 11 that remain alive, 5 have disease present. In total, only 3 patients have remained free from any disease relapse after initial treatment and are currently alive (Patients 12, 18, and 19). Retreatment involved surgery (wide local excision or lymph node resection) in 8 patients; 5 of these also received adjuvant radiotherapy. The 3 not receiving adjuvant radiotherapy had either received high dose radiotherapy previously (Patients 4 and 16) or did not require it (Patient 10 had a solitary lung lesion excised). Three patients (Patients 2, 3, and 5) relapsed a further second time; 2 of them were retreated with further surgery and radiotherapy (Patients 2 and 5). Both of these patients received further radiotherapy that partly encompassed previously irradiated fields. Four patients did not receive treatment on relapse because of either metastatic disease or poor performance status (Patients 3, 8, 9, and 17). Four patients received radiotherapy with palliative intent (Patients 1, 7, 11, and 13), with 2 of these also receiving chemotherapy.

Outcomes Based on Initial Treatment of Malignant Melanoma Patients (Patients 1–8)

The treatment and outcomes of these patients are described in Table 2. All 8 patients diagnosed with MM were treated initially with surgery alone; 2 had regional lymph node dissections at the time of primary excision, with only 1 having pathologically involved lymph nodes (Patient 5). No patient received radiotherapy in either the definitive or the adjuvant setting as part of the initial treatment. Four patients have maintained local control (Patients 3, 6, 7, and 8), 2 have experienced local failure (Patients 4 and 5), 1 has had regional failure (Patient 1), and 1 has had systemic failure (Patient 2). The mean time to relapse was 11 months (range, 4–15 months).

Table 2. Summary of Patients with Melanoma

Patient no.	Age (yrs)/ gender	Site	Time to diagnosis from transplant	Initial treatment	Result	Time to relapse from previous treatment	Type of retreatment	Outcome and time to death or length of follow-up (from diagnosis)	Site of metastases or residual disease
LC: local control; WLE: wide local excision; RR: regional relapse; SR: systemic relapse; LR: local relapse; WLE: wide local excision; DF: disease free; BND: bilateral neck dissections; IND: ipsilateral lymph node dissection. a These 2 patients had superficial spreading melanoma.
1	50/M	Temple	26 mos	WLE	RR	10 mos	BND	Dead (35 mos)	Brain/liver
2	56/M	Back	48 mos	WLE	SR	14 mos	No treatment	Dead (16 mos)	Liver/spleen
3	58/Ma	Abdominal	8 mos	WLE	LC	Alive DF (46 mos)
4	59/M	Forehead	63 mos	WLE/IND	LR	4 mos	WLE	Dead (8 mos)	Metastatic disease
5	36/M	Lower leg	13 mos	WLE/IND	LR	15 mos	WLE	Dead (36 mos)	Lung
6	42/Fa	Lower leg	75 mos	WLE	LC	Alive DF (12 mos)
7	43/M	Abdomen	33 mos	WLE	LC	Alive DF (5 mos)
8	36/M	Back	67 mos	WLE	LC	Dead (cardiac arrest) (8 mos)

Retreatment and Outcomes of Melanoma Patients (Patients 1–8)

Five patients diagnosed with MM have died (Patients 1, 2, 4, 5, and 8). One patient died from a cardiac arrest without experiencing a relapse of any sort (Patient 8). Four patients remain free from relapse, 2 of these had a diagnosis of superficial spreading melanoma (Patients 3 and 6). Of the 4 patients who have experienced a relapse, 3 were retreated with surgery (wide local excision in 2 and lymph node dissection in 1). The 1 patient who had bilateral neck dissections had pathologic involvement of multiple bilateral lymph nodes (Patient 1). The remaining patient relapsed with metastatic disease and was managed conservatively. All 4 patients who experienced a relapse have died of metastatic disease.

DISCUSSION

The etiology of malignancies following solid organ transplantation is multifactorial, but central to their development is the ongoing immunosuppression required as a consequence of allograft transplantation. Penn et al. first reported this correlation in 1969.5 The greater number of renal transplant recipients compared with CTT recipients and the longer follow-up period have provided supportive evidence for an increasing risk over time of malignancy development. Since 1963, malignancies developing in renal transplantation patients from Australia and New Zealand have been documented in a transplant registry. An Australasian study of 6596 renal transplant patients reported a 27% probability of developing a noncutaneous malignancy and a 66% probability of developing a cutaneous malignancy by 24 years posttransplantation.6 Similar cumulative incidences have been described by other authors in discussions of transplant populations in The Netherlands and Northern England.7, 8

Australia has the highest annual incidence of nonmelanoma skin cancer in the world: almost 1000 per 100,000.9 This is related to chronic sun exposure with higher levels of ultraviolet B radiation (UV B) (290–320 nm) compared with Northern Hemisphere countries. Malignant melanoma has an annual age-standardized incidence of 30.2 and 23.9 per 100,000 for Australian males and females, respectively.10 UV B radiation promotes epidermal mutagenesis and can selectively immunosuppress both circulating and noncirculating components of the immune system.11 This effect is heightened for the high proportion of Caucasians in the Australian population, who have a pale, lightly pigmented Celtic skin type that is prone to sunburn.

In the normal population, the ratio of BCC to SCC is approximately 5:1. This ratio is reversed in the transplant population, with studies reporting ratios of SCC:BCC from 1.8:1 up to 15:1.12, 13 A small proportion of renal transplant–related SCC are aggressive in behavior, displaying local infiltration, a tendency to recur, metastases to regional lymph nodes, and systemic spread. Sheil et al. reported a 7% incidence of recurrence and metastasis in renal transplantation patients diagnosed with SCC.6 Euvrard et al. reported on 50 French transplant patients treated for SCC, of whom 5 (2 with kidney transplants and 3 with cardiac transplants) were defined as having aggressive SCC because of multiple local recurrences and/or regional lymph node metastases.4 Three patients with regional lymph node metastases received adjuvant radiotherapy but subsequently died. All 5 patients had a marked reduction in immunosuppression, with cessation of cyclosporin and azathioprine; 2 remain alive 15 months posttreatment. It is noteworthy that no episodes of rejection were documented following the reduction in immunosuppression in the two surviving cardiac patients. Many of the 27 patients in our study had a reduction in overall immunosuppression of up to 25% without an apparent clinical benefit.

There is a proposed link between human papillomavirus (HPV) infection and the development of premalignant and malignant cutaneous lesions in transplant patients.14 A model for this is the rare lifelong disorder of epidermodysplasia verruciformis (EV), a condition characterized by multiple cutaneous lesions resembling flat warts and with an associated abnormal cell-mediated immune system. Up to 30% of these patients will develop cutaneous malignancies within sun exposed skin, and as many as 90% will have HPV 5/8 subtype DNA isolated from lesions.15 In a Scottish study of 202 renal allograft patients, 15 of 25 SCC contained HPV 5/8 DNA with the suggestion that these serotypes may contribute to the development of SCC in renal allograft patients.13 Further studies have isolated HPV serotypes from premalignant and malignant lesions in both cardiac and renal transplant patients. Berkhout et al. isolated HPV subtype DNA associated with EV in 80% of SCC from renal transplant patients using polymerase chain reaction.17 The contribution of HPV infection to the development of cutaneous malignancies in transplant patients remains unclear.

Both the dose of immunosuppressive drugs and the number used have been implicated in the incidence of malignancies.17 Cardiac transplant patients generally receive quantitatively more immunosuppression than their renal counterparts. The triple drug regime used to treat CTT recipients at St. Vincent's Hospital consists of cyclosporine, azathioprine, and prednisolone with later steroid minimization.18 Azathioprine and its two major metabolites, 6-mercaptopurine and methyl-nitro-thioimidazole, promote the development of ultraviolet radiation–induced skin cancers and premalignant dysplastic lesions.19 Sheil extensively reviewed the malignancies occurring in 6067 renal transplant patients in the Australian and New Zealand combined Dialysis and Transplant Registry. He was able to match a group of 339 patients treated with cyclosporine alone and compare them with a matched group of patients who received only azathioprine and prednisolone. Analysis of the data revealed no significant difference in the development of either cutaneous or noncutaneous malignancies between these two groups.20

Penn reviewed the Cincinnati Transplant Tumor Registry (CTTR) containing data on 7938 solid organ transplantation patients, of whom 769 were heart transplant recipients.17 He noted no difference in the overall incidence of malignancies among patients who received azathioprine, cyclosporine, or a combination of azathioprine and cyclosporine. Of note, 40.6% of the tumors in patients who received the azathioprine-based treatment were cutaneous and lip tumors, compared with 25.1% of those who received the cyclosporine-based treatment. Lymphomas comprised 10.2% and 22.5%, respectively. Penn concluded that it was not the cyclosporine per se but the intensity of overall immunosuppression, particularly in nonrenal patients, that has led to a changing pattern of malignancies.

Many authors fail to adequately separate or define the less aggressive cutaneous malignancies from the potentially lethal ACM. In a separate review of the first 455 cardiac transplant recipients at our institution, from which 55 patients were excluded due to early death or incomplete data, we identified a cumulative incidence for skin cancer of 31% at 5 years (Table 3).21 On the basis of this, we estimate that up to 15% of Australian cardiac transplant recipients who develop skin cancer will have an ACM. We feel justified in not including nonhematologic malignancies that occurred in the first 12 months posttransplantation. These almost certainly would have been present at the time of surgery. Other authors do not necessarily preclude such patients when reporting results, thereby potentially overcalling the true incidence of malignancies in this population of patients. We accept that 12 months is an arbitrary time period, but this acknowledges the concept that most solid tumors must undergo many cell doublings prior to becoming clinical entities (greater than 1 cm).

Table 3. Summary of Cutaneous Lesions in 455 Cardiac Recipients (Modified after Ong et al.21)

Cutaneous lesion	No. of patientsa	No. of lesionsb
SCC: squamous cell carcinoma; BCC: basal cell carcinoma; KA: keratoachanthoma. a Many patients were treated for more than one type of lesion. b Most patients had multiple lesions excised.
SCC	113	849
BCC	92	285
Melanoma	7	7
KA	19	28
Bowen disease	79	263
Total	152	1433

The aggressive and atypical natural history of ACM highlights the difficulty in approaching the management of these malignancies. The high rate of infield relapses and systemic failure (brain, lung) is unusual in the immunocompetent patient developing cutaneous malignancies. Whether the infield relapses following radiotherapy represent true infield second primaries or a failure to sterilize subclinical disease is unknown. The results of our study suggest that accepted adjuvant radiation doses (50–60 Gy) may be inadequate for our patients with an infield relapse rate of 53%. The mean time to infield relapse was 12 months, suggesting that there may have been a failure to adequately sterilize subclinical disease present at the time of radiotherapy. It should be noted that 5 of 7 patients treated for nonmelanoma ACM with surgery alone have experienced locoregional relapse. None of the MM patients received postoperative radiotherapy after initial surgery.

CONCLUSIONS

Malignancy is a major problem following CTT. As length of follow-up increases in the Australian CTT population, it is likely that the incidence of malignancies will increase over time. A small subgroup of these CTT recipients develops ACM. Standard treatment approaches have failed to control disease in a significant proportion of patients. Firm evidence that either an aggressive surgical approach, in combination with radiotherapy, or a major reduction in immunosuppression is beneficial or deleterious to the patient is lacking. The relevant interaction and prognostic significance of UV B radiation, skin type, HPV infection, and immunosuppression remains unknown.